PPPS 2019

DoubleTree at the Entrance to Universal Orlando

DoubleTree at the Entrance to Universal Orlando

5780 Major Blvd. Orlando, Florida, 32819, USA
Raymond Allen (NRL), Chunqi Jiang (Old Dominion University), Josh Leckbee (Sandia National Labs)

Welcome to the Indico interface for the 2019 IEEE Pulse Power and Plasma Science Conference. At this site, you will be able to perform the following actions related to the conference:

  • Abstract Submission
  • Manuscript Submission
  • View the online schedule of events

Further information about the conference, including the categories of the technical program, important dates, and contact information, we refer you to the main conference website.

    • 08:30 17:00
      Mini Course 8h 30m
    • 08:00 12:00
      Mini Course 4h
    • 12:00 18:00
      Registration 6h
    • 18:00 20:00
      Welcome Reception 2h
    • 07:30 08:15
      Continental Breakfast 45m Universal Center ()

      Universal Center

    • 08:00 08:15
      Announcements 15m
    • 08:15 09:15
      Plenary Mon AM - Martin Gundersen Seminole Ballroom ()

      Seminole Ballroom

      Convener: Dr Susan Heidger (U.S. Air Force Research Laboratory)
      • 08:15

        This talk will review a university program that overlaps pulsed power and plasma science, and will describe applications to industrial, environmental, biomedical and defense problems. It will present some background for the development of the research, and the ideas underlying transient plasma, or plasma in a formative phase, which is key to some of these studies. Transient plasma can produce volume ignition of various fuels and engines with lower energy cost, for example, considerably reduced delay to ignition in pulse detonation engines, higher peak pressure for internal combustion engines, and improved energy efficiencies in emissions abatement. Biomedical applications include studies of nanosecond pulsed electric fields for the induction of programmed cell death in cancer cells in vitro and in vivo which have led to animal studies conducted with catheter-based pulsed power delivery systems, and the formation of commercial entities translating the research to medical applications. Finally, a project (tenuously connected to pulsed power) to foster and encourage interest in science and engineering and improve perceptions of science in society through movies will be described.

        Speaker: Prof. Martin Gundersen (University of Southern California)
    • 09:15 09:45
      AM break 30m
    • 09:45 11:45
      1.2 Computational Plasma Physics I Seminole A/B (Double Tree at the Entrance to Universal Orlando)

      Seminole A/B

      Double Tree at the Entrance to Universal Orlando

      5780 Major Blvd. Orlando, Florida, 32819, USA

      Session Chair: John Luginsland

      Convener: John W. Luginsland (Confluent Sciences)
      • 09:45
        Fractional models in solving Maxwell equations and applications 15m

        In numerical modeling, it is common to solve coupled differential equations in one-, two- or three- dimensions (1D, 2D, or 3D) with corresponding boundary conditions. For some complicated objects, the dimension may be strictly 1D, 2D or 3D, where the normal computational approach may be expensive. By using fractional models that had been developed mathematicians, the complicated object is projected into a “fractional” dimension in order to solve the relevant equations in this non-integer dimension with the assumption that the effects at smaller scales can be ignored. In this talk, we will present some recent results of using such “fractional” models on Maxwell-equation based problems such as fractional Child-Langmuir law for high current cathode, Fractional Mott-Gurney law for space charge limited current transport in organic diode, Fractional Fowler Nordheim law for field emission from rough surface, Fractional Fresnel coefficients for laser absorption/reflection on a rough metal surface and Fractional capacitance of a planar capacitor. These new fractional models will provide useful fractional parameters that can be characterized by experimental measurement. Smooth transition between the fractional models and the traditional models is demonstrated. The calculated results will be compared with available experimental results or numerical results obtained from commercial solvers and the comparison shows good agreement.

        Speaker: Prof. Lay Kee Ang (Singapore University of Technology and Design)
      • 10:00
        Consistent BGK model for high energy density plasma mixtures 30m

        We derive a conservative multispecies BGK model that follows the spirit of the original, single species BGK model by ensuring pairwise conservation of momentum and kinetic energy and that the model satisfies Boltzmann’s H-Theorem. The derivation emphasizes the connection to the Boltzmann operator, which allows for direct inclusion of information from a molecular dynamics validated effective Boltzmann model. We also develop a complete hydrodynamic closure via the Chapman-Enskog expansion, including a general procedure to generate symmetric diffusion coefficients based on this model. We further extend the model to include the effect of degeneracy on the electron plasma species. We employ this model to investigate kinetic effects on interfacial mixing of the shell-fuel interface in inertial confinement fusion, as well as experiments performed on the Z pulsed power facility.

        Sandia National Laboratories is a multimission laboratory managed and operated by National
        Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell
        International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration
        under contract DE-NA0003525. SAND Number: SAND2019-1735 A

        This work was supported by the US Department of Energy through the Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract No. 89233218CNA000001).

        Speaker: Jeffrey Haack (Los Alamos National Laboratory)
      • 10:45
        Speed-limited Particle-in-cell for Fast Simulation of Slow-plasma Problems 15m

        Speed-limited particle-in-cell simulation (SLPIC) is a method of increasing the time-step in a PIC simulation by slowing down the fastest particles in such a way that the end state of the simulation is unaffected, while significantly reducing the number of time-steps required to reach this end state. SLPIC is useful when the simulation requires a kinetic treatment of fast particles (e.g. electrons) while the physics of interest occurs on the time-scale of slow particles (e.g. ions).

        In a SLPIC simulation, the true velocities and weights of particles are tracked, but particles are moved through the simulation at a lower speed specified by the "speed-limiting" function, and weighted to the grid with a reduced weight. By moving fast particles at a lower velocity, the time-step of the simulation can be significantly increased relative to that of a PIC simulation.

        We show that for steady-state problems, SLPIC can achieve the same accuracy as PIC with a computational speed-up that is bounded by $\sqrt\frac{m_\mathrm{ion}}{m_\mathrm{electron}}$. For an argon-electron plasma sheath simulation, a speed-up factor of approximately 200 for reaching steady-state is demonstrated.

        The trade-off of the large SLPIC time-step is an increased algorithmic complexity, since the equations of motion and grid-weighting are each modified by the velocity-dependent speed-limiting function. We discuss ways of dealing with these complexities and their effect on accuracy in certain cases, as well as the implications of choosing various speed-limiting functions.

        To demonstrate the limits of SLPIC in dynamic problems we simulate the interaction of a wave with speed-limited particles and show that SLPIC is accurate only when the speed-limit is sufficiently higher than the wave velocity. This implies that SLPIC is useful for problems where the wave speed is slower than the fastest particles, for example, in ion-acoustic Landau damping.

        Speaker: Dr Andrew Chap (Tech-X Corporation)
      • 11:00
        Transition of low-temperature plasma similarity laws from low to high ionization degree regimes* 15m

        Similarity laws are often employed when the characteristics of two or more discharge systems are compared. The classical similarity laws were previously validated and applied for weakly ionized plasma discharges [1, 2]. However, similarity relations are not valid for all plasma regimes [3, 4]. Especially for strongly ionized regimes, scaling laws are not well understood. In this study, we evaluate the transition characteristics of low-temperature plasma similarity laws from low to high ionization degree regimes. The similarity relations of plasma density and ionization degree in geometrically similar gaps are presented. It is found that deviations of classical scaling laws occur as the ionization degree increases from low to high. For low pressures, the similarity laws hold until a higher ionization degree than for high pressures. The time-dependent scaling of the charged species and the electron energy distributions in two geometrically similar systems are also compared.

        [1] Y. P. Raizer, Gas Discharge Physics (New York: Springer, 1991)
        [2] Y. Fu, P. Zhang, J. Krek, and J. P. Verboncoeur, Appl. Phys. Lett. 114, 014102, 2019.
        [3] A. Venkattraman and A. A. Alexeenko, Phys. Plasmas 19, 123515, 2012.
        [4] Y. Fu, G. M. Parsey, J. P. Verboncoeur, and A. J. Christlieb, Phys. Plasmas 24, 113518, 2017.

        *This work was supported by the Air Force Office of Scientific Research (FA9550-18-1-0062, FA9550-18-1-0061) and the Department of Energy (DE-SC0001939).

        Speaker: Dr Yangyang Fu (Michigan State University)
      • 11:30
        Numerical study of a coaxial electron sheath 15m

        A particle-in-cell (PIC), direct simulation Monte Carlo (DSMC) simulation is presented for a coaxial low-temperature plasma. Depending on the ratio of the area of the electron collector ($A_e$) to ion collector ($A_w$), different sheath structures may form$^1$. In the case of the $A_e / A_w < \sqrt{2.3m_e / m_i}$ where $m_e$ is and $m_i$ are the electron and ion mass respectively, an electron sheath will be observed. Here, a collisionless electron presheath is studied in a coaxial configuration. The center conductor is held at a positive bias and the outer conductor is fixed at ground potential. A clear electron sheath configuration is found in the model when the ratio of inner conductor area to outer conductor area meets the electron sheath criteria. It was observed that the azimuthal velocity distribution function experiences a loss as electrons with near zero velocity are lost to the center electrode. Radial velocity distribution functions are mostly Maxwellian except near the positively biased electrode where a drifted Maxwellian is observed. The analysis of the electron sheath has important applications in probe theory, probe diagnostics, and electron beam devices$^2$. This coaxial electron sheath problem demonstrates a fluid-like bulk plasma region and a kinetic sheath region. As such, this is an ideal problem for analyzing the transition region between the fluid and kinetic regime.

        $^1$S. Baalrud, et al., Phys. Plasmas, 14, 042109, 2007
        $^2$N. Hershkowitz, et al., US Patent num. 7398592B2, 2009.

        This work was supported by the Laboratory Directed Research and Development (LDRD) program and Sandia National Laboratories under project 209240. Sandia National Laboratories is a multi- mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

        Speaker: Dr Andrew Fierro (Sandia National Laboratories)
    • 09:45 11:45
      10.1/10.2 Converters, Components,Magnetics, Swiches and Capacitors Gold Coast I/II (Double Tree at the Entrance to Universal)

      Gold Coast I/II

      Double Tree at the Entrance to Universal

      Convener: Argenis Bilbao (Texas Tech University)
      • 09:45
        Study of Cockcroft-Walton Multipliers Driven by AC Sources with Limited Current 30m

        Cockcroft-Walton (CW) multipliers are common and simple DC high voltage sources; however, almost all literature on CW design assumes an ideal AC current source drives the multiplier. Our work on piezoelectric transformer driven CW multipliers for use in compact x-ray sources prompted a study of CW operation with limited supply current. Limiting the AC source current supplying a CW has been shown to significantly reduce its output voltage. Using SPICE, differing degrees of current limiting, varying stage capacitance, and variable loads all had a significant impact on total output voltage from a CW. CW multipliers are known to have an optimal number of stages to produce the highest output voltage, which is no different for multipliers with current limited sources. However, the optimum number of stages is dependent upon the current limiting and tends toward lower numbers of stages for reduced drive current. Varying stage capacitance also has a significant impact on output voltage, which is dependent on supply current. Both uniform stage capacitance and variable stage capacitance have been investigated, with marginal improvements found for the variable capacitance case. From simulation, the output voltage is shown to increase as the load increases, with the amount of increase dependent on the supplied current.

        Work supported by the Advance Research Projects Agency-Energy

        Speaker: Jacob Williams (University of Missouri Electrical Engineering and Computer Science)
      • 10:15
        Characteristic Analysis of Metal Oxide Resistor under Impulse of Different Wave-Head Time 15m

        ABSTRACT: This paper determines the waveform parameters of metal oxide varistor (MOV) test platform according to the IEC standards and the waveform characteristics of very fast transient overvoltage (VFTO) measured in gas-insulated metal-enclosed switchgear (GIS). The standard lightning impulse test circuit with 8 us wave-head time, the steep wave impulse test circuit with 100ns wave-head time and the very fast transient overvoltage impulse test circuit with 20ns wave-head time are designed and constructed respectively. Several typical MOVs used in 10kV and ultra-high voltage arresters are selected as samples. The volt-ampere characteristics of MOVs under the above three different wave-head time impulses are studied experimentally. The experimental results show that the residual voltage of MOV increases by 14.7% to 18.2% under the action of 20ns very fast transient overvoltage impulse compared with the standard lightning wave and the steep wave with the same amplitude. Under the action of standard lightning impulse with 8us wave-head time and the steep wave impulse with 100ns wave-head time of different amplitude, the voltage of MOV reaches its peak value earlier than current, showing the characteristics of inductance. Under the action of 20ns very fast transient overvoltage impulse, MOV has an obvious impedance transition voltage U0. Under U0 voltage, MOV shows resistance characteristics. When the peak voltage is less than U0, MOV shows capacitive impedance characteristics. When the peak voltage is greater than U0, MOV shows inductive impedance characteristics. U0/U1mA has little relationship with the shape and size of MOV.

        Speaker: Wei Zhang (Xi'an Jiaotong University)
      • 10:30
        Analysis of Commercial off-the-shelf 1200 V Silicon Carbide MOSFETs Under Short Circuit Conditions 15m

        Silicon carbide (SiC) power semiconductor devices are experiencing an increasingly widespread adoption in many power electronics and pulsed power applications such as high-power DC-DC converters and inverters, battery chargers, industrial motor drives, as well as high-power solid-state pulse generators such as a Marx generator or a linear transformer driver (LTD). The wide-bandgap (WBG) and thermal properties of SiC provide inherent advantages over silicon power devices especially in high power density applications. These advantages include higher blocking voltages, increased switching speeds, physically smaller implementations of power electronics and pulsed power circuits, improved system efficiencies, and higher operating temperatures. To improve the overall confidence in the ability of SiC devices to reliably replace equivalent silicon solutions, independent reliability testing and analysis must be conducted. In this research, a short circuit test board was developed to analyze the short circuit ruggedness of 1200 V MOSFETs. Using the test board, commercially available 1200 V / 10 A SiC MOSFETs from 3 different manufacturers were subjected to both single and repetitive short circuit events, and the short circuit ruggedness of each device was measured and analyzed. The purpose of this research is to independently measure and report on the short circuit capabilities of commercially off-the-shelf 1200 V SiC MOSFETs.

        Speaker: Jonathan Forbes (Texas Tech University)
      • 10:45
        The Current State of Custom Pulse Power Cores Supplied by Metglas Inc. 15m

        Metglas Inc. has a long history and large experience base in manufacturing Pulse Power Cores with amorphous metals for industrial and research applications. There are a wide variety of alloys for use as core materials which enables designs based on flux swing, core loss and rise/switching time requirements. The change in flux density can vary from 1.1 – 3.4 T depending on alloy selection. Custom cores range in size from grams to tons. Custom shapes, fabrication hardware, testing and handling devices are made as needed. Polyester film is the standard material used for interlaminar insulation but other coatings can also be used. Heat treatment, winding and other proprietary processing techniques result in products that meet requirements that might not be achieved using more conventional materials and these methods will be discussed.

        Speaker: Dr Eric Theisen (Eric)
      • 11:00
        Transient Loading of Ultracapacitors* 15m

        Ultracapacitors are of increasing interest in the high voltage community due to their ability to source high transient power while also offering a modest energy density. A market study of commercially available ultracapacitors finds several different models available with slightly different internal resistance, energy density, and power density parameters, among others. Hybrid ultracapacitor technologies, such as lithium-ion capacitors, have also been developed that have much higher energy density with nearly the same power density. In the work presented here, a few different commercially available off the shelf ultracapacitors and lithium-ion capacitors have been procured and evaluated into a low impedance load, few hundred micro-Ohms, in a transient manner. The design of experiments as well as the impedance and power density results obtained will be presented.

        Speaker: Mr Alexander Johnston (University of Texas at Arlington)
      • 11:15
        Investigation into the Reliability of Commercial 1.2-kV SiC MPS Diodes under Surge Current and Avalanche Events 15m

        With the prospect of wide-bandgap (WBG) devices such as SiC (silicon carbide) taking the forefront to replace Si (silicon) in commercial applications, analysis must be done on surge current and avalanche energy reliability to verify the viability of replacing Si with SiC in terms of long-term reliability. Properties of SiC theoretically show superior thermal conductivity, and higher breakdown field, giving SiC devices the edge in power applications where temperature and voltage hold-off are vital. Power converters and inverters are often exposed to these events due to a load short circuit or transients before reaching steady-state. It is common for power semiconductor devices such as MOSFETs (metal-oxide-semiconductor field-effect transistors) or diodes to experience a short duration of overcurrent or overvoltage in these power switching applications. The surge current can potentially damage devices if not properly rated. Extended duration of overvoltage leads to avalanche breakdown, ending in catastrophic failure of the device. This paper investigates the surge current and avalanche breakdown capabilities of commercial SiC MPS diodes rated for 1.2 kV reverse voltage and 20 A continuous forward current. The diodes are rated for 164 A of non-repetitive surge current and 220 mJ of total avalanche energy. A testbed is designed and developed to test the reliability of commercial WBG diodes in surge and avalanche events. Each device is initially characterized, exposed to testing conditions, and then characterized again to monitor signs of degradation. Analysis of the data collected during and after testing was conducted to determine the reliability in commercial applications.

        Keywords – SiC; WBG; wide-bandgap; MPS; merged pin; surge current; in-rush current; avalanche energy, pulsed power; reliability testing; power electronics

        Speaker: Fernando Salcedo
    • 09:45 11:45
      2.5 Codes and Modeling Space Coast I-III (Double Tree at the Entrance to Universal)

      Space Coast I-III

      Double Tree at the Entrance to Universal

      5780 Major Blvd. Orlando, Florida, 32819, USA

      This is the Codes and Modeling session within Microwave Generation and Plasma Interactions

      Convener: Ian Rittersdorf
      • 09:45

        Beam optics for sources, transport and depressed electron collectors for RF component design and performance are predicted using simulation codes with ever more fidelity. To meet these modern day challenges, the MICHELLE charged particle beam optics code [1,2] has a new official release of Version 7 (2019), which contains a host of improvements including the physics solvers, user environments, user interface, interfacing with 3rd party codes and data, and the installation package. The physics solvers include new advances in thermionic emission, and modifications to the algorithms. The user environments include the previously-reported AFRL Galaxy Simulation Builder, but also improvements in the Analyst-MP environment. There are now options to use a wider variety of computation mesh grid generators, along with some advanced meshing techniques. This is taken advantage of with the ability to run MICHELLE under SolidWorks for device design, and for subsequent parametric optimization. The new software installers support UNIX & Windows, up through Windows 10.

        This paper reports on the latest MICHELLE release and also will highlight the use of the new capability on advanced techniques for thermionic emission as well as extreme mesh examples of field emission arrays, illustrating how this capability can be used by the device designer.

        1. John Petillo, et al., IEEE Trans. Plasma Sci., vol. 30, no. 3, June 2002, pp. 1238-1264.
        2. John Petillo, et al., IEEE Trans. Electron Devices Sci., vol. 52, no. 5, May 2005, pp. 742-748.
        3. Stellar Science Ltd Co. Galaxy Simulation Builder (GSB) User Guide, Version 6.6. High Power Electromagnetic Division, Air Force Research Lab, Kirtland, NM, 2017.

        • Work supported by the ONR, NRL and Leidos
        Speaker: John Petillo (Leidos)
      • 10:15

        The injection and absorption of waves entering or escaping open boundaries in EM-PIC simulations is of interest provided the numerical reflections are low, and the boundary is stable. The Higdon [1] operator method provides the basis of a multi-phase velocity matching algorithm. The potential for high order implementation provides for (a) injection of low reflection incident waves into the interior of the simulation environment, (b) near perfect absorption of scattered outgoing waves, and (c) is insensitive in EM-PIC to particles traversing the boundary edge. This method has been applied with substantial success in several wave equation environments. These include dispersive electromagnetic wave modeling, as well as the shallow water equations and acoustic phenomena.

        The general Higdon operator of order J may be described as follows for wave propagation along the x axis, as the product of multiple uni-directional wave equations, each product uses a potentially unique value of phase velocity. For J=1, this reduces to the standard 1-dimensional wave equation in which the sign of the phase velocity indicate either a forward or backward traveling wave. As J, the number of product terms, increases it is necessary to capture information that is more remote spatially and temporally from the boundary edge. Givoli and Neta [1] suggested the method of recasting the solution in terms of auxiliary functions of arbitrarily high order. We will report on our implementation of this method for 1st and 2nd order FD approximations and the effectiveness on broadband transmission.
        1. Dan Givoli and Ben Neta, “High-Order Higdon Non-Reflecting Boundary Conditions for the Shallow Water Equations”, NAVAL POSTGRADUATE SCHOOL, Monterey, CA, NPS-MA-02-001, April 2002.

        Speaker: Larry Ludeking (OrbitalATK of NGC)
      • 10:30
        Elastostatics in Beam Optics Analyzer 15m

        Calabazas Creek Research (CCR) developed and has maintained and continuously added new capabilities to Beam Optics Analyzer (BOA). It provides several finite element field solvers for electrostatics, heat transfer, magnetostatics and Helmholtz fields. It can track particles relativistically in either static or harmonic fields with space charge effects. It provides sophisticated emission models for field and thermionic emission. Its Mesher generates unstructured mesh with fine-grained control.
        Using the power density generated by electrons depositing their energies on terminal electrodes, BOA can simulate the temperature profile of the device. This however does not provide a complete design loop. It would be ideal and much more efficient to compute the thermal stresses and predict the hot dimension of the electrodes on the same model and the same simulation platform. This would make a simulation tool, such as BOA, a complete multiphysics platform.
        In a multiphysics platform, the same CAD model should be used for all analysis types from beam simulation to heat transfer and stress analysis. Parts required in one analysis type could be redundant in another. Thus, the analysts should be able to enable/disable parts as needed. Changing material in one part in one analysis type should be automatically carried over to other analysis types. BOA currently includes all the above convenient features for its particle beam and heat transfer analyses.
        In the present work we extend BOA’s capability to include stress analysis. We will demonstrate seamless integration of the elastostatics field solver with beam simulation and heat transfer analysis. Integration of stress analysis into BOA provides a convenient, one-stop, multiphysics, simulation tool. It provides one package that can perform particle simulation, thermal and stress analysis using the same CAD model. We will present the finite element elastostatics formulation, particle, thermal and stress simulation results of a gridded triode gun.

        Speaker: Mr Thuc Bui (Calabazas Creek Research, Inc.)
      • 10:45

        Computational plasma models such as magnetohydrodynamics (MHD) and particle-in-cell (PIC) are essential tools in modern plasma physics. They can be used to complement physical experiments, inform future avenues of research, and provide insight into plasma phenomena that are difficult to create and control in a laboratory. The more detailed and physically realistic simulations can require the computational resources of entire supercomputers. As larger computing resources become available, we are able to conduct more refined plasma simulations, which can deepen our understanding of the behavior of plasmas. However, constraints in computer chip manufacturing are leading the next generation of supercomputers to employ a variety of novel architectures, usually with many more processing units. Until recently, each new architecture can require a separate, non-trivial rewrite of a simulation code. A current goal in computational science is the creation of programming paradigms for writing performance portable code: code that can run efficiently at high performance on many different supercomputer architectures. To explore the development of performance portable plasma simulation codes, we are currently modifying a CPU-only finite volume astrophysical MHD code to run efficiently on both CPUs and GPUs, using Kokkos, a performance portability library. I will present the strategies we used for implementing MHD using Kokkos and the challenges we encountered while attempting to achieve maximum performance on different platforms. In addition, I will discuss performance results for multiple architectures compared against the original code. The strategies, challenges, and results presented will allow other research groups to straightforwardly adopt this approach to prepare their own codes for the exascale era. (SAND No: SAND2019-2048 A)

        Speaker: Forrest Glines (Sandia National Laboratory)
      • 11:00
        Optimization of a Folded Waveguide Traveling Wave Tube Using Impedance Matrices 15m

        RF device design and performance are predicted using simulation. Fast design tools such as those that model beam dynamics in one dimension may be sufficient for initial design work or for RF devices where performance is not critical. However, in many cases these tools do not have the fidelity to meet performance objectives. Particle-In-Cell codes such as MAGIC can be used to model the complete device physics in 3D using Maxwell’s equations but these simulation tools are computationally intensive and time consuming. The GPU code NEPTUNE [1] is a faster alternative to MAGIC but its computational time is still not insignificant. An alternative approach is presented using impedance matrices and Tesla-Z [2]. In this approach the impedance matrix for a folded waveguide traveling wave tube (FWTWT) is calculated using the Analyst 3D field solver. The impedance matrix is used in Tesla-Z [2] to predict tube performance. This approach is faster than PIC simulation, where the 3D fields are taken into account through the computation of the Z-matrix as opposed to a simplified or analytic model, and the particle dynamics are modeled in 2D. Ultimately, the Tesla-Z matrix approach is used within the Galaxy Simulation Builder (GSB) framework to optimize the FWTWT using the DAKOTA optimization library. The approach and optimization results are presented.

        1. S. J. Cooke, I. A. Chernyavskiy, G. M. Stanchev, B. Levush and T. M. Antonsen, "GPU-accelerated 3D large-signal device simulation using the particle-in-cell code ‘Neptune’," IVEC 2012, Monterey, CA, 2012, pp. 21-22.
        2. I. A. Chernyavskiy T.M. Antonsen, Jr., J.C. Rodgers, A.N. Vlasov, D. Chernin, and, B. Levush, “Modeling Vacuum Electronic Devices Using Generalized Impedance Matrices,” IEEE Transactions on Electron Devices, Vol. 64, No. 2, pp. 536-542, Feb. 2017.
        Speaker: Aaron Jensen (Leidos)
      • 11:15
        Importing CAD-Generated Device Geometry to the Neptune EM-PIC Simulation Code 15m

        The 3D Electromagnetic Particle-in-Cell (EM-PIC) method is well known as a powerful simulation technique for modeling electron beam and/or plasma interactions with strong electromagnetic fields inside complex device structures. One of the first tasks for users of PIC codes is to precisely define the 3D device geometry for their simulation. To simplify this task for users of NRL’s Neptune code, we have created a new approach to import geometric models from commonly used Computer-Aided Design (CAD) tools.

        There are two primary approaches commonly used to define 3D geometry: (1) CAD modeling tools, and (2) Constructive Solid Geometry (CSG) methods. Conventional CAD tools represent and manipulate solid object surfaces using a boundary representation composed of 2D surface patches. CSG methods build complex geometry from a set of simple volumetric shapes (primitives, such as spheres or cylinders) composed using Boolean operations and geometric transformations.

        In Neptune, we implement the CSG approach using a mathematical representation of shapes as implicit functions in 3D that produce positive values inside the structure, zero on the boundary and negative values outside. A function is then mapped onto the Cartesian simulation grid using an accurate “cut-cell” algorithm that determines intersections of the grid with the zero-valued surface-contour of the 3D function. A full scripting language is available to construct shapes/functions of arbitrary complexity using simple operations, however for sufficiently complex geometries this becomes a challenging programming exercise, in which case importing CAD models would be a more convenient approach.

        We describe our new method, in which we transform the CAD model (using its surface triangulation) into Neptune’s function representation. This enables the imported geometry to be used as a new primitive shape in the CSG model and further combined with other shapes, providing considerable flexibility to the user.

        Supported by the US Office of Naval Research

        Speaker: Dr Simon Cooke (U.S. Naval Research Laboratory)
    • 09:45 11:45
      5.1 & 5.2 Opening and Closing Switches I Gold Coast III/IV ()

      Gold Coast III/IV

      Convener: Ryan Umstattd
      • 09:45
        Increasing the Pulse Repetition Rate for Solid State Thyratron Replacements 15m

        Silicon Power reports increased pulse repetition rate capability for Solid State Thyratron Replacements (SSTR) utilizing enhanced SolidTRON technology. Varying amounts of minority carrier lifetime killing in our semiconductors has enabled demonstration of 300ns wide capacitive discharge pulses up to 10kA/cm2 at pulse repetition rates up to 50kHz at a junction temperature of 110°C.

        While Silicon Power has found success in displacing some Thyratrons, the higher pulse repetition rates enjoyed by Thyratrons and Ignitrons had been a challenge to overcome. Finally, a well-documented procedure produces quantifiable tradeoffs for frequency capability versus conduction losses.

        The superior conduction efficiency and low leakage currents of SolidTRON products permits modest lifetime killing to increase operational pulse repetition rates without suffering detrimental increases in energy losses. Specifically, at 110°C a 20x improvement in frequency capability is achieved with a modest 75% increase in conduction losses (at 1kA/cm2). More importantly, comparing the peak current achieved in identical setups between as-fabricated devices and those with the highest frequency capability differ by only 1.5%.

        Additionally, these gains are achieved without a complicated gating scheme. The improved pulse repetition rate was demonstrated using a simple pulse transformer; where galvanic isolation is provided with magnetic coupling. The modest minority carrier lifetime killing allows the device to self-commutate without gate assisted turn-off.

        This paper includes plots of the effects of lifetime killing on the minimum period required between pulses, the impact of conduction losses as a function of current density, and leakage currents at junction temperatures ranging from 25°C to 110°C.

        Speaker: John Waldron (Silicon Power)
      • 10:00
        Study on aging characteristics of DC transmission line arrester considering impact load 15m

        Metal oxide arrester (MOA) is the main equipment to limit overvoltage in power system. It has been widely used in UHVDC project in China. The UHVDC system has longer deliver distance. Under the influence of various external factors, the state characteristic parameters of MOA will change and its performance will decline, which is called the aging problem. Unlike the arrester in AC system whose continuous operating voltage is power frequency voltage, the aging of MOA resistors in DC system is more complicated. Based on a ±1100kV UHVDC system in China, this paper studies voltage load waveforms of transmission line arrester in UHVDC system under different operating conditions, and analyzes its amplitude by mathematical methods. Based on simulation results, the impedance characteristics and power consumption characteristics of DC line MOA proportional components under multi-factors were studied. Considering the influence of impact load, the long-term integrated DC aging characteristics of MOA were studied. The comprehensive aging test results show that the DC reference voltage of the mainstream formula has an increasing trend of DC reference voltage after aging, which increases by a maximum of 6.05%; the DC leakage current shows a decreasing trend with a maximum reduction of 76%; the power loss shows a decreasing trend with a maximum of 71.4%. The AC reference voltage shows an increasing trend. The full current, resistive fundamental wave, and third harmonic show a decreasing trend. The capacitance and tanδ decrease slightly, and the residual voltage does not change significantly. The influence of different impact loads on DC aging of MOV is square wave>lightning wave>high current. It is found that there is a polarity effect in DC aging, and the regularity of the positive and negative characteristic parameters of MOV is opposite.

        Speaker: Mengzhen Li (Xi'an Jiaotong University)
      • 10:15
        High sensitivity HEH monitor 15m

        The LHC beam dumping system serves to safely abort 2 counter rotating proton beams each with energy of up to 360 MJ and its reliable operation is crucial for the accelerator safety. The system comprises 50 fast pulsed magnets and their associated pulse generators to fast extract and paint both beams on the surface of two 8 m long graphite blocks. The pulse generators operate at up to 29 kV and comprise 800 HV GTOs and 480 HV triggering IGBTs to deliver altogether more than 1 MA. All generators are installed underground in the galleries parallel to and shielded from LHC tunnel but some high energy hadrons (HEH) leak from the tunnel into the galleries via interconnecting cable ducts and can provoke Single Event Burnout (SEB) of HV semiconductors. This can lead to system malfunction and possible damage of the accelerator. In order to reduce the likelihood of SEB, the choice of HV semiconductors was based on their SEB cross-section and cable ducts shielding was improved by filling empty gaps with iron rods. To keep the probability of SEB failure low enough (< 0.1 per year), the requested HEH fluency in the galleries needs to be less than 5e4 HEH/cm2.year. The presently used HEH monitors are not sensitive enough subsequently the development of a new high sensitivity HEH monitor was necessary. It is based on protected SEB phenomenon in HV Si diodes. Depending on number of used diodes, it can be up to thousand fold more sensitive compared to present memory upset based instruments. It will allow confirming HEH fluency estimations used in failure rate evaluation and taking the most appropriate mitigation measures if necessary. The HEH monitor sensitivity measurement is ongoing under various conditions (cosmic rays at low and high altitudes, protons, neutrons, temperature variation).

        Speaker: Dr Viliam Senaj (CERN)
      • 10:30
        Packaging and Evaluation of 100 kV Photoconductive Switches 15m

        In practice, it is challenging to integrate a Photoconductive Semiconductor Switch (PCSS) capable of switching on the order of 100 kV into a package with small parasitic inductance such that the sub-nanosecond rise time of the PCSS is still achievable at current amplitudes of hundreds of amperes. The lateral geometry PCSS is built on a 2.5 cm x 1.3 cm GaAs sample with a gap distance of 2 cm to achieve rise-times less than one nanosecond. Another challenging aspect of a practical GaAs PCSS is the filamentary nature of the current which leads to shortening of the device lifetime. This is addressed by using an appropriate electrode profile at the contacts to produce a mostly uniform electric field between the electrodes, thereby decreasing field enhancement points and filament “hot-spots.” Fully 3D, transient electric field simulations of the switches, also incorporating the field dependent conductivity of the GaAs material, enable optimization of the switch package while keeping the electric field stress at acceptable levels during the fast, 10 µs application of the charging voltage. The switches are packaged such that the semiconductor is encapsulated in EFI dielectric to mitigate breakdown driven by the electric field across the GaAs surface. High-gain lock-on conduction mode is utilized so that the triggering system does not require high laser power to switch the PCSSs into their on-state. The high voltage design of the PCSS package is presented and initial switching results are discussed.

        Distribution Statement A: Approved for public release; distribution is unlimited (unclassified papers only)

        Speaker: Mr Jared Culpepper (Texas Tech University)
      • 11:00
        Experimental study of graphite electrode erosion under premixed atmosphere in spark gap switch 15m

        The spark gap switch with graphite electrodes is widely used in the high-power laser system. Affected by the heat flux of the high-current arc, the graphite will sublimate in a short time. The sublimation characteristic determines the lifetime of the graphite electrode. To study the effects of gas composition on the erosion rate, three kinds of mixture gas are chosen to work as the discharge atmosphere. All of them contain 20% oxygen, and other parts are nitrogen, helium, and argon respectively. In theory, the sublimation of the electrode is transferred into the gas state of carbon oxide and the solid state of simple substance carbon, but the too much solid state will damage the insulating property. The paper aims to research the effect of the atmospheres on the evolution of graphite. The current integral method is adopted to calculate the transfer charge of the pulsed arc, which is a basis for the measurement of arc power. Meanwhile, the concentration of gas products in the switch chamber is measured by a flue gas analyzer. It is found that, compared with the usual nitrogen-oxygen atmosphere, the discharge with inert gas can significantly reduce the ablation of the electrode. Especially in the case of the mixture of argon and oxygen, the sublimation of graphite electrode decreases significantly. The higher oxidation efficiency can directly reduce the mass of solid residue. In the inert gas atmosphere, the oxide proportion is higher, and more sublimation carbon translates into gaseous. The premixed gas with argon and oxygen is better than traditional gas in the switch chamber. In the method of optimizing premix gas, the electrode erosion rate can be reduced, which is beneficial to the lifetime of the electrode.

        Speaker: Mr Hongyu Dai (Huazhong University of Science and Technology)
    • 09:45 11:45
      6.5 Biological and Medical Applications I Seminole C (Double Tree at the Entrance to Universal)

      Seminole C

      Double Tree at the Entrance to Universal

      5780 Major Blvd. Orlando, Florida, 32819, USA
      Convener: Dayun Yan (The George Washington University)
      • 09:45
        Feedback from cells: how cancer cells could make cold atmospheric plasma jet selective during treatment 30m

        Currently, the plasma-based cancer treatment and the mechanism of plasma jet interacts with the target are hot topics. During the cell treatment, the cell feedback makes plasma jet inconsistent among different type of cells even with all other setups are the same. Therefore, such cell feedback on plasma jet cannot be neglected. In this work, we discovered how cancer cells change the plasma parameters at steady state during in-vitro treatments. Comparing with MDA-MB-231 (breast adenocarcinoma), PA-TU-8988T (pancreatic adenocarcinoma), and U87MG (glioblastoma), B16F10 (murine melanoma) make the lowest electron density in the helium plasma jet. When the jet is pointing at the edge of the cell colony, B16 also makes the most asymmetric self-organization patterns. Capacitance imaging of cell colonies indicates that the capacitance of the B16 colony is the highest one among these cell lines and also the permittivity. A finite element study of target permittivity shows that a dielectric target without ground electrode behind can decrease the electric field of streamer head in plasma jet when the permittivity is high. This agrees with the observation of this work. However, a dielectric target with a ground electrode behind results in an opposite permittivity effect which agrees with the previous simulations. The observation of this work reveals how the cancer cells can change the plasma jet due to their permittivity, which helps to determine the selectivity of plasma treatment.

        Speaker: Li Lin (The George Washington University)
      • 10:15
        Collective Effects of Nanosecond Pulsed Electric Fields on Cells Organized in a Monolayer 15m

        Basic research on the underlying mechanisms of pulsed electric field effects of sub-microsecond duration on mammalian cells has mostly focused on the study of individual cells in suspension. However, for cells that are organized in a tissue, connections and communication between cells are crucial. Accordingly, we investigated besides intracellular effects also extracellular effects and in particular the response on tight junctions and cell-cell communication and how both affect the development of cells in a tissue, such as their potential to metastasize. Distinct effects could be found that are primarily caused by a transient disassembly of respective membrane proteins that are only compensated by repair mechanisms over the course of one hour [1,2,3]. Conversely, these changes have an immediate effect on intracellular biomolecular pathways, elastic properties of cells and on the permeability of tissues. Some of these effects can be enhanced by combining the treatment with pulsed electric fields and exposures to non-thermal plasmas. Overall this allows for new possibilities for tumour treatment and potentially also tissue regeneration.
        [1] F. Shi et al., IEEE Trans. Biomed. Eng. (2019) in print. doi:10.1109/TBME.2018.2882299
        [2] A. Steuer et al., PloS one 13 (2018) e0204916. doi:10.1371/journal.pone.0204916
        [3] A. Steuer et al., European Biophysics J. 46 (2017) 567. doi:10.1007/s00249-017-1205-y

        Speaker: Prof. Juergen Kolb (Leibniz Institute for Plasma Science and Technology e.V. )
      • 10:30

        Exposure of biological cells and tissues to magnetic fields has not been studied much, and most of the research to date has focused on electrical stimulation. However, an even newer modality that might hold promise, is based on interactions of magnetic fields with living cells and tissues. Progress in experimental techniques has resulted in the burgeoning development of new approaches to target and observe effects of magnetic fields at the intracellular and molecular levels [1, 2]. Time-varying magnetic fields can also produce electric fields (and change transmembrane potentials) based on Faraday’s law of induction. n neural tissues. Also, while medical applications based on electric fields require direct application via electrodes inserted into tissue, pulsed magnetic fields would allow treatment without invasive electrodes. This advantage could lead to an expansion of bioelectric treatments by allowing clinicians to affect any target within the body in a contactless manner.

        An analysis into the time-dependent development of electric fields at cell membranes due to an externally applied magnetic field will be discussed. The parameter space for the magnetic stimulation for bringing about membrane poration will be discussed. Our results will focus on the time-dependent magnetic vector potential, and the resulting transmembrane potential, and poration based on the Smoluchowski equation.

        [1]. M. H. Cho, E. J. Lee, M. Son, J. H. Lee, D. Yoo, J. W. Kim, S. W. Park, J. S. Shin, and J. Cheon, Nat. Mater., vol. 11, pp. 1038–1043, 2012. [2]. S. Qin, H. Yin, C. Yang, Y. Dou, Z. Liu, P. Zhang, H. Yu, Y. Huang, J. Feng, J. Hao, J. Hao, L. Deng, X. Yan, X. Dong, Z. Zhao, T. Jiang, H. W. Wang, S. J. Luo, and C. Xie, Nat. Mater., vol. 15, pp. 217–226, 2016.

        Speaker: Qin Hu (Central Michigan University)
      • 10:45
        Cell permeabilization and exogenous molecule delivery via microwave treatment 15m

        Typical techniques to artificially introducing nucleic acids (DNA or RNA) into cells can be divided into three categories: viral, chemical, and physical. The application space targets usually production of recombinant proteins, the study of the function and regulation of genes, the production of transgenic organisms, and as a method for gene therapy in clinical workflows. Physical means, such as electroporation (via electric pulse treatment) or sonoporation (via ultrasound treatment), can avoid some of the adverse side effects potentially triggered by viral and chemical means, but may less efficient and induce deleterious effects on cell function or surrounding tissue. For instance, electroporation can cause pain to surrounding tissue for in vivo treatments and may reduce cell viability during either in vitro or in vivo treatments. Some cells, such as stem and neuronal cells, remain difficult to transfect by any means. Thus, a physical method that can improve transfection efficiency while eliminating many of the side effects of chemical or viral techniques, particularly for these difficult to transfect cells, would significantly improve workflows requiring nucleic acid delivery into cells.

        This paper presents experimental results for plasmid and siRNA delivery to CHO cells using 2.45 GHz microwave exposures in a single mode cavity, and potential mechanistic pathways for molecule uptake. While plasmid delivery efficiency is minimal, siRNA results are promising. This could indicate that microwave exposures may permeabilize the plasma membrane, but not the nuclear membrane.

        Speaker: Allen Garner (Purdue University)
      • 11:00
        Microorganism inactivation with Electric Pulses and Drugs 15m

        Overuse of antibiotics in agriculture and healthcare has reduced the effectiveness of commonly used drugs for treating infections, motivating the development of methods to overcome antibiotic resistance to inactivate microorganisms. This study combines 300 ns electric pulses (EPs) with clinical and subclinical doses of drugs used to treat two common infectious bacteria, S. aureus and E. coli. We applied 20, 30 and 40 kV/cm, 300 ns EPs at 1 Hz with the number of EPs chosen to deliver the same energy density to 0.2, 2, and 20 μg/mL of tobramycin for 10 minutes of total drug exposure time. Applying 30 and 40 kV/cm EPs alone caused 2 to 4 log reduction; adding tobramycin induced a 7 to 9 log reduction. While this synergy occurred for smaller dosages with some antibiotics, it generally increased with larger doses. Minimal inactivation occurred with antibiotics alone over these timescales since these treatments typically require hours or days to effect cell kill off. These results indicate that combining EPs with drugs may potentially increase the effectiveness of drugs for treating local bacterial infections.

        Speaker: Agni Dhanabal (Purdue University)
      • 11:15
        Modeling of Fluxes and Surface Coverage of Plasma-Produced Species on Artificial Bone Scaffolding 15m

        Porous ceramics are used as scaffolding for tissue engineering and bone regeneration. Plasma treatment of porous calcium hydroxyapatite increases cell attachment and adhesion, improving performance as a bone substitute.[1] The mechanisms producing improvement are not well known, but surface coverage of chemisorbed O, OH and N have been correlated with increased wettability, possibly leading to increased cell proliferation. These plasma processes are typically performed at low pressure. Using atmospheric pressure plasmas (APPs) would lower cost, but are challenged to produce uniform plasmas inside the pores.
        APPs propagating through pores tens of microns in size in ceramics were computationally investigated using the 2D modeling platform, nonPDPSIM.[2] Plasma was produced in a co-planar dielectric barrier discharge (DBD) by negative ns voltage pulses. The bottom dielectric had chains of pores with 100% interconnectivity. Discharges in air and He/O_{2} mixtures were studied while varying the width and angle of the pore-chain.
        Plasma propagating from the DBD into the pores initially produces a Townsend-like discharge and surface charging. The surface charging is sensitive to the angle of the pore-chain and can produce restrike-like discharges propagating back towards the plasma. The alignment of the pore-chain with the applied electric fields impacts plasma properties. Surface ionization waves (SIWs) along the curved surfaces of pores most readily form when the pore-chain has a large angle. The high electron temperatures at the head of the SIWs produce higher radical fluxes onto pore surfaces which are progressively less uniform as the pore-chain angle increases. Narrower openings between pores produce less uniform fluxes as propagation of the plasma is inhibited.
        Work supported by the National Science Foundation and the DOE Office of Fusion Energy Science.

        [1] Y. Moriguchi et al., PLoS ONE 13, 3 (2018).
        [2] S. Norberg et al., Plasma Sources Sci. Technol. 24, 035002 (2015).

        Speaker: Mr Juliusz Kruszelnicki (University of Michigan)
      • 11:30
        Plasma polymerization of N,N-dimethylacrylamide: cell-repellent or cell-adhesive coatings? 15m

        Several precursors for plasma polymerization have been studied at length, including carboxylic acids, amines, siloxanes and ethers. This is in stark contrast with amide-based precursors, for which only a limited amount of studies are available [1-5], which notably differs with the abundance of biomedical research focussing on amide-based surface modification using wet chemistry [6-8]. This indicates that plasma polymerization of amide-based precursors has still unexplored potential, even more so because the previously performed chemical analyses were not extensive and the stability examinations were limited to only 3 hours of water incubation. Therefore, the plasma polymerization of a novel amide precursor (N,N-dimethylacrylamide) was explored in this study. The effects of varying discharge power on the plasma active species (OES), hydrophilicity (WCA), chemical composition (FT-IR, Raman spectroscopy, XPS) and stability (up to 1 week of water incubation, with AFM scratch tests) were examined. Additionally, the interactivity between cells (MC3T3) and the deposited coatings were studied in-vitro through life/dead fluorescent imaging and MTS assays. In contrast to the unstable coatings obtained at lower powers, the stable coatings showed a reduced preservation of the precursor structure and therefore a lower hydrophilicity. The plasma fragmentation resulted in coatings with a complex N-rich chemistry that could be directly linked to the observed plasma species. XPS $C_{60}$ depth profiling indicated a difference between the top layer and bulk of the plasma polymer due to spontaneous oxidation and/or post-plasma deposition. Stable coatings were found to have cell-interactive behavior, showing a cell viability of up to 71% as compared to tissue culture plates after 1 day of cell culture.

        [1]: Cheng et al., DOI:10.1021/la050417o. [2]: Chu et al., DOI:10.1016/j.surfcoat.2007.08.076.
        [3]: Griesser et al., DOI:10.1163/156856294X00194. [4]: Pan et al., DOI:10.1021/bm0000642.
        [5]: Bullet et al., DOI:10.1002/sia.2318. [6]: Morgese et al., DOI:10.1016/j.eurpolymj.2016.11.003.
        [7]: Lin et al., DOI:10.1021/bm200368p. [8]: Liu et al., DOI:10.1021/bm201814p.

        Speaker: Mr Tim Egghe (Ghent University)
    • 09:45 11:45
      8.5 Power Supplies and Modulators I Seminole D/E (Double Tree at the Entrance to Universal)

      Seminole D/E

      Double Tree at the Entrance to Universal

      Convener: David E. Anderson (Oak Ridge National Laboratory)
      • 09:45
        Pulsed Resonant Charging Power Supply for the Spallation Neutron Source Extraction Kicker PFN System 30m

        Pulsed Resonant Charging Power Supply for the Spallation Neutron Source
        Extraction Kicker PFN System

        R. Saethre, B. Morris, Oak Ridge National Laboratory

        The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) uses fourteen pulsed modulators in the extraction system to deflect the proton beam from the accumulation ring to the target. The SNS is executing the Proton Power Upgrade (PPU) project to increase the beam energy from 1.0 to 1.3 GeV, which requires an increase in the extraction kicker magnetic field intensity. Each pulse modulator consists of a pulse forming network (PFN), located in a service building external to the ring tunnel, along with a charging power supply and related controls and interlocks. Increasing the magnet current by charging the PFN to a 20% higher voltage will provide the required deflection. The existing capacitor charging power supply is incapable of charging the PFN to higher voltages between 60 Hz pulses, and therefore a new resonant charging scheme has been developed to charge to the required voltage within the available time. This paper describes the resonant charging power supply design and presents test results from a prototype operating on a full system test stand.

        Speaker: Mr Robert Saethre (Oak Ridge National Lab)
      • 10:15

        As pulse modulators are increasingly built more compact, so do the ancillary supporting electronics systems have to decrease in size. The trigger system presented, utilizes a 10kV 10kA capable solid state switch to conduct energy through a ferrite loaded air core transformer. The final output is capable of reaching 60-70kV in less than 100nS, with extremely low jitter with reliable (or consistent) performance. Depending upon the charging supply this system is capable of repetition rates of 88 Hz in continuous or burst mode operation. Higher voltages or faster rise-times are also supported based on lower repetition rates limited by the size and capability of the compact 10 kV power supply.

        • Work is sponsored by Air Force Research Laboratory (AFRL)/RDH, Kirtland Air Force Base, under FA9451-17-D-0070
        Speaker: Joshua Gilbrech (Leidos)
      • 10:30
        Design of Solid-state Marx Modulator with Fast Rising Time and Short Pulse Width 15m

        This paper describes the design of the solid-state Marx modulator with fast-rising time and short pulse width for various applications such as accelerator and plasma application. By stacking of SiC-MOSFETs, the designed specifications are satisfied as 10 kV, under 50 ns of pulse width, under 15 ns of rising time. The designed circuit consists of the ON switches for applying pulse to the load and the OFF switches for pull-down the pulse that is closely related to the rising and falling time of output pulse, respectively. Compared to conventional Marx generator using diode, the OFF switch connected in parallel with load provides discharging path for the stored energy on the parasitic capacitance and allows short pulse width owing to fast falling time. In order to provide complementary driving signal and power for ON/OFF switches, the simple control algorithm with minimum component count and reliable drive circuit against to the noise is proposed. Besides the circuit design, compact configuration for minimizing the inductance and synchronizing all the signal is essential to shorten the rising, falling and pulse width. Based on the proposed circuit, the detailed design and implementation of ns Marx modulator is presented such as the layout of Marx cell for minimizing inductance, the artwork of PCB for synchronizing fast gate signal, and resonant converter based charging circuit for compact arrangement.
        In addition to the experiment with resistor load for verifying the performance of developed modulator, the results of application study including PAW (plasma activated water) application as well as the kicker system for accelerator will be introduced in the following paper.

        Speaker: Jung Soo Bae (University of Science & Technology)
      • 10:45

        Stangenes Industries has designed and delivered a 5U 19” rackmount modulator for driving high impedance capacitive loads such as an electron gun. An FPGA controls the delays between successive triggering of 18 Marx stages to match the modulator output to loads of various impedances. With a single HV supply providing a fixed charge voltage the modulator can generate output voltages from 5KV to 50KV with 0.1KV of output resolution commanded pulse-to-pulse. The pulse width is adjustable between 0.5µs and 5µs in steps of 0.1µs and the pulse delay from trigger signal is adjustable from 0µs to 3µs in steps of 0.1µs, also commanded pulse-to-pulse.

        The system is operated remotely by a client-computer via an ethernet protocol which also displays real-time diagnostic data. The modulator is equipped to count and recover from vacuum arcs and maintains a time-stamped fault log. Fast signals such as the trigger, faults, interlocks and waveform select are hardwired through low-latency optically isolated circuits. All the components: power supply, Marx, pulse-transformer, 40W DC filament, current/voltage diagnostics and controls are contained within the box which is powered by a single phase 120/240V wall plug. The system is entirely air cooled with no insulating oil.

        This paper described the modulator as tested into an electron-gun. Waveform tuning procedures as well as heat data are presented. The system is capable of operation at a repetition rate of 1000HZ and a modulator average output power of 300W. The internal components are modular with emphasis placed on rapid MTTR for enhanced serviceability.

        Speaker: Christopher Yeckel (Stangenes Industries)
      • 11:00
        Re-orientation of BN nanosheet induced by pulsed electric field and its effect on thermal properties of epoxy resin-based nanocomposites 15m

        In this study, low level of boron nitride (BN) nanosheets filler loading (5wt.%, 10wt.%) were incorporated in epoxy resin matrix to improve thermal conductivity of the nanocomposites by performing the alignment of BN nanosheets. The orientation of BN nanosheets in epoxy resin matrix are controlled by applying pulsed electric field during the curing process of epoxy resin/BN nanocomposites. The pulsed electric field has a pulse width of 1 μs, a field strength of 30 kV/mm, and a repetition frequency of 50 Hz to 50 kHz. Under the application of the pulsed electric field, the BN nanosheets are polarized and are subjected to electric field induced force, thermal motion and viscous drag. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) show that the BN nanosheets are oriented parallel to the direction of the electric field. When the BN content is 5wt.%, the thermal conductivity of the nanocomposite with the application of the pulsed electric field is several times that without the pulsed electric field (0.215 W/mK). The results also show that the repetition frequency affects the thermal conductivity of the nanocomposites. The higher the repetition frequency, the higher the thermal conductivity of the nanocomposites. This paper provides guidance for the preparation of epoxy resin-based nanocomposites with high thermal conductivity at low BN loadings.

        Speaker: Ms Lulu Liu (Chongqing University)
    • 11:45 13:00
      Lunch break 1h 15m
    • 13:00 14:30
      Posters Fundamental Research and Basic Processes and Power Electronics Universal Center ()

      Universal Center

      Conveners: Emily Schrock (Sandia National Laboratories), Dr John W. Luginsland (Confluent Sciences)
      • 13:00
        1P02 - Implications of surface roughness on microscale gas breakdown theory 1h 30m Universal Center

        Universal Center

        Predicting gas breakdown for micro- and nanoscale dimensions increases in importance as devices continue to shrink. A recent study derived a single universal theory to predict breakdown voltage for any gas and pressure for breakdown characterized by Townsend avalanche or field emission at microscale [1]. At microscale, the gas breakdown model more strongly depends on the electrode conditions, such as work function and field enhancement, than geometric or gas parameters [2]. Thus, more accurately predicting breakdown voltage requires further elucidating how physical electrode parameters, such as surface roughness, may alter the work function. This presentation theoretically examines the impact of sinusoidal surface roughness, specifically its amplitude and period, on work function [3] and breakdown voltage. Comparison to microscale gas breakdown experiments assessing cathode surface roughness and implications for nanoscale devices will be discussed.
        Work supported by a Directed Energy Professional Society Scholarship and by the Office of Naval Research under Grant No. N00014-17-1-2702.
        [1] A.M. Loveless and A.L. Garner, “A Universal Theory for Gas Breakdown from Microscale to the Classical Paschen Law,” Phys. Plasmas, vol. 24, 2017, art. no. 113522.
        [2] S.D. Dynako, A.M. Loveless, and A.L. Garner, “Sensitivity of Modeled Microscale Gas Breakdown Voltage due to Parametric Variation,” Phys. Plasmas, vol. 25, 2018, art. no. 103505.
        [3] W. Li and D. Y. Li, “On the correlation between surface roughness and work function in copper,” J. Chem. Phys., vol. 122, 2005, art. no. 064708.

        Speakers: Ms Jacqueline Malayter (Purdue University), Ms Amanda Loveless (Purdue University), Dr Allen Garner (Purdue University)
      • 13:00
        1P03 - Phase mixing and collisionless dissipation at the boundary sheath of magnetized low temperature plasmas 1h 30m Universal Center

        Universal Center

        High power impulse magnetron sputtering (HiPIMS) is an important example for the technical application of magnetized low temperature plasmas. The spontaneous emergence of self-organized structures (spokes) and the presence of anomalous transport - probably two sides of the same coin - play an important role in HiPIMS and related E$\times$B discharges (e.g., Hall thrusters) [1]. These phenomena are not fully understood at present, although they seem to have a strong impact on the overall discharge behavior. Due to their symmetry breaking nature, in principle, a three dimensional kinetic simulation is required. Especially, for the HiPIMS regime ($n_{\mathrm{e}}\leq10^{20}\, \mathrm{m}^{-3}$ and $p\approx 0.5\, \mathrm{Pa}$), conventional kinetic approaches like particle-in-cell (PIC) methods are too resource consuming to simulate relevant time scales. One possible alternative makes use of the fact, that the electron Larmor radius $r_{\mathrm{L}}$ of the thermal electrons is small compared to the typical length scale $L$ of the system. This ansatz, however, breaks down at the plasma boundary sheath in front of the target. A hard wall model might be an effective boundary condition for the interaction of magnetized electrons with this interface [2,3]. In this work, we present numerical and analytical investigations to relate the incoming and the outgoing electron velocity distribution function (EVDF) for different inclination angles of the magnetic field. An interesting feature, which can be observed in the outgoing EVDF, are fractal type structures, which disappear due to phase mixing about a distance of a few Larmor radii away from the sheath edge.

        1 Anders et al., Journal of Applied Physics 111, 053304 (2012)
        2 Krüger et al., Plasma Sources Sci. Technol. 26, 115009 (2017)
        3 Krüger et al., Plasma Sources Sci. Technol. 27, 025001 (2018)

        This work is supported by the German Research Foundation in the
        frame of the Collaborative Research Center TRR 87.

        Speaker: Mr Dennis Krüger (Ruhr University Bochum, Germany)
      • 13:00
        1P04 - RF Gas Breakdown Theory and Experiment as a Function of Gas, Gap Size, Frequency, and Pressure 1h 30m Universal Center

        Universal Center

        Although gas breakdown for radiofrequency (RF) and microwaves has been extensively studied, a consistent and accurate model for AC breakdown voltage independent of unknown fitting parameters remains incomplete. While the RF breakdown model derived by Kihara theoretically justifies a fitting parameter based on various molecular constants, the magnitude used to match experimental results differs [1].
        This study aims to elucidate the physical meaning behind the material parameter and derive a relationship dependent on frequency and pressure. We measured breakdown voltage at 0.38, 0.76, and 1.52 Torr for argon at a ~1 cm gap with frequencies of 96, 140, and 191 MHz and helium at a ~0.8 cm gap and frequencies of 92, 140, and 185 MHz. By using Kihara’s RF equation [1] to fit the data, we demonstrate how the fitting parameter depends on the different experimental parameters, elucidating the underlying physical mechanisms involved.

        1. T. Kihara, “The mathematical theory of electrical discharges in gases,” Rev. Mod. Phys., vol. 24, pp. 45-61, 1952.
        Speaker: Amanda Loveless (Purdue University)
      • 13:00
        1P05 - Investigation of Electron Emission using Molecular Dynamics Simulations 1h 30m Universal Center

        Universal Center

        Continued miniaturization of electronic devices requires a comprehensive understanding of electron emission behavior at micro- and nanoscales for applications involving micro- and nanoelectromechanical systems (MEMS and NEMS, respectively) and microplasmas. While Paschen’s law (PL) traditionally governs gas breakdown, field emission (FE) causes experimental deviation from PL at microscale [1]. Further reducing gap distance makes electron emission space-charge limited as defined by Mott-Gurney with collisions and Child-Langmuir (CL) at vacuum [2]. While previous work has unified PL with FE [1] and FE, MG and CL [2], more detailed simulations showing ion interactions are necessary to develop a comprehensive theory. This presentation applies molecular dynamics (MD) simulations to assess FE [3,4] with ionization and collisions and determine its impact on microscale breakdown. Understanding and quantifying the interplay and transitions amongst these emission regimes is vital for optimizing device design and ensuring reliable results.

        Speaker: Ms Amanda Loveless (Purdue University )
      • 13:00
        1P07 - Density rise away from the antenna in a helicon plasma source following resonance cone absorption in a diverging axial magnetic field 1h 30m Universal Center

        Universal Center

        A Helicon wave heated hydrogen plasma is created by applying 800 W RF power to a Nagoya-III antenna at 13.56 MHz frequency, which excites m = ±1 azimuthal mode in the plasma. A permanent ring magnet provides the axial field required for the helicon wave excitation. The plasma expands from the source chamber to the expansion chamber in a diverging magnetic field. Diagnosis of the wave field through a b-dot probe confirms the helicon wave excitation in the hydrogen plasma. The dc magnetic field is varied from 40 G to 85 G in the source region by changing the ring magnet position to study the effect of the diverging field in the expansion chamber. The field in the expansion chamber reduces down to < 10 G within a distance of 6 cm from the source boundary. The axial profile of the density attains a maximum value under the antenna and decays as we move away from the antenna into the expansion chamber. In the expansion chamber a second peak in density, weaker than the first one, is observed for a particular magnetic field configuration but is absent for other configurations. The phase and amplitude of the axial component of the wave field is measured and a wave damping is observed for the magnetic field configuration in question. This wave damping corresponds to the resonance cone absorption of the helicon wave. No such damping is observed for other cases where there is no dramatic increase in the downstream density.

        Speaker: Arun Pandey (Institute for Plasma Research)
      • 13:00
        1P08 - Investigation of Electron Emission Characteristics of Multi-finger Ferroelectric Trigger Source for Pseudospark Switch 1h 30m Universal Center

        Universal Center

        State of the art low pressure cold cathode pseudospark switches (PSS) for high pulsed power applications require special kinds of trigger electron source, which possess long life, reliable, durable, uniform electron emission, economical, etc. [1-4]. A high dielectric (dielectric constant ~2000) ferroelectric based trigger source is one of the best suited trigger mechanism for the PSS [2-4]. In this paper electron emission and breakdown characteristics of the ferroelectric trigger source has been presented for the generation of required trigger electrons for fast, low jitter and reliable switching. Electrodes with different finger tips and PZT ferroelectric dielectric disc has been analyzed. The electrical and optical diagnostics of ferroelectric trigger source have been performed to analyse electron emission and breakdown characteristics at different operating parameters, such as, gases and pressures, voltages, resistors and inductors. Electrostatic and plasma simulation studies have also been performed to investigate the electric field lines and electron emission processes. An electric field ~107 V/cm in vacuum as well as in gases has been observed between the tips of the electrode and ferroelectric disc. The emission characteristics are strongly dependent on the gases, pressures, voltages and resistances. It is showing better emission characteristics ~30 A at higher pressure ~60 Pa in case of helium and hydrogen gases for 1 kΩ resistance while in case of argon and nitrogen gases at comparable lower pressure ~10 Pa, ~20 A emission current has been observed for 20 kΩ resistance.

        [1] K. Frank, et al., IEEE Trans. Dielectr. Electr. Insul., vol. 14, pp. 968–975, 2007.
        [2] H. K. Dwivedi, et al., IEEE Trans. Plasma Sci., vol. 30, pp. 1371-1375, 2002.
        [3] V. Pathania, et al., IEEE Trans. on Dielectric Elec. Ins., vol. 22, pp.3299-3304, 2015.
        [4] R. P. Lamba, et al., Plasma Sources Sci. Technol., 27, 035003, 2018.

        Speaker: Udit Narayan Pal (CSIR-Central Electronics Engineering Research Institute, Pilani, India)
      • 13:00
        1P09 - Electron emission in liquids 1h 30m Universal Center

        Universal Center

        Discharge formation and breakdown in water have critical implications for water sterilization and biomedical applications [1]. Several studies demonstrate current scaling in liquids following field emission by the Fowler-Nordheim law (FN) and space charge-limited emission (SCLE) by the Mott-Gurney law (MG) with collisions [2]. Recent theoretical work for gases has unified the asymptotic solutions of FN and MG with the Child-Langmuir law (CL) for SCLE at vacuum, even demonstrating a triple point where all three as [2]. This presentation assesses the feasibility of applying a similar unification of MG and FN for liquids. Experimental implications will be discussed.

        1. J. E. Foster, “Plasma-based water purification: Challenges and prospects for the future,” Phys. Plasmas, vol. 24, no. 5, 2017, Art. no. 005501.
        2. K. Dotoku et al., “Field emission into nonpolar organic liquids,” J. Chem. Phys., vol. 69, no. 3, pp. 1121-1125, 1978.
        3. A. M. Darr, A. M. Loveless, and A. L. Garner, “Unification of field emission and space charge limited emission with collisions,” Appl. Phys. Lett., vol. 114, no. 1, 2019, Art. no. 014103.
        Speaker: Ms Sarah Lang (Purdue University)
      • 13:00
        1P10 - Unification of thermionic, field and space charge limited emission 1h 30m Universal Center

        Universal Center

        A recent theoretical study unified field emission modeled by Fowler-Nordheim (FN) and space charge limited emissions (SCLE) with and without collisions modeled by Mott-Gurney (MG) and Child-Langmuir (CL), respectively [1]. This study showed the existence of a triple point, where the three asymptotic solutions matched, and the ubiquitous nature of CL at high voltages, even at high pressure [1]. Theoretical work has also connected FN to thermionic emission modeled by the Richardson-Laue-Dushman (RLD) equation using a general thermal-field emission (GTFE) equation [2]. This presentation explores the potential connection of the GTFE with SCLE, particularly the existence of the triple point as GTFE approaches FN. Experimental implications will be discussed.

        Work supported by a Purdue Doctoral Fellowship and the Air Force Office of Scientific Research under award number FA9550-18-1-0218.

        1. A. M. Darr, A. M. Loveless, and A. L. Garner, “Unification of field emission and space charge limited emission with collisions,” Appl. Phys. Lett., vol. 114, no. 1, 2019, Art. no. 014103.
        2. K. L. Jensen, “A tutorial on electron sources,” IEEE Trans. Plasma Sci., vol. 46, no. 6, pp. 1881-1899, 2018.
        Speaker: Ms Sarah Lang (Purdue University)
      • 13:00
        1P11 - Investigating transport properties of collisionless magnetized plasmas in pulsed power systems via high-order kinetic simulations 1h 30m Universal Center

        Universal Center

        Pulsed power experiments rely on magnetically insulated transmission lines to deliver mega-amps of current to a load to produce and study high energy density matter. Experimental results show that the formation of low-density plasmas in the power feeds gives rise to parasitic currents, which affect load dynamics and prevent scaling of load parameters. To understand the inimical transport properties of these low-density, magnetized, collisionless plasmas and how they affect experimental outcomes, the cross-field environment within the power feeds is studied using high-order time-dependent continuum kinetic simulations, which offer enhanced solution accuracy and can robustly capture equilibria. The effects of drifts, anisotropies, finite Larmor motion, charge separation, and sheared flow instabilities are examined. The computational study is facilitated in part through the development of machinery for constructing self-consistent kinetic equilibria and through the generalization of existing fluid theory analysis. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and was supported by the LLNL-LDRD Program under Project No. 18-ERD-048. LLNL-ABS-767941

        Speaker: Genia Vogman (Lawrence Livermore National Laboratory)
      • 13:00
        1P12 - Quantifying Streamer Dynamics for Azimuthally Swept 3D Wedges in Pin-to-Plane PIC-DSMC Simulations 1h 30m Universal Center

        Universal Center

        The dynamics of streamers in PIC-DSMC simulations of 3D pin-to-plane wedge geometries are formally quantified for several azimuthally swept wedges in terms of electron velocity and density as temporal functions of spatial direction and coordinates r,$\phi$,z. Particles are tracked with picosecond temporal resolution out to 1.4 nanoseconds, spatially binned, and averaged over six independent simulations each sourced with a random plasma seed. An air model$^1$ comprised of Townsend breakdown and streamer mechanisms via tracking excited state neutrals that can either undergo quenching or spontaneous photon emission collisions$^2$ is employed. A 100 $\mu$m radius 1 eV plasma with a 10$^1$$^8$ m$^-$$^3$ particle density placed at the tip of a 100 $\mu$m hemispherical pin electrode (at 6 kV) in a 600 Torr air filled gap, 1.5 mm above a planar grounded cathode, seeds the domain. Prior 2D studies have shown that the reduced electric field, E/n, can significantly impact streamer evolution$^3$. We extend the analysis to 3D wedge geometries (to limit computational costs) with wedge angle azimuthally swept in 15$^o$ increments from 15$^o$ to 45$^o$ to examine the wedge angle’s effect on streamer branching, propagation, and velocity. Initial results suggest that solution convergence in terms of the parameters described above may be achievable.

        1. C. Moore et al., Development of PIC-DSMC Air Breakdown Model in the Presence of a Dielectric, ICOPS, 2016.
        2. A. Fierro et al., Discrete Photon Implementation for Plasma Simulations, Physics of Plasma, 23, 2016.
        3. A. Jindal et al., Streamer Formation Near a Dielectric Surface with Variable Quantum Efficiency, ICOPS, 2017.

        Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

        Speaker: Ashish Jindal (Sandia National Laboratories)
      • 13:00
        1P15 - Determination of First Townsend Ionization Coefficient by Simulation 1h 30m Universal Center

        Universal Center

        In 1963, L.M. Chanin and G.D. Rork[1] measured the first Townsend ionization coefficient for various gasses and a range of pressures experimentally in a vacuum tube. A Townsend discharge is an ionization avalanche that occurs between two electrodes when secondary electron emission caused by ion impact on the cathode is negligible. The first Townsend coefficient, α, is essentially a measure of how many ionization events a single electron will cause when subject to a uniform electric field. In this study, we reproduce the experimental setup of Chanin and Rork’s device in VSim[2], a highly parallelized particle-in-cell/finite-difference time-domain code. Various particle interactions are included, and the first Townsend coefficient is calculated and compared to the reported value. This Work supported by U.S. Department of Energy, SBIR Phase II award DE-SC0015762.

        [1] L.M. Chanin and G.D. Rork, Phys. Rev. 133, A1005 (1964)
        [2] C. Nieter and J. R. Cary, “VORPAL: a versatile plasma simulation code”, J. Comp. Phys. 196, 2004, pp. 448-473.

        Speaker: Nathan Crossette (Tech-X Corporation)
      • 13:00

        Universal Center

        Nanosecond pulsed plasmas (NPPs) can efficiently generate ionized and excited species. While numerous studies have examined local flow field effects [1], characterization of the induced flow field and electrode geometry and the induced flow field remains incomplete. We hypothesize that altering the electrode configuration to modify the electric field will strongly influence plasma species generation and the induced flow field, motivating the development of a more comprehensive model.

        This study couples a quasi-one dimensional model for a parallel plate geometry [2] to BOLSIG+ to better characterize plasma species generation [3]. The implication of electrode configurations, such as pin-to-pin and pin-to-plate, on the induced electric field and generated species will be examined. The long-term incorporation of this model into a high fidelity computational fluid dynamics (CFD) model and comparison to spectroscopic results under quiescent and flowing conditions will be discussed.

        1. A. V. Likhanskii, M. N. Shneider, S. O. Macheret, and R. B. Miles, “Modeling of dielectric barrier discharge plasma actuator in air,” J. Appl. Phys., vol. 103, 2008, Art. no. 053305.
        2. I. V. Adamovich, M. Nishihara, I. Choi, M. Uddi, and W. R. Lempert, “Energy coupling to the plasma in repetitive nanosecond pulse discharges,” Phys. Plasmas, vol. 16, no. 11, 2009, Art. no. 113505.
        3. G. J. M. Hagelaar and L. C. Pitchford, “Solving the Boltzmann equation to obtain electron transport coefficients and rate coefficients for fluid models,” Plasma Sources Sci. Technol., vol. 14, no. 4, pp. 722–733, 2005.

        • This material is based upon work supported by the U.S. Department of Energy, Office of Fusion Energy Sciences under award DE-SC0018156.
        Speaker: Nancy Isner (Purdue University)
      • 13:00
        1P17 - Spontaneous density variations observed in steady-state plasmas sustained using focused microwaves. 1h 30m Universal Center

        Universal Center

        The US Air Force is investigating the dynamics of plasmas sustained for long times using focused microwaves laboratory in conditions which approximate free-space. When the plasma density is sufficiently below the cut-off density the plasma develops regular desnity variations with a wavelength equal to one half the wavelength of the drive beam. We hypothesize that these density variations are the result of standing waves generated by multiple reflections of the drive beam within the plasma. Preliminary simulations taking into account beam diffraction, ionization and diffusion support this interpretation.

        Speakers: Dr Remington Reid (US AFRL), Mr Adrian Lopez (US AFRL)
      • 13:00
        1P21 - Submicroscale Gas Breakdown as a Function of Cathode Protrusions 1h 30m Universal Center

        Universal Center

        Nano- and microscale surface features can have drastic impact on field enhancement and work function, altering field emission from the material. This can significantly change gas breakdown voltage for microscale gaps at atmospheric pressure [1], where field emission drives breakdown rather than Paschen’s law. This presentation reports the nanofabrication of surface feature protrusions [2] and assessment of their impact on field enhancement and breakdown. Nanoscale devices with gaps ranging from 10s to 100s of nanometers were constructed to simulate individual surface protrusions. The devices were made of gold layered onto silicon wafer material to yield nanoscale and microscale gaps with protrusions of various aspects ratio to assess the impact of protrusion shape on field enhancement, as previously reported theoretically [2]. We report DC breakdown of these nanogaps at atmospheric pressure and discuss extensions to other pressure conditions.
        [1] S. Dyanko, A. M. Loveless, and A. L. Garner, “Sensitivity of modeled microscale gas breakdown voltage due to parametric variation,” Phys. Plasmas, vol. 25, 2018, Art. no. 103505.
        [2] J. Lin, P. Y. Wong, P. Yang, Y. Y. Lau, W. Tang, and P. Zhang, “Electric field distribution and current emission in a miniaturized geometrical diode,” J. Appl. Phys., vol. 121, no. 24, 2017, Art. no. 244301.
        This material is based upon work supported by the Office of Naval Research under Grant No. N00014-17-1-2702. A. M. L. gratefully acknowledges funding from a graduate scholarship from the Directed Energy Professional Society and a fellowship from the Purdue Research Foundation.

        Speaker: Russell Brayfield (Purdue University)
      • 13:00
        1P23 - Remote plasma assisted graphene growth for designing graphene/Si hetero-interfaces 1h 30m Universal Center

        Universal Center

        Graphene has gained a lot of attention due to its exception properties of high electron mobility (104-5 cm2/Vs), electric current carrying capacity (~108 A/cm2), optically transparent (> 95 %) and considered as an excellent candidate for next-generation optical, electrical and spintronic devices. Graphene properties can be modified by altering the graphene/substrate interfaces by changing the chemical potential gradient, thereby, induced effective field at interfaces. Here, in the present research, we demonstrated the graphene/silicon interfaces to design the Schottky barrier with a large potential gradient. Graphene was synthesized using the plasma assisted chemical vapor deposition method and was transferred on the silicon substrates. The quality of graphene was confirmed using the Raman spectroscopy technique. The presence of equally intense G and 2D peaks shows the growth of high-quality graphene. Thereafter, the graphene/silicon interface was exposed to the hydrogen plasma at 30 Watt and distance between the plasma electrode and graphene/silicon was kept 15 cm to avoid any direct damage on the graphene/silicon interfaces. Plasma was monitored using optical emission spectroscopy during the graphene synthesis and plasma assisted hydrogen functionalization of graphene/silicon interface. The hydrogen desorption was performed by the thermal annealing of graphene/silicon interface at 150 C. The adsorption and desorption hydrogen on graphene/silicon was also estimated with the help of a probe station by measuring the resistance of graphene keeping distance between the two electrodes fixed. Electrical measurements show the improved diode type behavior at graphene/silicon interface after hydrogen functionalization. Results will be useful to design graphene interfaces for spintronics device to improve the effective field at the graphene hetero-interfaces.

        Speaker: Rohit Medwal (Nanyang Technological University)
      • 13:00

        Universal Center

        The field of study of the explosion of metallic conductors has great scientific interest for researchers [1] because of its adjustable input power, low-temperature plasma generation, and production of nanoparticles for materials science applications. X- and Z-pinch configurations of wires are studied across a broad range of pulsed power drivers. The diagnostics for such experiments typically demand very expensive instrumentation due to the specific requirements for operation and control of the system.

        Plasma spectroscopy [2] is a noninvasive method capable of characterizing the spectral emission off copper and conductive polymers neutrals and ions during the experiment. A Marx bank generates voltages up to 100 kV in a microsecond time frame and supplies the energy to the wire explosion. Two different materials, copper and polylactic acid (PLA) conductive were evaluated in an air atmospheric pressure during wire explosion. A low-cost lab made spectrometer was manufactured applying the fused deposition modeling (FDM) process[3]. The 3D printed parts for the prototype were designed under constraints of basic optics and plasma spectroscopy diagnostics. The intensity of the light from the explosion was recorded by a fast video camera attached to the spectrometer.

        The incorporation of 3D printers for the design of pieces of equipment reduces the cost of manufacturing and acquisition as well as the time for delivery. Furthermore, this tool gives the developer the ability to customize solutions for each experiment more effectively. Experimental data collected with 3D printed spectrometer will be compared with a manufactured commercial version in order to evaluate its performance.
        1. J. Jadidian et al., “Visualization of a Copper Wire Explosion in Atmospheric Pressure Air,” IEEE Trans. Plasma Sci., vol. 39,2842,2011.
        2. H.J. Kunze Introduction to Plasma Spectroscopy (Springer 2009).
        3. D.G. Schniederjans, “Adoption of 3D-Printing Technologies in Manufacturing: A Survey Analysis,” IJPE, vol. 183, Part A, 287, 2017.

        Speakers: Prof. Mark D Johnston (Sandia National Lab), Prof. Edl Schamilogly (The University of New Mexico)
      • 13:00
        1P25 - Dielectric Elastomers: An Investigation in Strain Dependent Electrostatic Pressure of Soft Compliant Dielectrics 1h 30m Universal Center

        Universal Center

        Dielectric Elastomers have the potential of exhibiting large strains when the material is in the presence of an electric field. The dielectric in this case is a soft acrylic (VHB 4910) film that is sandwiched between two compliant electrodes that are subjected to a high voltage potential. When high voltage is applied, attracting charges lead to a contractive force known as electrostatic pressure. The electrostatic pressure deforms the elastomer membrane and the contraction occurs in the thickness (z plane) direction, which leads to actuation and expansion of the film. The pressure in the z direction is usually governed by electrostatic pressure from Pelrine, $\rho=\epsilon_0\epsilon_rE^2$. In the Pelrine model the relative permittivity is held constant when calculating for pressure. Recent research has found that relative permittivity in this case is not at all constant. New literature has shown that permittivity decreases as stretch rates increase. From a microscopic level, polarization within the DEA determines the material's permittivity, which makes it deformation dependent. This work looks to investigate, theoretically and experimentally, a polarization dependent electrostatic pressure model. Using the Tettex 2822 measuring system we can back-solve for the permittivity of the DEA as a function of stretch. This data was collected and used to find a best fit curve to determine various permittivity ranges of the DEA from its relax state to its max deformation.

        Speaker: Mr Barnard Onyenucheya (Primary Contributor)
      • 13:00
        1P26 - Catalytic and Acoustic Nano/Micromotors 1h 30m Universal Center

        Universal Center

        Nano/micromotors that can convert local chemical fuels or external physical inputs into autonomous motion and perform a variety of advanced functions ranging from active drug delivery to environmental remediation and nanofabrication [1]. Synthetic micro/nanomotors can be self-propelled or externally powered in the liquid phase by different types of energy source such as catalytic, electro/magnetic or acoustic [2]. Several methods including electrochemical/electroless deposition, physical vapor deposition, strain engineering and three-dimensional direct laser writing have been fabrication to create the micro/ nanomotors. Compared with other methods, the plasma-based process is fast and environmentally friendly. By using plasma method the materials can be rapidly prepared onto different substrates at room temperature without using any solvent. Besides, in nanomotors fabrication experiments, plasma methods provide homogeneous coating.
        In this study, multiple approaches to powering nanomachines, utilizing individual chemical or physical stimuli, were investigated. In accordance with this purpose, Gold (Au) nanowire prepared by template electrodeposition method for acoustic propulsion and Platinum (Pt) coating for catalytic propulsion RF magnetron sputtering method was used. Fabricated nanomotors were analyzed by Scanning Electron microscopy (SEM) and Mapping analysis. Catalytic and ultrasound propulsion effects were examined on nanomotors speed and direction. For this purpose, speed and direction of nanomotors were investigated by using Optic Microscope (Nikon Ti Eclipse) according to different fuel concentration and acoustic wave power.

        [1]. Chuanrui Chen, Fernando Soto, Emil Karshalev, Jinxing Li, Joseph Wang, 2019, Hybrid Nanovehicles: One Machine, Two Engines, Advanced Functional Materials, 29, 1806290.
        [2]. Dekai Zhou, Yuan Gao, Junjie Yang, Yuguang C. Li, Guangbin Shao, Guangyu Zhang, Tianlong Li, Longqiu Li, 2018, Light-Ultrasound Driven Collective “Firework” Behavior of Nanomotors, Advanced Science, 5, 1800122.

        Speaker: Prof. Aysegul Uygun Oksuz (Suleyman Demirel University, Chemistry & Bioengineering Department)
      • 13:00
        1P30 - Progress Porting VPIC to Several Modern Architectures 1h 30m Universal Center

        Universal Center

        VPIC is being ported and optimized on several modern architectures. These include KNL processors available on Trinity, Cori and Stampede2, Skylake processors available on Mare Nostrum and Stampede2, IBM Power 9 processors and Volta GPUs available on Summit and Sierra and ARM ThunderX2 processors, available on Astra at Sandia and ARM clusters at Los Alamos National Laboratory. VPIC is in production on several of these systems. These architectures vary in many ways including available memory bandwidth, vector length, threads per core, clock frequency and overall node architecture. This work is focused on single node performance. Current efforts to optimize single node performance are exploring changes to data layout of key data structures, use of performance portability frameworks such as Kokkos and performance profiling with a variety of performance analysis tools. Results will be presented which compare the performance of VPIC on these different architectures.

        This work was supported by the US Department of Energy through the Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of U.S. Department of Energy (Contract No. 89233218CNA000001).

        Speaker: Dr William Nystrom (Los Alamos National Laboratory)
      • 13:00
        1P31 - Speed-Limited Particle-in-Cell Modeling of Low-Temperature Plasma Discharges 1h 30m Universal Center

        Universal Center

        Speed-limited particle-in-cell (SLPIC) modeling is a new simulation technique [Werner et al., PoP 25, 123512 (2018)], potentially much faster than conventional PIC, for modeling plasmas characterized by low-velocity kinetic processes. Numerical constraints (e.g. timestep limitations associated with particle cell-crossing times or stability limits) often place challenging restrictions on PIC models of these plasmas; even though the kinetic physics of interest predominantly involves slow particle evolution, the fastest particles dictate the maximum allowable timestep. For high-Z plasmas, large ion/electron mass ratios separate the species timescales to the point that kinetic simulation may be prohibitive, and computational costs can be high even in hydrogenic plasmas. SLPIC provides a possible solution. SLPIC (like PIC) retains a fully kinetic description of the plasma, but imposes an artificial speed limit on fast particles whose kinetics do not play a meaningful role in the system dynamics. Larger simulation timesteps, which enable faster simulations of such discharges, are thus permitted. The speed-limiting is done in a mathematically rigorous sense to maintain accuracy over longer timescales; we may, for instance, speed-limit the bulk of the electron distribution to evolve only on characteristic ion timescales (and use larger simulation timesteps, which need only resolve these scales, to simulate the discharge).

        In this poster we'll demonstrate the use of SLPIC methods using the VSim code [Nieter & Cary, JCP 196, 448 (2004)], moving from simple models of collisionless sheath formation (for which SLPIC has achieved >150x overall speedup relative to PIC with comparable accuracy) to more general low-temperature plasma discharges that include collisional effects and complex geometries. We'll also demonstrate how SLPIC can rapidly model plasma discharge evolution through transient or fluid-like phases, and then continuously transition to a smaller-timestep conventional PIC model as kinetic processes in the discharge become important.

        Funded by US DoE, SBIR Phase II Award DE-SC0015762.

        Speaker: Dr Tom Jenkins (Tech-X Corporation)
      • 13:00
        1P33 - Advanced optimization and machine learning for magnetron design 1h 30m Universal Center

        Universal Center

        High-power microwave source design has evolved from analytic scaling laws to advanced computational methods that can virtually prototype devices before metal is cut in the laboratory. However, with this success has come the requirement that the DOD have the capability to quickly design novel HPM sources for applications with different power and frequency requirements. This need for rapid design capability has pushed both optimization and machine learning techniques into the field of high power RF sources.
        We perform 2D simulations of the Rising Sun magnetron using kinetic modeling framework VSim and optimization engine Dakota. We simulate operation of the magnetron in a series of conditions as a part of the optimization study with the goal to identify the optimum device geometry that operates in a specified frequency bandwidth and at the same time produces adequate output power. We investigate a broad spectrum of control parameters, multiple optimization formulations and a combination of linear and non-linear constraints to fully describe magnetron’s physical state of operation. Furthermore, we apply machine learning techniques to investigate modes of operation of the magnetron with optimized geometry, to efficiently navigate through the physical parameters space, predict and mitigate its performance.

        Speaker: Dr Anton Spirkin (Tech-X Corporation)
      • 13:00

        Universal Center

        In current and future pulsed-power devices, it has become increasingly important to have predictive capability for determining the amount of energy coupled through the magnetically-insulated transmission line (MITL) to the load. Because of the high magnetization and low densities of electrode plasmas in the MITL gap, extended-MHD effects may play a critical role in power-flow physics. In this presentation, we show simulations from PERSEUS [1]/FLEXO and HYDRA [2], which are both capable of modeling MHD and extended-MHD effects in inner MITL power-flow. Specifically, we focus our attention on relevant power-flow plasma quantities, such as plasma density and current density, as predicted by both codes. This problem has recently been simulated with PERSEUS in Hamlin and Seyler [3], and serves as a relevant test problem for understanding the role that extended-MHD plays in power-flow systems.

        • The work of N. D. Hamlin and M. Hess is funded in large part by Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-NA0003525. The work of N.D. Hamlin and C.E. Seyler is supported by the National Nuclear Security
          Administration stewardship sciences academic program under Department of Energy
          cooperative agreements DE-FOA-0003764, DE-FOA-0001153 and DE-NA0001836.

        [1] C. E. Seyler and M. R. Martin, vol. 18, 012703, 2011
        [2] M. Marinak et al, Physics of Plasmas, vol. 8, 2275, 2001
        [3] N. D. Hamlin and C. E. Seyler, Physics of Plasmas, vol. 25, 102705, 2018

        Speaker: Nathaniel Hamlin (Sandia National Laboratories)
      • 13:00
        1P37 - Diode Design For Increased Radiation Dose in HERMES III Far-Field 1h 30m Universal Center

        Universal Center


        Troy C. Powell, Andrew Biller, Keith L. Cartwright, Timothy J. Renk, and Timothy D. Pointon
        Sandia National Labs, 1515 Eubank SE
        Albuquerque, NM 87123 USA

        Increasing radiated dose in the HERMES III far-field region is both a matter of mitigating current loss in the MITL and electron incident angle on the Bremsstrahlung converter. The self-magnetic field of the high currents in the diode causes the electrons to pinch at steep angles once the radial electric field drops to zero near the converter. Since a lower incident angle (closer to norm) increases the dose nonlinearly, diode design should minimize this pinch angle. Designs include extended and indented anode configurations. Indented anode designs are compared with results shared by T. Renk and P. F. Ottinger (using LSP) as well as older results from Sanford (using MAGIC). All designs are reported along with their associated simulation results.

        Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525

        Speakers: Mr Troy Powell (Sandia National Labs), Mr Andy Biller (Sandia National Labs)
      • 13:00
        1P38 - Thermodynamic properties and transport coefficients of C4F7N/CO2 thermal plasma as an alternative to SF6 1h 30m Universal Center

        Universal Center

        The paper is devoted to the calculation of equilibrium compositions, thermodynamic properties (mass density, enthalpy and specific heat at constant pressure) and transport coefficients (electrical conductivity, viscosity and thermal conductivity) of C4F7N/CO2 thermal plasma. Assuming local thermodynamic equilibrium, the species composition is determined using the principle of minimization of the Gibbs free energy. The transport properties are calculated by the Chapman-Enskog method. Some recently updated cross-sections or interaction potentials in the literature is adopted to obtain collision integrals. These data are computed in the temperature range between 300 K and 30 kK, for a pressure between 0.1 MP and 1 MPa and for several CO2 proportions. Transport coefficients of pure CO2 plasma are also compared with previously published values. The results clarify some basic chemical process in C4F7N/CO2 mixtures and provide reliable reference data for the arc simulations.

        Speaker: Mr Lisong Zhang
      • 13:00
        1P39 - A High Order Convected Scheme Solution of the Wigner-Poisson System 1h 30m Universal Center

        Universal Center

        We present a new code for solving the Wigner-Poisson system. Drawing from techniques for solving Vlasov-Poisson, we employ Strang splitting to divide Wigner-Poisson into an advection equation and an integral equation describing velocity space. A forward, Semi-Lagrangian scheme based on the Convected Scheme handles the advection piece while a Fourier transform handles the velocity integral operator. High order is achieved by using WENO to calculate small corrections to the displacement of the Convected Scheme and through Spectral Deferred Correction. Since Wigner-Poisson is the quantum analog of Vlasov-Poisson, we study traditional problems such as Landau damping and the two stream instability in 1D-1V and compare the two models. Eventually, we seek to incorporate our Wigner-Poisson code in a study of stopping power in DT fusion.

        Speaker: Mr Matthew Link (Michigan State University)
      • 13:00
        1P40 - Data storage of particle-in-cell simulations for big data analysis of capacitively coupled plasma reactors 1h 30m Universal Center

        Universal Center

        Capacitively coupled plasmas (CCPs) are widely utilized in etching and deposition processes in semiconductor manufacturing. Nowadays, the nonuniformity of plasma density and temperature distributions is a critical issue near the wafer edge which results in non-uniform etching or deposition profiles. Computer simulation is a good tool to understand the background physics of the nonuniformity and to find a better condition to keep uniformity at the edge. Notably, a particle-in-cell (PIC) simulation is an excellent method to investigate the electron energy probability function (EEPF) which is essential for the control of electron heating mechanism and chemical reactions. In this presentation, we report the spatial changes of EEPF in a two-dimensional PIC simulation for CCP reactors. In order to find out the tendency of the plasma properties over wide parameter variations, a big data analysis is designed to utilize the data storage of PIC simulation results. HBase that runs on top of Hadoop eco system is used to store a large amount of PIC simulation data, which organizes the essential data into two kinds of tables for effective data management.

        Speaker: Ms Yeun Jung Kim (Pusan National University)
      • 13:00
        1P41 - A Fluid Solver approach via Discontinuous Galerkin Methods to Viscoelastic Models for dense plasmas 1h 30m Universal Center

        Universal Center

        We seek a nonequillibrium, heterogenous, large-scale model for strongly coupled plasmas. We generate a generalized hydrodynamic model for strongly coupled plasmas using density functional theory closures of BBGKY hierarchies via hypernetted chain theory. We formulate these equations in the form of a balance law, thereby providing a ''memory'' effect, facilitating correlation. This isothermal ``single fluid'' form of the electrostatic limit is modelled in a fluid context with an exact form of the functional term that is a non-local integral term rising out of hypernetted chain theory. These models, dubbed the Visco-Elastic Density functional (VEDF) equations, provide the first continuum models that match the dispersion of waves in electrostatic ultra-cold correlated plasmas. The resulting equations admit no quasilinear homogenous form and thus we recast the equations as a balance law system treated regionally-implicitly with DG-FEM. We use the DG cell sizes to represent a spatial scale over which model parameters are constant. The generalized pressure and dissipative stress are handled explicitly via a multi-cell reconstruction, as we assume the relevant length scales are different than the length scales of the cells. Here we present work towards a 3D VEDF Solver software package, highlighting our approach to computing the correlation contribution to the generalized pressure.

      • 13:00
        1P42 - Momentum Coupling in Magnetized Plasmas 1h 30m Universal Center

        Universal Center

        Many interesting and important problems in plasma science involve the differential motion of regions of plasma, threaded by the same magnetic field lines. In all reference frames, there is a difference in the motional electric field between regions, a difference in potential parallel to the magnetic field, and currents that couple the different regions, thus transferring momentum by J x B forces. Examples of this dynamic interaction include the coupling between the ionosphere magnetosphere and solar wind; plasma generation in space, charged or large extended spacecraft including tethers, the penetration of plasma jets into plasma-sphere, and the interaction of Jupiter with its moon Io. The standard model for interaction is a MHD-like picture where oblique Alfvén waves mediate the coupling current. If the currents are large and spread over the largest possible volume the coupling is strong. If sheaths and charge layers form, the coupling is diminished. In the years leading up to the flight of the Tethered Satellite System, TSS-1R, in 1996, this standard model was applied but it never lead to definite predictions of the tether current, and the mission results showed no dependence on the Alfvénic parameters. The results did indicate a plasma probe-type interaction, but unlike anything known at that time and still not fully explained today. We are exploring the lessons learn from TSS, to see if the standard model of momentum coupling needs repair, and to better understanding of the problem of a positively charged probe in a flowing plasma. Our approach includes a unique ExB drift plasma chamber, numerical simulation, and theoretical review. Results from all fronts will be discussed.

        Speaker: Dr David COOKE (Air Force Res Lab)
      • 13:00
        1P43 - Experimental Investigation of Colliding Plasma Flows 1h 30m Universal Center

        Universal Center

        This experimental investigation of colliding plasma flows which are frozen-in opposing magnetic fields was conducted to better understand and describe microscopic instabilities and macroscopic-flow patterns. The Plasma Physics and Sensors Laboratory (PPSL), located at Wright-Patterson AFB, developed an experimental setup where a pulsed power system is used to create colliding flows through an Ohmic explosion process from a parallel arrangement of fine-metallic wires creating conditions similar to that of colliding winds in binary star systems. These laboratory experiments create collisionless and collisional shocks in succession. The collisionless shock forms through interpenetration and mixing of the initial portion of flows emitted from fine metallic wires. The latter collisional shock is formed by the colder and denser partially-ionized metallic gases from the wire. This collisional period is conducive to observations of radiative shocks. To characterize these experiments, several optical diagnostics were utilized. This effort shows light emission imaging results collected during experiments of colliding flows agree with theoretical developments in (M. A. Malkov, V. I. Sotnikov 2018), progress on numerical interferometric analysis to extract atomic and electron volumetric densities when symmetry is not available, and progress on a non-equilibrium spectroscopic model under developed to analyze spectral data and provide electron density measurements to complement the other diagnostics.

        Speaker: Andrew Hamilton (Air Force Research Laboratory)
      • 13:00
        1P45 - Laboratory Simulations of Solar Wind Interactions with Airless Bodies: Magnetic Anomalies and Wakes 1h 30m Universal Center

        Universal Center

        The Colorado Solar Wind Experiment (CSWE) simulates solar wind plasma with ion energy up to 1 keV. We present two sets of experimental results related to the interaction of the solar wind with airless bodies: 1) solar wind interactions with lunar magnetic anomalies (LMAs); and 2) wake formation with various Debye lengths relative to the object size. In the first experiment, a permanent magnet was used to create a vertical dipole field behind an insulating surface that faces a plasma flow. Potential profiles on the upstream side were measured using an emissive probe. It was found that surfaces in the dipole lobes are charged to a large positive potential by unmagnetized beam ions and the surface in the cusp is charged negatively by magnetically focused electrons. At lower ion beam energies (< 200 eV), the surface potential in the dipole lobes follows the ion beam energy. However, at higher ion beam energies (200-800 eV), the surface potential becomes significantly lower than the ion beam energy. The exact mechanism is not well understood though various tests have been performed. In the second experiment, an obstacle was inserted in the plasma flow and the wake formed behind it was characterized, including both potential and electron density profiles. The wake is filled by electrons because supersonic beam ions stream by. The ratio of the Debye length to the size of the obstacle was varied from larger than to smaller than 1 to simulate wakes behind bodies with various sizes in space, from small asteroids/rocks to the Moon. Potential and electron density features are shown in preliminary results.

        Speaker: Tobin Munsat (University of Colorado)
      • 13:00
        1P46 - Numerical simulation of a spark channel expansion in water and its comparison with an experimental result 1h 30m Universal Center

        Universal Center

        The main difficulties in verification of numerous mathematical models of an electrical discharge in water is a lack of experimental studies observing a spark channel expansion and its comparison with an experimental result. As a rule, the authors compare an acoustical signature produced by an underwater discharge and also how a bubble cavitation matches the simulation. The presented investigation numerically studies an incompressible approach for a spark expansion and compare it to experimental results obtained by a high speed camera and optical hydrophone. An electrical energy delivered to a plasma channel through Joule heating is divided between an internal energy of the plasma–vapor mixture inside the channel and a mechanical work done by an expanding channel. The total energy deposition is described by an energy balance equation [1]. The pressure in liquid on a spark wall p(t) is described using two mathematical expressions for the pressure p(t) proposed by Naugolnych / Roy [1] and Braginskki [2]. The temporal variations of a spark channel radius, its velocity and pressure wave generated by an expanding spark have been obtained and compared with the experiment. The spark expansion process is perfectly predicted by incompressible approach when the input energy is less than 16 J and can be used for practical applications under this limit.

        This work was performed under auspices and with the support of the Grant Agency of the Czech Republic (contract 18-12386S) and of the Ministry of Education, Youth, and Sports of the Czech Republic (INTER-EXCELLENCE/Inter-transfer contract LTT17015).

        [1] Naugolnych K A and Roy N A 1971 “Spark Discharges in Water” (Moscow: Nauka) translation: Foreign Technology Division, Wright-Patterson AFB, OH, 1974).
        [2] Braginskii S.I., “Theory of a spark channel”, Soviet Physics JETP, vol. 34(7), December

        Speaker: Vitaliy Stelmashuk (Institute of Plasma Physics of Czech Academy of Sciences)
      • 13:00
        1P47 - Electron heating mode transitions in a capacitively coupled oxygen discharge 1h 30m Universal Center

        Universal Center

        Using particle-in-cell Monte Carlo collision simulations we have demonstrated an electron heating mode transition from drift-ambipolar (DA) mode to $\alpha$-mode in the capacitively coupled oxygen discharge as the operating pressure [1,2], electrode separation, and driving frequency [3] are increased. Here we explore further the transition as pressure and electrode separation are varied. When operating at low pressure (10 mTorr) the electron heating is a combination of DA- and $\alpha-$mode heating while at higher pressures ($>$ 30 mTorr) electron heating in the sheath regions dominates. At fixed discharge pressure varying the electrode spacing the electron heating is a combination of DA- and $\alpha-$mode heating for small electrode spacing and it transitions to pure $\alpha-$mode heating as the electrode spacing is increased. We relate the transition to increased electronegativity and generation of drift and ambipolar electric field within the electronegative core when the discharge pressure is low or electrode spacing is small. It is important to note that the addition of the single metastable molecules and secondary electron emission to the oxygen discharge model has a significant influence on the discharge properties and in particular to lower the effective electron temperature [3,4].

        [1] J. T. Gudmundsson and B. Ventéjou, The pressure dependence of the discharge properties in a capacitively coupled oxygen discharge, Journal of Applied Physics, 118(15) (2015) 153302

        [2] H. Hannesdottir and J. T. Gudmundsson, Plasma Sources Science and Technology 25(5) (2016) 055002

        [3] J. T. Gudmundsson, D. I. Snorrason and H. Hannesdottir, The frequency dependence of the discharge properties in a capacitively coupled oxygen discharge, Plasma Sources Science and Technology, 27(2) (2018) 025009

        [4] A. Proto and J. T. Gudmundsson, The influence of secondary electron emission and electron reflection on a capacitively coupled oxygen discharge, Atoms, 6(4) (2018) 65

        Speaker: Prof. Jon Tomas Gudmundsson (University of Iceland)
      • 13:00
        1P55 - Pulsed power-induced CO2 dissociation for CO production 1h 30m Universal Center

        Universal Center

        Carbon dioxide CO$_2$, as thermodynamically stable end product of fossil fuel based combustion, is an interesting source for carbon monoxide CO, if a sustainable energy source is used. CO is a major precursor in chemical synthesis, e.g. Fischer-Tropsch based CO hydrogenation to synthetic hydrocarbons and future fuels.

        In this study, pulsed power technology has been applied for generating a non-thermal plasma for CO$_2$ dissociation; the CO$_2$ average bond dissociation energy is 8.3 eV. A capacitor-spark gap pulsed power source has been utilized, typically delivering 200 mJ pulse energy (at repetition rates up to 100 Hz) to a concentric wire-cylinder electrode geometry in an atmospheric pressure and ambient temperature operated reactor. Power-to-gas coupling has been studied by varying gas flow and pulse repetition rates. CO$_2$ conversion and CO production efficiencies have been determined as a function of the energy density, for plasma batch operation of CO$_2$ mixtures with nitrogen, argon or helium.

        CO$_2$ conversion has appeared to be most favorable using Ar as buffer gas. This is explainable by the average metastables energy which decreases according to the order He>Ar>N$_2$, combined with the assumption that He metastables production under the applied plasma conditions is less favorable. Although CO$_2$ conversion levels increase with energy density, also the probability of CO$_2$ back oxidation increases, when oxygen is not removed from the system. Additionally, it has appeared that the plasma provides enough energy to even split CO with a bond dissociation energy of about 11.2 eV; in other words, carbon deposition has been observed.

        Speaker: Dr Wilfred Hoeben (Eindhoven University of Technology)
      • 13:00
        1P56 - Modeling of the Plasma Chemistry in an Electron Beam Induced Discharge* 1h 30m Universal Center

        Universal Center

        A simulation code is under development for the solution of the time-dependent Boltzmann equation, self-consistently coupled with time-evolving nitrogen plasma chemistry of an electron beam driven discharge. The application is to a N2 filled chamber of pressure 0.1 to 10 Torr driven by the NRL 90 kV Febetron generator with a e-beam current pulse of 100 ns, peak current of 4 kA, and beam current density ranging from 25 to 300 A/cm$^2$. The code follows the time–dependent response of the electron energy distribution function (EEDF) to the changing electromagnetic field induced by the e-beam. During the e-beam pulse, the gas undergoes ionization and excitation due to collisions with the beam electrons with subsequent plasma chemistry. Processes included in the kinetics part of the model are electron-neutral and Coulomb collisions, vibrational excitation, electron collisional excitation, and ionization by plasma and beam electrons. The plasma chemistry module includes collisions with electrons, dissociation and dissociative ionization of N2 molecules. Conditions are such that the creation of secondary as well as later generations of plasma electrons by the e-beam are important. A circuit model of the 2D axisymmetric gas target solves for the electromagnetic fields and the return current density is computed from a generalized Ohm’s law. Plasma conductivity and the return current are studied in a wide range of gas pressure and e-beam current densities. The importance of individual plasma chemical processes and species production is evaluated.

        *Work supported by NRL 6.1 Base Program.

        Speaker: Dr Tzvetelina Petrova (NRL)
      • 13:00
        1P57 - Generation of carbon monoxide from carbon dioxide using nanosecond pulsed discharge 1h 30m Universal Center

        Universal Center

        Carbon monoxide is a gas generated by incomplete combustion and carbon compounds, and generally recognized as a toxic gas. Also, it is known as greenhouse gas. The emission amount of carbon dioxide is increasing year by year, which is recognized as a severe environmental problem. On the other hand, it is considered to be an industrially useful material used for the synthesis of methanol. Carbon dioxide was converted to carbon monoxide using nanosecond pulsed discharge which created by a very short, high voltage with pulse width is 5 ns. The nanosecond pulsed discharge was generated in a coaxial cylinder type rector with a large discharge volume. The applied voltage from the nanosecond pulse generator to the reactor was adjusted to 30, 40, 50 kV and the pulse repetition rate was adjusted 50 - 400 pps. The initial carbon dioxide concentration of simulated gas was regulated at 100 %; and gas flow rate was controlled to 2, 5, and 10 L/min. The results showed that carbon monoxide was successfully generated and by products was not detected. Detailed results will be presented at the conference.

        Speaker: Tatsuya Ichiki (Graduate School of Science and Technology, Kumamoto University)
      • 13:00

        Universal Center

        Low temperature atmospheric pressure non-equilibrium plasma was generated in liquid hydrocarbon with gas bubbles to characterize the reformed gases formed by hydrocarbon processing. A 10g sample of hexadecane was used as liquid hydrocarbon while 90% CH4, 10% H2 gas bubbles of 50 sccm was flown throw it. A RC circuit was used to generate nano-second pulsed plasma with 20pF capacitor and 1.5M ohm resistor. Energy deposited was 500KJ/kg over a duration of 3 hours. The electrodes were on a pin-plate configuration with the top being a quarter inch rod and the bottom electrode being the capillary. The bottom electrode was set to high voltage while the top electrode acted as ground. A closed system loop was developed that recirculated the gases through the plasma reactor and into a gas chromatography in real time to characterize and analyze them. Preliminary results show that 3.9% of the gases captured were reformed gases formed from methane, hydrogen and hexadecane cracking by plasma reaction. The rest of the gas sample was Methane and hydrogen whose percentage concentration will be quantified in future. Of the 3.9% reformed gases, Ethane is 10%, Ethylene 15%, Propane 1%, Propylene 2%, Acetylene 40%, C4 – 3%. C5 – 15%, and C6+ 14%. To quantify the above results, an independent fast refinery gas analysis method was created to detect hydrocarbons in vapor phase using Shimadzu gas chromatography 2014. A complete mass balance is required to understand the thermodynamics and reaction kinetics of non-equilibrium plasma processing and reforming of hydrocarbons.

        Speaker: Mr Shariful Islam Bhuiyan (Texas A&M university)
      • 13:00
        1P61 - Effects of non-Maxwellian electron energy distribution function on plasma chemistry in Cl$_2$ and CF$_4$ 1h 30m Universal Center

        Universal Center

        Control of low-temperature plasmas for materials processing is critical to the quality of the product. Therefore, it is required to further refine and customize reactive fluxes. In this regard, a global model was used to study the electron kinetic effects in Cl$_2$ and CF$_4$ plasmas. The model was benchmarked against another global model by using the same set of gas phase and wall surface chemical reactions assuming a Maxwellian electron energy distribution function (EEDF). In this model the reaction rate coefficients are calculated by integrating EEDFs with cross sections; the wall recombination coefficients are approximated using available experimental data. The model was validated by comparison with experimental data for chemical composition and plasma density. We used a bi-Maxwellian EEDF (with the hot and cold electron temperatures) to investigate electron kinetic effects on the chemical composition. As the temperature of hot electrons increases at the given constant power, densities of Cl$^+$, Cl$^-$ as well as Cl increase gradually in Cl$_2$ plasmas. By contrast, the chemical component in CF$_4$ plasmas is affected less by variation of hot electron temperature at given power absorbed in plasma. This is due to the relatively high dissociation energy thresholds. However, chemical composition can be significantly modulated by increasing power. Therefore, discharge power and temperature of hot electrons should be used simultaneously to control the densities of chemical species.

        Speaker: Xifeng Wang (University of Michigan)
      • 13:00
        1P63 - Spectroscopic investigation of air excited and ionized by an electron beam* 1h 30m Universal Center

        Universal Center

        Plasma chemistry induced in air by an electron beam is being studied at the Naval Research Laboratory. An electron beam is produced in vacuum using a Febetron pulsed-power generator modified to produce a peak voltage of 80 kV, a peak current of 4 kA, and a pulse width of 100 ns. The beam then passes through a thin anode followed by a thin pressure barrier into a cavity filled with low-pressure dry air. Visible and near-ultraviolet spectral lines are used to diagnose the presence of excited and ionized states induced as the beam transits the air. The time dependence of these excited states at different pressures is compared with the electron density and current within the cavity, as well as framing camera images of the visible emission.

        *Supported by the Naval Research Laboratory Base Program.

        Speaker: S. L. Jackson (Plasma Physics Division, Naval Research Laboratory)
      • 13:00
        1P65 - Development of a plasma source to accommodate an LIF dip measurement system 1h 30m

        Understanding air plasma chemistry requires accounting for the myriad of gas phase reactions initiated and mediated by electrons and excited species in the presence of an applied field. In the case of a time-varying electric field, this task of tracking reactions is even more complex. Models developed to track these physical processes require physics verification, model benchmarking and ultimately experimental validation. This effort aims to produce experimental measurements of the time-varying electric field associated with a pulsed air plasma. The effort is two-pronged, including diagnostic development and a plasma source to validate the diagnostic with a controlled and well-defined electric field. Here we describe a glow discharge hollow cathode source that provides a source of excited species which flow into a well-controlled, plasma- free electric field. These species will be probed by an LIF dip diagnostic for calibration and validation. Preliminary optical emission spectra of the source plasma as a function of pressure and power with argon and air as well as absorption spectroscopy of argon excited species using a white light source are presented along with specific features of the plasma source. Argon metastable density as a function of discharge power and pressure is also presented.

        Speaker: Ms Jenny Smith (University of Michigan)
      • 13:00
        1P66 - What different effects can be taken by different liquid-dissolved gases on the concentration of aqueous RONS? 1h 30m Universal Center

        Universal Center

        The chemical process that occurs in plasma-liquid interaction is a key issue in plasma biomedical applications and clinical treatment processes. The researchers have discussed the effects of different plasma sources and liquid components on the generation of aqueous RONS [1-3] while almost no one cares about the different liquid-dissolved gases have what different influence on the concentration of aqueous RONS. Since the aqueous oxygen is involved in the formation and transformation of the aqueous RONS according to the researches, and the proportion of the gas component in the living body is different from the proportion of the gas component in water body in the atmospheric environment. Therefore, we designed an experiment, in which we dissolved different kinds of gases in double distilled water (DDW), including CO2, O2, N2 and air, to separate the effects of different gas components in the liquid on the formation of liquid active particles.
        The experimental results show that the presence of aqueous oxygen plays an important role in the formation of ROS and RNS, which can promote the generation of various aqueous RONS, and nitrogen has a certain influence on the formation of RNS. CO2 also has a positive impact on the formation of RNS while it is slightly detrimental to the formation of OH. Taking H2O2 and NO2—as examples, the rule of H2O2 concentration in various DDWs shows: unprocessed ( µM) > N2 ( µM) (or O2 ( µM)), and CO2 ( µM) > O2 (or N2) > air-free( µM), and the rule of NO2— is: unprocessed ( µM) > O2 ( µM) (or N2 ( µM)) and CO2 ( µM) > O2 (or N2) > air free ( µM).

        Speaker: YING YANG (Huazhong University of Science and Technology)
      • 13:00
        1P67 - Design of High-Voltage Pulse Generator Control System for CSNS Linac RF System 1h 30m Universal Center

        Universal Center

        China Spallation Neutron Source (CSNS) is the first neutron source facility in developing countries. it includes a powerful linear proton accelerator, a rapid circling synchrotron, a target station and three neutron instruments. As one of the largest science and technology infrastructure projects in China, CSNS is expected to have positive effects in promoting the sciences, high-tech development and national security. Klystron power supply of CSNS is an important part of linac. high-voltage pulse generator provides for Klystron.
        High-voltage pulse generator is composed of 400Hz series-resonant DC high-voltage power supply and solid state modulator. it includes variable frequency power supply (50Hz converted to 400Hz), boost transformer (0.5kV-6kV), 400Hz series-resonant LC, energy storage capacitor and pulse high-voltage modulator.
        AC power transfer to the 500V of 400 Hz square wave,and then LC series-resonant circuit inspires high-voltage ,AC High-Voltage transfer to DC High-Voltage by Silicon Stack and capacitance, at last realize the output of DC high-voltage. The core component of high-voltage pulse modulator is composed of 150 MOSFET power switches, which are synchronously triggered by the optical pulse,the pulse width and repetition rate of optical signal is variable, so the DC supply is modulated pulse High-Voltage supply.
        This paper introduces the structure of the pulse high-voltage generator system and the principle of the system, Focusing on the introduction of high-voltage pulse generator control system, the operation of high-voltage pulse generator.

        Speaker: Mr Maliang Wan (Dongguan Branch,Institute of High Energy Physics,Chinese Academy of Sciences)
      • 13:00
        1P68 - Compact Rapid Capacitor Charger for Mobile Marx Generator Applications 1h 30m Universal Center

        Universal Center

        Abstract - The purpose of this paper is to show the work being performed to develop a compact rapid capacitor charger suitable to charge Marx banks to voltages ranging between 5 kV to 10 kV. The capacitor charger is being constructed with mobility in mind; for this reason it is powered using a series of high current LiPo batteries. A detailed description of the different system components is presented to familiarize the reader with rapid capacitor chargers. Additionally, a sketch of the proposed system packaging is shown along with volumetric and specific power densities. These figures are of paramount importance in mobile applications. Results of the preliminary tests are discussed outlining the performance achieved while charging test capacitor banks. Future improvements to the system are shown and their implementation paths discussed.

        Speaker: Dr Argenis Bilbao (U.S. Army Research Laboratory)
      • 13:00
        1P69 - A Compact 100 kW high-voltage power supply with balanced bipolar output 1h 30m Universal Center

        Universal Center

        A passively balanced, bipolar high voltage power supply was created to drive a compact pulsed power system. The supply was designed to average 100 kW when charging a capacitive load to ±50 kV with a total volume of less than 15 L. The supply is sourced from a low-ESR ultra-capacitor bank charged to 285 V and operates with runtime durations less than a few seconds. The converter employs a novel high turn ratio, low leakage inductance transformer to achieve a resonant impedance less than 200 mΩ at a switching frequency of 20 kHz. The step-up transformer output is converted to a balanced bipolar output by a dual polarity diode multiplier circuit with a common ground reference. The resonant converter is based on an LCL-T configuration and operates in constant current mode. Design details and experimental measurements of the converter will be presented.

        Speaker: Dr Jordan Chaparro (Naval Surface Warfare Center)
      • 13:00
        1P71 - Ablation and Breakdown Characteristics of High Current Gas Spark Switch with Different Profiles 1h 30m Universal Center

        Universal Center

        In the field of high voltage discharge device and pulse gas laser, electrode profiles, in the determination of the polar electric field, have a direct impact on the quality of the product. From the demand of practical engineering, for the high-current two-electrode self-breakdown repetitive spark switch, in this paper, we selected the ball electrode, flat bulb electrode, Chang and Bruce, four different types of electrodes which were made of 90WNiFe in the same geometry except its profile. Under ~20kV average self-breakdown voltage, ~40kA average peak current and dry air environment, these four pairs of electrodes are tested for 5000 shots short circuit discharge in standard atmospheric pressure, respectively. During the experiment, the distance between two electrodes were set to be 2,3,4,5,6 mm respectively and obtain the self-breakdown voltage curve to provide guidance for engineering practice. Self-breakdown voltage statistics and discharge stability are investigated. Furthermore, after the experiment, the high voltage electrodes were scanned by electron microscopy to observe the distribution of erosion area and ablation morphological characteristics of different electrodes.

        Speaker: Mr Yu Wang
      • 13:00
        1P72 - Measurement of Diode Reverse Recovery Losses as a Function of Switching Frequency* 1h 30m Universal Center

        Universal Center

        In compact pulsed power applications, high voltage is often sourced by a DC to DC resonant converter that draws power from a manageable, lower voltage source. As the converter’s switching frequency is increased, the size of the magnetics decreases increasing overall power density. Despite its many advantages, increasing the frequency can increase the switching losses within the rectifying diodes and solid-state switches. The research presented is focused on the studying the reverse recovery losses associated with Silicon Carbide Schottky diodes used in a high voltage full-bridge rectifier at switching frequencies ranging from 10 kHz to 65 kHz. The testbed assembled to study the diodes will be presented along with the results collected to date.

        Speaker: Mr Christopher Martinez (University of Texas at Arlington)
      • 13:00
        1P73 - High Rate Charge and Discharge of High Voltage Capacitors* 1h 30m Universal Center

        Universal Center

        In the work presented here, a well-controlled study has been performed to characterize the performance of a high-voltage, pulsed-power capacitor when it is recharged to 100 kV in 100 μs. A CLC testbed has been assembled to supply the high rate pulsed recharge current to the capacitor being studied. Experiments are being performed in a controlled temperature environment ranging from 20 deg C to 60 deg C. The capacitors are of interest for use in compact, repetitive rate, Marx generator sources used to supply pulsed power to a few different loads. The testbed will be discussed along with the experiments planned in the coming months.

        Speaker: Mr Christopher Martinez (University of Texas at Arlington)
      • 13:00
        1P75 - Multi-pulse performance of amorphous metal magnetic cores at high magnetization rates 1h 30m Universal Center

        Universal Center

        Amorphous metal magnetic cores are essential in developing multi-pulse solid state systems due to their high magnetic saturation value. In order to operate in multi-pulse mode, the magnetic core must provide enough volt-seconds before reaching saturation. They must prove to be reliable and maintain little to no load loss during the high rate pulses. This paper presents the efforts to characterize the performance of various MetGlas cores at high magnetization rates and use this data to develop models for simulation. Results of the test are used to match the magnetic cores and to assemble cells with identical volt-second ratings.

        Speaker: Daisy Acosta-Lech (Lawrence Livermore National Laboratory)
      • 13:00
        1P76 - Compatibility of SLA and FDM printed components with common insulating oils 1h 30m Universal Center

        Universal Center

        To enable the use of additively manufactured polymer materials as structural and insulating components within high voltage pulsed power systems, better understanding of material compatibility with common pulser material environments is required. The present work examines the effect of long-duration contact of printed polymeric components with three high voltage insulating oils: Diala, Luminol, and Royco 66. Test pieces printed using stereolithography (SLA) were evaluated for changes in mechanical properties and dielectric strength after submersion in insulating oil for a nine-month duration. Test pieces printed using fused deposition modeling (FDM), also submerged in oil for a nine-month duration, were evaluated for changes in mechanical properties only, due to known problems with highly anisotropic dielectric strength. GC-MS was used for oil composition analysis on neat and plastic-exposed oils to identify any chemical leeching that occurred during the exposure period and to correlate changes in chemical composition with bulk mechanical and dielectric properties that result from oil immersion. Data and analysis of experimental results will be presented.

        Speaker: Casey Ottesen (COSMIAC at University of New Mexico)
      • 13:00
        1P79 - An Eigenvalue Approach to Study SPIDER RF Oscillator Operating Space 1h 30m Universal Center

        Universal Center

        The SPIDER experiment features four radiofrequency (RF) circuits to heat the plasma generated in its inductively coupled ion source. Each circuit includes a tetrode-based Colpitts push-pull oscillator (200kW rated power) operating at 1 MHz frequency, a coaxial transmission line to feed the load composed of a couple of RF antennas and a resonant matching network. The SPIDER operation has shown two phenomena that affect the performance of the RF circuit: the so-called “frequency flip” that prevents the operation at the best load impedance matching condition and a limitation on the maximum RF power delivered by the RF generators.
        Theoretical models of the SPIDER RF circuits have been developed able to predict the frequency flip occurrence, that has been also experimentally observed. By using the validated models, an operational setup to avoid the frequency flip occurrence has also been synthesized and successfully implemented. However, limitation in the maximum delivered RF power are still present, thus the RF circuit modelling approach has been further developed exploiting the state space analysis to achieve a deeper comprehension of its operation.
        The results of the eigenvalue analysis of the circuit gives as outputs both the operating frequency of the system and equivalent load seen by the oscillator at that frequency. The equivalent load is used as input for an steady state electrical model of the push pull connection of the tetrodes, which exploits the algebraic model of the tetrode and permits the identification of the RF power delivered by the oscillator as a function of the tetrodes polarization voltages.
        A validation of the models developed is presented on the base of the SPIDER experiments.

        Speaker: Ferdinando Gasparini (Consorzio RFX (CNR, ENEA, INFN, Università di Padova, Acciaierie Venete Spa), Corso Stati Uniti, 4, Padova, Italy)
      • 13:00
        1P81 - E-band Power Combining Experiment for High Power Millimeter Waves* 1h 30m Universal Center

        Universal Center

        Millimeter wave summation is the only way to achieve high power generation due to the limited power handling capability of a single high power microwave source. In this work, a 12-way power combiner was experimentally studied. The amplitude and phase of each input are adjustable using 12 attenuators and 12 phase shifters. The output of the power combiner was received by an open-ended waveguide probe. The experiment was performed in E-band where the designed frequency was at 78 GHz.

        The power combiner converts the fundamental mode of the rectangular waveguide to the TE01 mode in overmoded circular waveguide. The TE01 output mode is a reasonable option due to the fact that conductive losses decreases for this mode as frequency increases.

        • Work supported by DARPA Grant #N66001-16-1-4042
        Speaker: Ahmed Elfrgani (University of New Mexico)
      • 13:00
        1P82 - High Power Ćuk Converter for Fusion Science Applications 1h 30m Universal Center

        Universal Center

        Eagle Harbor Technologies (EHT), Inc. is developing a Ćuk converter for local helicity injection and magnet driving and control for the Pegasus Toroidal Experiment at the University of Wisconsin – Madison. A Ćuk converter has low output ripple; high efficiency; voltage gain greater than one, allowing for deeper energy storage utilization; continuous power flow that lowers output EMI, reducing noise generation; continuous input and output current – energy flow from the series capacitor allows for greater control of the injector currents; and series arrangements can be utilized that isolate individual switch modules so a failure does not potentially damage all solid-state switches. EHT will utilize previously developed precision gate drive technology that allows for high frequency switching, which reduces the capacitor and inductor values significantly, making the design more compact and lower cost. EHT has designed and built a first-generation Ćuk converter that was tested at Pegasus. We will present the Phase I project plan and the results of the project.

        Speaker: Mr Alex Henson (Eagle Harbor Technologies, Inc.)
      • 13:00
        1P84 - Plasma water treatment and oxidation of organic matter in water 1h 30m Universal Center

        Universal Center

        Low temperature plasma microdischarges in contact with aqueous solutions which include organic dyes are studied. Plasma treatment of samples over a duration of time encompassing one hour are observed. The Ultraviolet–visible (UV-Vis) spectrum of select samples is analyzed to assess and measure the change in organic dye content. Results are presented which indicate the efficacy of small scale plasma systems to oxidize organic matter in water. The color change in the water is observed and the associated changes in absorbance and reflectance are characterized. With the present system, dye-containing samples were made visibly clear during testing. Further tests with water treatment parameters such as total organic carbon are also being pursued. The efficacy of using plasmas for cleaning water, specifically related to the presence of oil in water is discussed.

        Speaker: Dr Kamau Wright (University of Hartford)
    • 14:30 15:30
      Plenary Mon PM - Manfred Thumm (2018 NPSS Merit Award) Seminole Ballroom ()

      Seminole Ballroom

      Convener: Edl Schamiloglu (University of New Mexico)
      • 14:30
        The Wendelstein 7-X Stellarator: Plasma Generation, Heating and Current-Drive with the Worldwide Largest Electron Cyclotron Heating Facility 1h

        The stellarator Wendelstein 7-X (W7-X) is equipped with a steady-state capable 10 MW ECH system, operating at 140 GHz, which corresponds to the 2nd cyclotron harmonic of electrons in the magnetic field of 2.5 T. Ten megawatt-class gyrotrons, two 5 MW quasi-optical transmission lines (94 % transmission efficiency) as well as 4 directly steerable and 2 novel remotely steerable launchers inside the plasma vessel are operational and already delivered more than 7 MW X2-mode heating to W7-X plasmas. Besides the reliable plasma start-up and routine ECH wall conditioning, stationary discharges up to 100 s have been achieved, which were only limited by the maximum test divertor energy load. Combined with pellet injection, the highest triple product $(0.68\times 10^{20} keV m^{-3} s)$, observed up to now in stellarators, was achieved, exclusively by electron heating heating [T Sunn Pedersen et al 2019, Plasma Phys. Control. Fusion 61, 014035]. The corresponding plasma parameters were $T_{i0}=T_{e0}=3.8$ keV, $n_{e0}=0.9 \times 10^{20} m^{-3}$ and $\tau_E=0.22$ s. For the first time, dense W7-X plasmas were sustained by 2nd harmonic O-mode (O2) heating, approaching the collisionality regime for which W7-X was optimized [T Stange, submitted to PRL]. O-mode heating needs high $T_e$ and multi-pass absorption that was obtained by tungsten covered Mo mirror tiles with holographic gratings at the inner wall. After boronization of the plasma vessel, stationary O2-heated plasmas above the X2 cut-off with hydrogen gas fueling only; hydrogen plasmas with 6 MW ECRH for 30 s at only 1 MW divertor heat load (detached plasma), thermalization $(T_i=T_e=1.5 keV)$, density $1.6 \times 10^{20} m^{-3}$ and radiation control with $W_{dia}=800$ kJ were achieved, which is a reference scenario for later long-pulse high density discharges. Power deposition scans did not show any indication of electron temperature profile resilience. In low-density, low-power plasmas compensation of the bootstrap current with electron-cyclotron current drive (ECCD) was demonstrated [RC Wolf et al 2019 Plasma Phys. Control. Fusion 61, 014037]. The long discharges were used to demonstrate current control and bootstrap current compensation by ECCD. Until 2018, the plasma vessel was equipped with an uncooled divertor, which allowed to extend the integrated heating energy from 4 MJ to 80 MJ. The full steady-state capability will be reached in 2021, after an actively cooled high heat-flux divertor has been installed which can tolerate steady-state heat fluxes of up to 10 MW/m$^2$. Plans for later upgrades include a further increase of the ECH power to 18 MW and the introduction of tungsten as a first-wall material.

        This work was carried out within the framework of the EUROfusion Consortium and has received funding from the EURATOM research and training programme 2014-2018 and 2019-2020 under Grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

        • W7-X Team: see author list of T Klinger et al., Nucl. Fusion 2019 doi.org/10.1088/1741-4326/ab03a7
        Speaker: Prof. Manfred Thumm (Karlsruhe Institute of Technology, IHM)
    • 15:30 16:00
      PM Break 30m
    • 16:00 17:30
      1.3 Space Plasmas Gold Coast I/II (Double Tree at the Entrance to Universal)

      Gold Coast I/II

      Double Tree at the Entrance to Universal

      Convener: David Cooke (US Air Force Res Lab)
      • 16:00
        Parametric Interaction of VLF and ELF Waves and Impact on Energetic Electrons in a Radiation Belt 15m

        Different sources for Very Low Frequency (VLF) whistler wave generation including parametric mechanisms of excitation with involvement of Extremely Low Frequency (ELF) waves, Ion Acoustic (IA) waves as well as conventional loop antennas will be analyzed. Whistler waves interact with Radiation Belt (RB) electrons via cyclotron resonance. This interaction leads to enhanced pitch angle diffusion and shifting energetic electrons towards the loss cone. In order for this interaction to be efficient it is necessary to create certain level of finite amplitude VLF electromagnetic whistler waves in the interaction region. In the case of conventional sources a great deal of the source power is radiated not as a whistler wave but as a quasi-electrostatic Low Oblique Resonance (LOR) mode which does not propagate on great distances from the source region. Only a small percentage of the power ~ (3 – 5)% is radiated as an electromagnetic whistler wave. We present new results on parametric interaction of LOR waves with IA waves and ELF waves to demonstrate the possibility to overcome this difficulty. It will be shown that interaction of LOR waves with low frequency waves gives rise to electromagnetic whistler waves on combination frequencies. It is shown in this work that the amplitude of these waves can considerably exceed the amplitude of whistler waves directly excited by a loop source. Additionally, particle-in-cell (PIC) simulations, which demonstrate the excitation and spatial structure of VLF waves excited by conventional and parametric sources will be presented.

        Speaker: Dr Vladimir Sotnikov (Air Force Research Laboratory)
      • 16:15
        Modulational instability and study of freak waves in an ion beam plasma with two temperature superthermal electrons 15m

        From past few decades, there has been a great deal of interest in understanding different types of nonlinear solitary structures in various kind of plasmas. The study of ion-acoustic solitary waves in multi-component plasmas holds an mportant place in both theoretical and experimental point of views. The satellite observations in space plasma confirm that these waves generally occur in association with ion beams. It was also shown that the ion acoustic solitary waves are associated with up flowing ion beams. The propagation of an ion beam into a plasma can strongly affect the conditions for the occurrence of solitary waves and may modify their properties. Most of the space and astrophysical environments show the existence of superthermal particles. These particles are well modelled by the kappa type distribution. We have derived the nonlinear Schrodinger equation (NLSE) using multiple scale perturbation technique. From the nonlinear and dispersion coefficients, we have studied the instability conditions. Both dark and bright envelope solitons are observed. It is observed that physical parameters (e.g., beam concentration, superthermality of electrons and beam velocity) play a significant role to modify the envelope solitary structures associated with these low frequency waves. The characteristics of first- and second-order freak waves has been studied in detail in the presence of ion beam. The findings of the present study may be useful in understanding the amplitude modulation of low frequency solitary waves in various space/astrophysical plasma environments such as Saturn's magnetosphere, polar cap region of Earth's magnetosphere where ions, two temperature superthermal electrons and ion beams are present.

        Speaker: Ms Nimardeep Kaur (Guru Nanak Dev University)
      • 16:30

        One of the fundamental questions in space plasmas still not fully understood is how the energy is transported from solar wind to the Earth’s magnetosphere. In the present study, we observe electron velocity distribution (EVD) functions in the Earth’s magnetosphere using the CLUSTER data. The EVD functions are observed at different times when the CLUSTER traversing the southern cusp region, such as when the electron density is very low typical of cusp values and when it suddenly increases due to some solar wind disturbances at the magnetopause. We found that the observed EVD functions are flat top distributions and have two populations; a cold bulk magnetospheric population and a hot solar wind tenuous population. Observed EVD functions are then fitted by generalized (r,q) distribution which is the generalized form of kappa and Maxwellian distribution functions and can be reduced to kappa and Maxwellian distributions in the limit r =0, q = k+1 and r =0, q goes to infinity, respectively. We derive the expressions of real frequency and damping/growth rates by employing generalized (r,q) distribution function and plot them using the observed plasma parameters and fitting values of spectral indices r and q. When solar wind electrons with flat top distribution enter into the Earth’s magnetosphere, we obtain enhnced growth causing the Whistler waves to grow and hence responsible for the transport of energy from solar wind to the magnetosphere.

        Speaker: M N S Qureshi (GC University, Lahore)
      • 16:45
        Low energy electron irradiation induced charging of dielectric materials: measurements and analyses. 15m

        Charging of dielectric materials under electron irradiation is a commonly encountered problem in many space applications. Spacecraft charging due to solar and cosmic radiations may lead to critical discharge phenomenon. Indeed, under irradiation (especially electron irradiation), insulators as well as floating conductors may charge negatively or positively depending on the incident electron properties (energy, incidence angle , flux ) and on the specific material properties (composition, surface roughness, contamination, temperature, etc.) The knowledge of the electrical properties electron emission yield, conductivity and radiation induced conductivity) under electron irradiation for each material of the spacecraft is needed for spacecraft plasma interaction software. The aim of our contribution is to present the results of charging proprieties (magnitude and kinetic) under low incident electrons energy (from few eV to keV) for Teflon and alumina. The energy distribution of the emitted secondary and backscattered electrons was measured dynamically with the help of high speed hemispherical electron energy analyzer. The evolution of the surface potential of the irradiated sample was derived from the energy shift of the secondary electron pic.

        Speaker: Dr Mohamed Belhaj (ONERA)
      • 17:00
        Modeling DSX Plasma Interactions Using Nascap-2k 15m

        Wave Particle Interaction Experiment (WPIx) will be conducted on the DSX (Demonstration and Science eXperiments) mission to be launched in 2019. WPIx broadcasts VLF (Very Low Frequency) into the MEO (Medium Earth Orbit) environment using an 80 meter tip-to-tip dipole antenna biased to kilovolt potentials. The spacecraft-plasma interactions code Nascap-2k is used to model the dynamic plasma sheath during VLF transmission. Simulations have been performed for several cases of plasma density, plasma temperature, VLF frequency, and applied voltage. Because a positive antenna element easily collects electrons from the environment, most of the time one antenna element is near zero potential and the other is negative near the full applied bias. The sheath around the negative element is very large at MEO densities. While a small ion current is collected by the antenna, most of the ions accelerated into the sheath orbit the antenna and leave at high energy when the potential is relaxed. The current flow in the antenna is computed from the surface electric field using a pseudopotential method. This current is used to obtain the antenna impedance (capacitance and phase shift) and its dependence on plasma and operational parameters. A similar pseudopotential approach gives the electron current in the near-field plasma, along with the energy imparted to plasma electrons and their flow along magnetic field lines. Finally, the spectrum of ions to the LEESA (Low Energy ElectroStatic Analyzer) particle detector during VLF transmission is obtained from ion macroparticles striking the surface containing LEESA. These analyses will aid in planning operations and understanding results for WPIx.

        Speaker: david cooke (US Air Force Res Lab)
    • 16:00 17:30
      3.1 Plasma, Ion, and Electron Sources I Seminole D/E (Double Tree at the Entrance to Universal)

      Seminole D/E

      Double Tree at the Entrance to Universal

      Convener: Peng Zhang (Michigan State University)
      • 16:00
        Transients on Arc and Convertor currents in the Multicusp Cesiated Surface Conversion H - Source at LANSCE 15m

        The Multicsup Cesiated Surface Conversion H- Ion Source at the Los Alamos Neutron Science Center (LANSCE) has provided beam at ~14 mA, 120 Hz, and 10% D.F. for many years of neutron science research. Recently, random high current transients were discovered in the arc current used to ionize hydrogen in the LANSCE H- ion source, and in the convertor current used to convert protons to H- ions. Most have no effect, but more severe transients can cripple beam output. Hypothesized causes are related to cesiation effects, plasma potential changes, tungsten filament evaporation/sputtering, or from the arc modulator circuitry. A dedicated study was recently done on the LANSCE H- Ion source test stand to determine the cause of these transients. Current understanding indicates that the more severe transients come from a combination of cesiation effects and plasma potential changes. The status of these current transient studies on the LANSCE H- ion source will be discussed.

        Speaker: David Kleinjan (Los Alamos National Laboratory)
      • 16:15
        Atmospheric Pressure Breakdown and Evidence for Field Emission in GHz Split-Ring Resonators 15m

        Microplasmas generated using microwaves have generated interest in recent years since they have promising qualities such as high-electron densities, breakdown occurring at voltages lower than predicted by Paschen’s law, and the potential for remote excitability. One prominent method for generating microwave microplasmas is to employ a microwave split-ring resonator (SRR). However, while some comparative materials studies have been performed at low to moderate pressure, studies of the effect of different materials and fabrication methods at atmospheric pressure has remained absent. In addition, as field emission is expected to play a larger role as gap size is decreased, it is believed that the study of breakdown voltage vs. gap size for sub-100um may help clarify breakdown mechanisms.
        Here we study plasma generation in silver and gold SRRs with gap sizes ranging from 100um to less than 10um fabricated using screen-printing, fs- and ns- laser ablation, and focused ion beam milling. Minor forays are also made with regard to SRRs with copper oxide nanowires and e-beam evaporated Cu. Breakdown is studied both in dark conditions and when SRRs are illuminated with deep ultraviolet irradiation. In order to characterize breakdown voltage distributions, we utilize Weibull statistics. Significant differences are found in the performance of SRRs fabricated using different techniques – differences occur due to Q-factor as we found in previous studies, but also independent of Q-factor. In addition, we find that there is a relationship between Weibull modulus and breakdown voltage. Differences in Weibull modulus and breakdown voltage are investigated by analyzing SEM micrographs of the SRR structure near the region of plasma generation. The large differences in SRR breakdown performance under dark conditions is attributed to the lack or presence of seed electrons which could result from field emission.

        Speaker: Mr Zane Cohick (The Pennsylvania State University)
      • 16:30
        Experimental, analytical and computational studies of electron gun grid heating 15m

        Electron guns are at the heart of various vacuum tubes used for applications that include communications, RF accelerators, industrial nondestructive inspection and medical imaging. An electron gun is typically comprised of the electron emitter and electrodes/electron gun elements used for beam extraction, control and focusing. A critical component of an electron gun for linear vacuum tubes is the mesh grid, which is typically placed at a certain distance from the emitting surface for beam extraction and control. For microwave tubes, the grid electrode serves the important function of beam pre-bunching. A linear microwave tube benefits from having a pre-bunched beam before entering the microwave circuit by having increased interaction efficiency, considerable reduction in vacuum tube length, and enhanced output power. Modulating the voltage on the grid electrode with a hundred microns of the emitting surface pre-bunches the electron beam and intercepts a fraction of the incident electron beam. Grid deformation, grid heating, secondary emission from the grid, and precise control of grid position and placement during electron gun assembly when designing a mesh grid-based electron gun.

        Understanding and avoiding mesh grid overheating in electron guns is critical since it may critically impact electron beam optics. This work uses experiments, analytic calculations from first principles and 3D finite element (FE) simulations to assess electron gun mesh grid heating. Analytic and FE studies recover the experimental trends; in experiments, we have determined electron beam mesh grid intercepted power that causes grid destruction using a dispenser cathode as the electron emitter and a Mo grid placed 250 microns away. 3D FE simulations accounting for temperature-dependent material properties and the analytic theory predict mesh grid heating in reasonable agreement with experimental results.

        Speaker: Allen Garner (Purdue University)
      • 17:00
        Power Consumption in a Miniature Microwave Inductively Coupled Plasma Source 15m

        Miniature Microwave Inductively Coupled Plasma (MMWICP) source is a novel and versatile non-thermal plasma source, which profit of high electron density and high power efficiency. In its compact version a single MMWICP source comprises a quartz tube of 5 mm inner diameter enclosed by a copper resonator of 8 mm thickness. This basic unit can be combined in an array of two (double), four (Quadriga) or more sources. Here, the single source is characterized by Optical Emission Spectroscopy (OES). A continuous stream of nitrogen gas is running through the glass cylinder at a pressure of 2000 Pa. This specific pressure is chosen to satisfy the Local Field Approximation (LFA), which is used in the latter data analysis. For the OES measurements nitrogen as a test gas is selected for its well-known population kinetics. In particularly, the second positive system of neutral nitrogen (380 nm line) and first positive system of nitrogen molecular ion (391 nm) are monitored, for which the population kinetics can be described by a simple collision radiative model. The OES measuring unit consists of a macro objective, CCD camera and two narrow band-pass filters, which isolate the corresponding emission lines. With previously absolutely calibrated OES unit, the radially resolved absolute line intensities are collected with a 28 micrometer resolution. Simultaneously, an absolutely calibrated high resolution Echelle spectrometer monitors the rotational lines distribution form respective emissions. Using the rate equations of collision-radiative model and BOLSIG+ for solving a Boltzmann equation under the assumption of LFA, it is possible to measure the spatially resolved electron density and electric field. Moreover, the spatially resolved deposited power density is calculated. In the presentation we will discussed the power dissipation in CCP, ICP and hybrid mode of operation. In respect to power efficiency MMWICP will be compared to other microwave plasma sources.

        Speaker: Ilija Stefanović (Ruhr-University Bochum)
      • 17:15
        Test beds for electron emission studies 15m

        For electrodes coated with thin insulating layers, material properties like emissivity, stability, radiation endurance are key parameters for successful application in plasma physics, space industry or accelerator technology. In this paper, we report on test beds used at CEA for basic physics experiments dealing with material properties. Two studies are presented:
        - The first one is dedicated to the characterisation of material under controlled field emission conditions. The experimental set up is presented as well as the results of the characterisation of pure metallic material or insulating coated metallic electrode.

        • The second study deals with Electron Induced Electron Emission (EIEE) from a thin insulating layer deposited on top of a metallic electrode. The main experimental results are reported. They confirm that emission properties depend on the layer thickness, on the bias voltage and on the primary current intensity. All these dependencies can be qualitatively explained by surface charging effects which occur when the metallic back electrode cannot supply a sufficient amount of current to balance out the charge in excess. The experimental set up and results are presented and discussed via two mechanisms: Radiation Induced Conductivity (RIC) and electron injection from the back barrier into the conduction band of the insulator governed by the internal electric field strength.
        Speaker: Michel CARON (CEA)
    • 16:00 17:30
      5.2 & 5.3 Transmission Lines and Transformers and High Energy Density Storage Gold Coast III/IV (Double Tree at the Entrance to Universal)

      Gold Coast III/IV

      Double Tree at the Entrance to Universal

      Conveners: Joel Ennis (NWL), Jose Rossi (National Institute for Space Research)
      • 16:00
        Development of a 1MW pulsed air core electromagnetic toroidal coupler for wireless power transmission with reduced stray emission 15m

        Wireless power transmission (WPT) has seen rapid growth during the last decades. It is a promising technology which has gained worldwide attentions in several applications (smart grid, defense systems…). WPT has proved its capability to be convenient, safe and autonomous. Nevertheless, this technology requires a high power and large area coupler which increases the human exposure to the electromagnetic field. In this context, an air core electromagnetic coupler with reduced stray emission was developed. It is a part of a system which ensures a proper DC / DC conversion between a low-voltage bus (three-phase rectified, fuel cell, battery) and a bus regulated to another voltage value while ensuring the galvanic insulation between both. The transfer of very high power and the galvanic insulation can only be achieved in sinusoidal mode via an air core electromagnetic coupler. So, the innovative proposed architecture confines the magnetic radiation in a torus in order to have an efficient coupling factor (k=0.72) without any use of shielding plate. Our toroidal coupler works mainly in pulsed regimes (1MW) and it is a light solution for several needs. The operating frequency of the compact coupler is 200kHz, the expected average power is 200kW resulting in a satisfactory efficiency of 98%. The effectiveness of the proposed novel system was first investigated by CST 3D numerical modeling then tested with an experimental step-up at low and high level of power. The simulation and the experimental results will also be discussed.

        Speaker: Mrs Fatima Zahra Boudara (Laboratory SIAME)
      • 16:15
        Assessing Effective Medium Theories for Designing Composites for Nonlinear Transmission Lines 15m

        Nonlinear transmission lines (NLTLs) are interest because they can sharpen pulses to produce oscillations from 100 MHz to low GHz once their permittivity and permeability have saturated and the electromagnetic shockwave has formed [1]. NLTLs typical use nonlinear dielectrics such as barium strontium titanate (BST), nonlinear magnetic materials such as nickel zinc ferrites (NZF), or both nonlinear dielectric and magnetic materials to provide these shock waves. An alternative approach involves designing composites comprised of BST and/or NZF inclusions in a host material, analogous to electromagnetic interference designs incorporating inclusions of various shapes in a plastic to tune the composite’s electromagnetic properties [2]. Appropriately designing NLTL composites requires predicting these effective properties and, eventually, the high power microwave systems comprised of them. This study benchmarks various effective medium theories (EMTs) [3] to predict the permittivity and permeability of various composites of BST and/or NZF inclusions in the linear regime (for a fixed voltage and current). We first apply EMTs to predict DC permittivity of BST composites and then to AC measurements of permeability and permittivity for BST, NZF, and BST/NZF composites. We describe preliminary applications of CST Microwave Studios to predict the effective permittivity and permeability and compare to experiment and EMTs.

        This material is based upon work supported by the Office of Naval Research under Grant No. N00014-18-1-2341.

        [1] J.-W. B. Bragg, J. C. Dickens, and A. A. Neuber, “Ferrimagnetic nonlinear transmission lines as high-power microwave sources,” IEEE Trans. Plasma Sci., vol. 41, pp. 232–237, 2013.
        [2] A. L. Garner, G. J. Parker, and D. L. Simone, “A semi-empirical approach for predicting the performance of multiphase composites at microwave frequencies,” IEEE Trans. Dielectr. Electr. Insul., vol. 23, pp. 1126-1134, 2016.
        [3] A. Sihvola, Electromagnetic Mixing Formulas and Applications (IEE Electromagnetic Waves Series, Vol. 47), IEE, London, 1999.

        Speaker: Xiaojun Zhu (Purdue University)
      • 16:30
        Development and Diagnostics on Composites for Nonlinear Transmission Lines 15m

        Nonlinear transmission lines (NLTLs) utilize nonlinear dielectrics to sharpen the input pulse to generate an electromagnetic shockwave for compact, high repetition rate, high power microwave sources. This shock wave generates microwave oscillations that can be propagated as a directed beam toward the intended target. Materials such as barium titanate (BT), barium strontium titanate (BST), and nickel zinc ferrites (NZF) have been used in NLTLs for their nonlinear dielectric or magnetic properties. Alternatively, one may adjust a composite’s effective bulk parameters by adding inclusions of different permittivity, conductivity, or permeability. For instance, adding 2-3% of stainless steel fibers to a plastic makes the composite resemble stainless steel electrically for electromagnetic shielding [1]. Others have evaluated the dielectric properties of composites using nickel zinc ferrites coated in BT in a ceramic base medium for high energy density capacitors. The BT coated ferrite composite has a higher permittivity and lower magnetic loss than a bare ferrite composite [2].
        This study evaluates the permittivity of composites comprised of various volume loadings of BST, the permeability of composites comprised of various volume loadings of NZF, and the permittivity and permeability of materials comprised of inclusions of both materials. Measurement system development for measuring nonlinear permittivity and permeability at microwave frequencies will also be discussed.

        This material is based upon work supported by the Office of Naval Research under Grant No. N000014-18-1-2341.
        [1] A. L. Garner, W. Lafayette, G. J. Parker, and D. L. Simone, “A Semi-Empirical Approach for Predicting the Performance of Multiphase Composites at Microwave Frequencies,” IEEE Trans. Dielectr. Electr. Insul., vol. 23, pp. 1126–1134, 2016.
        [2] R. Curry, A. Pearson, K. Noel, and S. Mounter, “Development of Metamaterial Composites for Compact High Power Microwave Systems and Antennas,” Columbia, MO, 2016.

        Speaker: Andrew Fairbanks (Purdue University)
      • 16:45

        Compact pulsed power systems are often limited by the size and shape of the capacitors required for high voltage energy storage. Marx banks, pulse forming networks, and other devices requiring multiple capacitors are larger than necessary due to the size and shape of the capacitors as well as the geometry of the connection terminals. The size and weight of the capacitor are determined by the energy density of the capacitor dielectric and the dielectric strength of the surrounding insulation. While doorknob-style capacitors are commonly implemented in these devices, the cylindrical shape does not permit efficient packing of multiple capacitors to fill the available volume. Alternative capacitors, such as those based on mica films, have an improved form factor but have end terminations that add inductance in many assemblies. A new effort is addressing these design issues by building the capacitor upon nanodielectric composites. The nanodielectric composites combine high dielectric constant ceramic particles with low dielectric constant, high dielectric strength polymers to produce materials with both high dielectric constant and high dielectric strength. The combination of these two properties enables higher energy densities than possible with conventional ceramics. The composite materials can also be formed or machined into complex shapes to improve capacitor packing density while maintaining low inductance connection points. This effort is focused on development of 40 kV, 2.5 nF capacitors for compact capacitor banks in a vehicle stopping system. In this contribution, the tradeoffs are discussed with the design and simulated performance. The test requirements are described, and preliminary test data is analyzed.

        • This effort was sponsored by the U.S. Government under the DoD Ordnance Technology Consortium (DOTC) Other Transaction Agreement (OTA) (W15QKN-18-9-1008) with the National Armaments Consortium (NAC). The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.
        Speaker: Kevin O'Connor (NanoElectromagnetics LLC)
      • 17:15
        Research on Distribution Problem of Overvoltage Online Monitoring Device on Distribution Lines 15m

        The safety of the power system is related to the magnitude of its overvoltage. Therefore, an overvoltage online monitoring device needs to be installed for monitoring. In order to obtain the lightning overvoltage information of distribution lines and realize effective monitoring of lightning parameters in the area, an overvoltage monitoring device needs to be installed on distribution lines. Considered the wide distribution of distribution lines and many branches, the overvoltage monitoring device cannot achieve full coverage monitoring on distribution lines. Therefore, it is necessary to select lines suitable for installing an monitoring device.
        Combined with the data collected by the lightning positioning system and the lightning trip rate of distribution lines, a method for selecting distribution points in distribution lines of monitoring devices is proposed.
        Calculate the influence of ground lightning density on p-value and m-value of lightning current amplitude of each line, and counts lightning strike failure rate q of each line.And calculate ω1, ω2, ω3 of the three influencing factors by analytic hierarchy process according to the key factors of the actual lines, and calculates the priority c of each line (c = ω1 * p + ω2 * m + ω3 * q), and lines the device installed on are determined by c. The p-value is calculated according to the flash density grid distribution map of the line region, and the total ground density value p of each line is calculated.The m-value is calculated according to the magnitude of lightning current of the line region, and the total magnitude of lightning current value m of each line is calculated. The q-value is to consider the historical lightning strike failure rate, and the lightning strike failure rate of each outlet is counted q. The final p,m, q is then the result of the normalization process.

        Speaker: Xin Feng (Xi'an Jiaotong University)
    • 16:00 17:30
      6.4/6.5 Environmental, Biological, and Medical Applications Seminole C (Double Tree at the Entrance to Universal)

      Seminole C

      Double Tree at the Entrance to Universal

      Convener: Katharina Stapelmann (North Carolina State University)
      • 16:00
        On the VUV optical emission of N-APPJs 30m

        In the past several decades, to understand the fast propagation behavior of nonequilibrium atmospheric pressure plasma jets (N-APPJs), many simulations have been reported. It is believed that photoionization plays key role for such behavior. In all the simulation model, it is either assumed that high energy photons are emitted by N2 in the wavelength range 98–102.5 nm, which induce the ionization of oxygen molecule, or simply replaces the photoionization by a given background electron density. However, no N2 emission from N-APPJs in the wavelength range 98–102.5 nm has been reported by experiment. In this paper, It is found that, for all the working gas, i.e. He, He+0.1%, 0.5% of O2 or N2, Ar, Ar+0.1%, 0.5% O2 or N2, and for the plasma jet driven by either pulsed DC power supply or kHz AC power supply, no N2 emission between 98 – 102.5 nm can be measured. On the other hand, the O II 83.3 nm emission is detectable, which is able to ionize O2, O, and N. Therefore, the widely used photoionization model for N-APPJs need to be revised.

        Speaker: Prof. XinPei Lu (Huazhong University of Science and Technology)
      • 16:30
        Modification of the Hodgkin-Huxley wave behavior by electroporation 15m

        The Hodgkin-Huxley equations [1] have long been used to assess membrane current and its impact on conduction and excitation in nerves for action potential initiation and propagation by modeling the ion channels as parallel combinations of voltage and voltage-dependent conductance that yield a set of nonlinear differential equations. Applying sufficiently intense electric pulses (EPs) create membrane pores that form an additional, parallel, cell membrane potential-dependent shunt conductance that can arrest the action potential [2]. While a self-consistent theory provides the most thorough means of relating the applied EP conditions, cell membrane potential, and resulting cell membrane pore formation [2], it is not readily amenable to assessing the EP induced changes in the wave behavior. This study provides an initial assessment of a simpler approach to specifically examine the EP-induced wave behavior by using a semi-empirical approach to assess EP-induced cell membrane conductivity due to pore formation [3]. We report the impact of various EP conditions on the wave and chaos behavior of the Hodgkin-Huxley equations and potential implications on therapy and nonlethal defense.

        1. A. L. Hodgkin and A. F. Huxley, “A quantitative description of membrane current and its application to conduction and excitation in nerve,” J. Physiol. vol. 117, pp. 500-544, 1952.
        2. R. P. Joshi, A. Mishra, Q. Hu, K. H. Schoenbach, and A. Pakhomov, “Self-consistent analyses for potential conduction block in nerves by an ultrashort high-intensity electric pulse,” Phys. Rev. E, vol. 75, 2017, Art. no. 061906.
        3. B. Mercadal, P. T. Vernier, and A. Ivorra, “Dependence of electroporation detection threshold on call radius: an explanation to observations non compatible with Schwan’s equation model,” J. Membr. Biol., vol. 249, pp. 663-676, 2016.
        Speaker: Amanda Loveless (Purdue University)
      • 17:00
        Downstreaming of valuable compounds from microalgae with spark discharges, instigated by 100-ns high voltage pulses 15m

        Microalgae have become an important resource for blue-green biotechnologies. In addition to their potential for biofuel production, microalgae comprise also valuable metabolites for pharmaceutical and nutritional purposes, such as fatty acids, proteins, carbohydrates, and pigments. However, microalgae are distinguished by a sturdy cell wall, which affords a remarkable mechanical and chemical strength that often translates into inadequate extraction yields. Therefore, conventional extraction methods have shown to be often energy and/or time consuming and consequently are associated with unreasonable economic costs [1]. Accordingly, improvements and alternatives to current extraction methods are needed for successful commercialisation.

        In our previous works [2], [3], we could show that spark discharges, instigated in microalgae suspensions with 100-ns high-voltage pulses, offer a gentle and yet effective extraction method, especially for heat sensitive compounds. When comparing with extraction by burst microwave heating, proteomic analysis revealed commonalities and differences in the protein distribution pattern. Although the yields and number of extracted proteins were similar, notably valuable heat-sensitive proteins, e.g. photosystem-related proteins could be extracted abundantly in comparison to the reference method. Schlieren diagnostics and atomic force microscopy were conducted to elucidate the responsible spark properties for successful cell wall disintegration. Results show that for the generation of sparks by short high-voltage pulses, in particular the energy that is dissipated by shockwaves could easily overcome the stiffness of the microalgae.

        1. Lee, A.K., et al.; Biomass and bioenergy, 2012. 46: p. 89-101.
        2. Zocher, K., et al., Plasma Medicine, 2016. 6(3-4): p. 273-302.
        3. Zocher, K., et al., Algal Research 39, 2019. 101416.
        Speaker: Katja Zocher (Leibniz Institute for Plasma Science and Technology)
      • 17:15

        Underwater electrical wire explosion (UEWE) has attracted the attention of researchers in the field of warm dense matter physics and applied physics in the past several decades. Current pause is a common phenomenon in UEWE, however, the physical process of the re-strike in UEWE is still not very clear till now due to the difficulty of direct diagnostics, so is the formation mechanism of the re-strike shock wave. In this work, a “multilayer structure” of the compressed water flow generated by re-strike in underwater electrical explosion of Cu wire was reported, and corresponding experimental and numerical researches were carried out. It is believed that the partial heating of the re-strike arc initiate a pressure wave that propagates back and forth inside the wire, resulting in the oscillating of pressure on the wire boundary and consequently resulting in an oscillating radial density distribution of the compressed water flow generated by re-strike. This special compressed water flow leads to a “multilayer” structure in the shadow images and schlieren images. The simulation results of a one-dimensional hydrodynamic model supported the above explanations, and indicated that the radius of re-strike arc was about 0.3 times of that of the expanding wire. As the re-strike energy deposition rate increased with the charging voltage, this kind of compressed water flow with oscillated density distribution evolved into one single shock wave finally.

        Speaker: Dr Huantong Shi (Xi'an Jiaotong University)
    • 16:00 17:30
      7.1 Explosive Power Generators Space Coast I-III (Double Tree at the Entrance to Universal)

      Space Coast I-III

      Double Tree at the Entrance to Universal

      Convener: Bucur Novac (Loughborough University)
      • 16:00
        Pulse Compression Considerations for High Current Ranchero Generators 30m

        The Los Alamos Ranchero Flux Compression Generator (FCG) has been tested at the 76 MA level, and a 36 MA experiment was recently conducted demonstrating the functionality of an improved performance design. That test was performed with 3.5 MA initial current, which is the limit available from the capacitor bank at our high explosive pulsed power (HEPP) firing point. Computations show that the improved “Ranchero-S” FCG can generate currents of over 80 MA with 10 nH loads given sufficient initial flux. A program to develop the MK-X generator, which will be a replacement for the MK-IX FCG fielded through the 1980s and 1990s, is on-going at Los Alamos to provide initial flux for Ranchero FCGs and facilitate these very high current experiments. The function time of the improved Ranchero FCG is ~25 µs, and for powering loads requiring short pulses, pulse conditioning is required. In this paper we investigate issues relating to switching the output of a Ranchero-S FCG into meaningful loads given peak currents of 80 MA while desiring load pulses in the 1 µs range for realistic inductance loads. 2D-MHD computations are used to assess design configurations, and significant issues include flux diffusion through any conductor that can hold the 80 MA and rupture rapidly, closing switches that can isolate the load from the diffused flux and then carry the desired current, and dynamic effects seen on vacuum transmission lines operating at such currents.

        Speaker: Mr James Goforth (Los Alamos National Lab)
      • 16:30
        3D Magneto-Hydrodynamic Modelling of an Overstressed Helical Magnetic Flux Compression Generator 15m

        Lawrence Livermore National Laboratory (LLNL) is actively engaged in an experimental program using two-stage magnetic flux compression generators (MFCGs) as pulsed power sources for equation of state measurements. These MFCGs amplify a current pulse in two stages. The first stage uses a helical MFCG and the second stage uses a coaxial MFCG. In support of this program, LLNL recently conducted an overstress test of a megajoule class helical flux compression generator. This experiment was intended to push the generator into a regime where losses would become nonlinear. One of the goals of this experiment was to serve as a benchmark for the suite of computational tools used to predict the performance of these devices. This paper will focus on analysis of this experiment using the LLNL developed magneto-hydrodynamic (MHD) code ALE3D. Analysis of a helical MFCG with an MHD code is particularly challenging because the magnetic field is inherently three-dimensional and does not lend itself well to computational domain reductions using symmetry conditions. This means that large-scale simulations are required to analyse even the simplest helical generators. Regardless of these challenges, this paper will show that ALE3D is capable of predicting the overall behaviour of the generator as well as allowing one to see the source of the nonlinearity in gain. For this particular experiment, the analysis suggests that a significant portion of the loss in compression was due to excessive magnetic pressure on the armature and stator of the generator changing the phasing behaviour of the contact.

        *This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-690731

        Speaker: Anthony Johnson (Lawrence Livermore National Laboratory)
      • 16:45
        A 2-D Numerical Model for the Estimation of the Time Varying Inductance of an Explosively-Driven Helical Flux Compression Generator 15m

        There is a considerable interest worldwide in the development of compact and single-shot expendable pulsed power sources for various applications. Explosively-driven helical magnetic flux compression generators (HFCGs) are natural contenders to provide the high power requirement for such applications. They consist of a hollow cylindrical metal tube called the armature filled with high energy explosives, placed within a helical coil that forms the stator. The explosives when detonated cause the armature to expand leading to the compression of the magnetic flux present within the space between the armature and the stator. This flux is initially set up by the seed current flowing in the circuit using a separate current source. These can be used to achieve very high magnetic fields accompanied with very high currents.
        The present interest in the use of HFCGs brings about the need for a fast computer code for use in the preliminary design stage which can provide results with good accuracy. Using these codes, the performance of the generator can be analyzed before conducting actual experiments, which are difficult to conduct. The electrical equivalent circuit of HFCG can be represented by a first-order series R-L circuit with time-varying elements. During armature expansion, equivalent resistance and inductance of the generator tend to vary with time and geometry. A 2-D numerical model described in this paper presents an approach to evaluate the time-varying inductance of the generator by simulating the chemical explosion occurring within the armature using a commercially available package (AUTODYN) and using these results, a numerical code has been developed to model the time-varying inductance of the generator. The results obtained using the numerical model are compared with the ones available in the literature to validate the code developed. The results will be presented and discussed in detail in the final manuscript.

        Speaker: Ashish Sharma (Indian Institute of Science)
      • 17:00
        Ignition Mechanisms of Polymer Bonded Explosives during Drilling 15m

        The drilling behavior of polymer bonded high explosives (HE) is investigated by varying drilling parameters and analyzing resultant forces and thermal response. A modified drill press enables remote operation and precise control of cutting speed, feed, and depth. To acquire temperature at the cutting interface a K-Type thermocouple is inserted in the coolant holes of thru-coolant drill bits and epoxied flush with the drill’s flank face. The signal is amplified and digitized with an AD8495 Thermocouple Amplifier and MSP430G2553 Microcontroller with a sensing accuracy of ±1°C and a resolution of 0.48°C. Temperate data are relayed real time via blue tooth link to the control computer. Cutting forces and torques are acquired with an ATMI MC3A Force and Torque Sensor up to a sampling speed of 2,000 Hz. The comparison of downward directed forces across cutting operations is indicative of which speed and feed rate combinations limit excessive stressing of the HE, while cutting axis torques give indication in the case of drilling obstructions such as insufficient chip clearance. Drilling conditions in excess of the existing DOE-STD-1212-2012 limitations are tested to determine safe but efficient machining limits for these materials. Drilling speed, feed rate, and peck depth are varied for drilling cycles with a 5 mm diameter drill bit, and further cycles are performed to determine the effect of increasing cut diameter. In peck drilling, clearance of chip from the drill flute is crucial and governs the drill’s temperature rise.

        Speaker: Raimi Clark (Center for Pulsed Power and Power Electronics (P3E))
    • 16:00 17:30
      9.1 Optical, X-ray, FIR, and Microwave Diagnostics and 9.3 Pulsed Power Diagnostics Seminole A/B (Double Tree at the Entrance to Universal)

      Seminole A/B

      Double Tree at the Entrance to Universal

      Convener: Clayton Myers (Sandia National Laboratories)
      • 16:00
        Hyperfine structure and isotopic shift analysis of uranium transitions using LIF of laser-produced plasma 15m

        Optical spectroscopy in conjunction with laser-produced plasma (LPP) is a very promising tool for in-field and non-contact isotopic analysis of solid materials. Both emission and fluorescence spectroscopy of laser ablation plumes can be used for isotopic analysis. However, the reported isotopic shifts of U I and U II transitions in the visible spectral regime are in the range ~ 1-25 pm which necessitate the requirement of an extremely high-resolution spectrograph with a resolution ⪖60000 for using emission based diagnostic tools (eg. laser-induced breakdown spectroscopy) for isotopic analysis. In addition to this, the emission spectral analysis requires thermal excitation by electrons which happen at early times of plasma evolution when the lines are broader due to various line broadening mechanisms (Stark, Doppler etc.). Laser-absorption/ laser-induced fluorescence spectroscopy (LAS/LIFS) can be used to marginalize the effect of instrumental broadening. LAS and LIFS probe the ground state atoms existing in the plasma when it is cooler, which inherently provides narrower lineshapes. We recently reported the linewidths of U transitions using LAS/LIFS of laser-produced plasmas are ~ 1 pm which is significantly lower than the average isotopic shift of U atoms/ions (~ 9 pm). In addition to isotopic splitting, the hyperfine structures (hfs) may influence the lineshape of a transition. Hyperfine splitting’s are usually small; however, in certain cases, they can be larger than isotope splitting. In that scenario, isotope shifts of atoms and molecules can be entangled with hyperfine structure. Here we report the isotopic shifts between U-238 and U-235 transitions and hyperfine structures of U-235 using laser-induced fluorescence (LIF) of laser-ablation plumes. We used a collinear laser geometry for isotopic detection, where both the plasma generation beam and LIF excitation beams propagate near normal to the target, which is a prerequisite for any standoff analytical detection tool.

        Speaker: Dr S. S. Harilal (Pacific Northwest National Laboratory)
      • 16:15
        Propagation process of streamers and time history of reduced electric field during nanosecond pulsed discharge in coaxial electrode in atmospheric air 15m

        Pulsed discharge plasmas which are one of the non-thermal plasma have been actively studied for industrial and environmental applications. The observation of discharge plasma formation is beneficial for better understanding of the plasma physics of this growing field. Generally, a pulsed discharge with time duration of 100s ns is divided into two phases, primary streamer and secondary streamer discharges. The primary streamer discharge has streamer head with the largest electric field among entire discharge phases and thereby produces a variety of radical species with high efficiency. Meanwhile, secondary streamer discharge is capable to produce many radicals by high density electron, however, causes much larger heating loss. Therefore, pulse duration of the applied high-voltage pulse has a strong influence on the energy efficiency of the plasma processes. In the recent study, a nanosecond (ns) pulsed power generator which can generate a pulsed voltage with 5 ns of duration was developed and achieved the higher efficiency on several applications (e.g. ozone generation, NO removal, and water cleaning). However, the fundamental mechanisms of these high efficiencies are not very well studied and understood. Therefore, the present study focused on obtaining the propagation process of streamers and time history of reduced electric field by measuring the intensity ratio of spectral bands of molecular nitrogen during ns pulsed discharge in atmospheric pressure air. In the experiment, the discharge propagation process and time history of reduced electric field in the vicinity of coaxial electrode were observed by using a high-speed gated emICCD camera and time-resolved spectroscopy, respectively. As the result, the reduced electric field of ns primary streamer near the high voltage inner electrode was estimated more than 1000 Td. This can be explained from the effect of significantly fast pulse rise rate of ns pulse voltage with exceeding 10 kV/ns.

        Speaker: Terumasa Ryu (kumamoto university)
      • 16:30
        Importance of RF Measurements in Pulsed-Plasma Applications 15m

        The capability to measure RF power in plasma systems has existed for many years but key parameters, such as impedance of the plasma and delivered power, can be difficult to measure with high accuracy. This is especially true in pulsed RF systems, where it is difficult for the system to achieve a conjugate match from the rapidly changing plasma impedance to the RF source, resulting in reflected power. Actively monitoring the power and shape of the pulses is critical to develop and maintain consistent and repeatable processes. Changes in these measurements can indicate problems like equipment wear, drift, and instability and serve as a great starting point for process improvement. In this work, RF pulsing is studied in different pulsed-plasma applications and it is then demonstrated how directing attention to and gaining understanding of the RF measurements can assist in improving the processes.

        Speaker: Mr Stephen Heagy (Bird)
      • 16:45
        Influencing Factors and Error Analysis of Pulse Current Measurement With Air-core Rogowski Coil 15m

        The error of an air-core Rogowski coil caused by eccentricity or tilting of the conductor flowing through the measured current and interference magnetic field outside the area is analyzed under uniform and uneven coiling conditions. The error which is a function of the density of the windings along the bobbin is given. In the aspect of extraction of distribution parameters, a method combining finite element simulation modeling and data fitting is proposed, which solves the problem of extracting the inter-turn capacitance of a Rogowski coil. Under high frequency conditions, the influence of the current of gap on shielding container on the measurement results is discussed. The function of the error caused by the current of shielding container is derived, and factors that affect the error are given.

        Speaker: Yao Xu (Tsinghua University)
      • 17:00
        A Multi-Material Velocimetry Detector for Pulsed Power Flow Studies 15m

        Pulsed power experiments at Sandia National Laboratories' Z Pulsed Power Facility have traditionally utilized single material velocimetry flyers for diagnosing magnetic pressure, and hence current, within the magnetically insulated transmission lines (MITLs). More recent experiments at Sandia suggest that energy absorption through various mechanisms, such as charged particle loss, can contribute to the measured motion of flyers as well. In order to further test this hypothesis, we have fielded flyers that are both single material, such as aluminum, and multi-material flyers, such as a combined substrate of gold and aluminum, to detect particle energy absorption on the MITL. Since each flyer material has a unique charged particle stopping power, as well as a unique sound speed, then the simultaneous velocimetry response of two different detectors can provide information about the types of charged particles depositing their energy, as well as their time-dependent energy deposition. Our presentation explains the physics of these detectors, and shows the experimental measurements from these detectors.

        Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

        Speaker: Mark Hess (Sandia National Laboratories)
      • 17:15
        Plasma Kinetics Study of a Repetitive 10-ns Pulsed Plasma Ignition for Combustion 15m

        Combustion efficiency and rate of ignition were shown to be improved when fuel-air ignition was initiated with highly non-equilibrium plasmas generated by high-voltage, nanosecond pulses, also known as transient plasma ignition (TPI). In order to optimize the pulse power parameters for plasma ignition for combustion, detailed experimental investigations of the effect of rise time and pulse repetition frequency (PRF) were conducted for atmospheric pressure static methane/air ignitions. Plasmas driven by 10 ns, 12 kV pulses at a range of PRF from 1 kHz to 10 kHz were generated for combustion ignition with a pin-to-plate electrode configuration. Experiments revealed that a different mode in the plasma was initiated when the fuel/air mixture was ignited. At constant pulse duration and PRF, this plasma mode change occurred earlier for the faster rise time (e.g. 4 ns) compared to (e.g. 8 ns) [1]. In addition, faster PRF favored the earlier plasma mode change or earlier ignition. In this study, the kinetics of reactive plasma species that generated during the transient plasma ignition and combustion were investigated using optical emission spectroscopy (OES). Gated and filtered imaging in combination with electrical diagnostic techniques are to help understand the plasma chemistry related to combustion that is initiated with different pulse rise times and PRFs at a constant pulse width of 10 ns. Gas temperature of the PRF plasma ignition for combustion is discussed by measuring the rotational temperature of the second positive systems of nitrogen N2 (C-B).

        *The work was supported in part by the Department of Energy (Phase I STTR, DE-SC001788) and the Air Force Office of Scientific Research (Award No. FA9550-17-1-0257).

        [1] D. Alderman, C. Tremble, J. Sanders, D. Singleton, and C. Jiang, "Initial Evaluation of Pulse Risetime on Transient Plasma Ignition for Combustion," in IEEE International Power Modulator and High Voltage Conference, 2018.

        Speaker: Mr David Alderman (Old Dominion University)
    • 18:00 19:30
      WIE Reception 1h 30m
    • 19:30 21:00
      Plasma Decadal Town Hall Meeting 1h 30m
    • 07:30 08:15
      Continental Breakfast 45m Universal Center ()

      Universal Center

    • 08:00 08:15
      Announcements 15m
    • 08:15 09:15
      Plenary Tues AM - John Verboncoeur (2019 Plasma Science and Applications Award) Seminole Ballroom ()

      Seminole Ballroom

      Convener: Joseph Schumer (Naval Research Laboratory)
      • 08:15

        Multipactor and its transition to gaseous ionization breakdown remain one of the most significant limitations in RF device operation, particularly at high power. Nonlinear effects can couple multiple carrier frequencies, cause instabilities and dispersion, and result in temporary failure as well as permanent damage. These phenomena are relevant to conducting and dielectric surfaces, in devices ranging from communications to high power microwave sources, to accelerators and even high gradient microwave circuits and devices.

        In this ongoing work, carried out over the past decade, we examine the process of initial multipactor growth, surface heating and gas desorption, and subsequent evolution to ionization breakdown. We look at a variety of mitigation schemes, from spatio-temporal signal modulation to surface morphology and materials properties.

        This work is part of a larger effort which includes development of standardized platforms in planar, coaxial, and stripline configurations, with both computational and experimental analogs to enable validation and develop analytic and predictive capability integrated with well-tested experiments. The test cells enable study of multipactor susceptibility and transition to ionization breakdown, as well as novel material, geometric, and electrical mitigation schemes in isolation or as a system. Test cell designs allow variations in gap spacing, driving frequency, waveform shape, ambient and desorbed gas, surface morphology, and many other key parameters. The cells will motivate integration and development of novel diagnostics, such as direct multipactor electron detection, optical/VUV emission spectroscopy, and X-ray imaging, at ns timescales and sub-mm- spatial scales.

        The test cells and corresponding models will be published in detail and made available to the community as standard reference platforms on which repeatable basic physics results can be studied, validated, benchmarked, and openly published.

        *This work was supported by AFOSR MURI Grant No. FA9550-18-1-0062. The contributions of the entire MURI team are gratefully acknowledged: Michigan State University (Y. Fu, A. Isqbal, D.-Q. Wen, P. Wong, P. Zhang), Texas Tech University (J.C. Dickens, R.P. Joshi, A.A. Neuber, J. Mankowski), University of Michigan (R.M. Gilgenbach, N.M. Jordan, Y.Y. Lau), University of New Mexico (M. Gilmore, S. Portillo, E. Schamiloglu), and University of Wisconsin (N. Behdad, J.H. Booske2, D. Morgan), as well as contributions by C. Chang and R. Temkin.

        Speaker: John Verboncoeur (Michigan State University)
    • 09:15 09:45
      AM Break 30m
    • 09:45 11:45
      1.1 Basic Phenomena I Seminole A/B (Double Tree at the Entrance to Universal Orlando)

      Seminole A/B

      Double Tree at the Entrance to Universal Orlando

      Convener: L. K. Ang (SUTD)
      • 09:45
        Hot electron emission processes in waveguide integrated graphene 30m

        Photoemission plays a central role in a wide range of areas, from electronic structure measurements to free electron laser sources. In metallic emitters, photons with energy lower than the material workfunction can only drive photoemission through the multi-photon, or strong-field processes, both of which require large optical power densities, limiting the integration and deployment of these photoemitters despite their favorable properties. Here, we demonstrate a graphene emitter that is excited via a waveguide with 3.06 eV photons from a continuous wave (CW) laser exhibits two hot-electron processes that drive photoemission at peak powers orders of magnitude lower than previously reported multi-photon and strong-field metallic photoemitters. We use optical power dependent experiments combined with modeling to suggest that the observed behavior can be explained by considering two hot-electron emission processes: (i) non-equilibrium electron heating, and (ii) direct emission of excited electrons. These processes are dramatically enhanced in graphene due to the relatively weak electron-phonon coupling and the single layer structure.
        By observing the optical power dependence as a function of electric field, we show that there is a cross-over from non-linear at low electric fields to linear at high electric fields. This crossover in optical power dependence is also reproduced in our model only when we consider both electron heating and direct emission of excited electrons. When LaB6 nanoparticles are used, the power dependence is linear regardless of the electric field, due to the low workfunction of the LaB6. Additionally, the integrated photonics approach demonstrates an efficiency three orders of magnitude greater than free space excitation. These results suggest the approach of integrated photonics combined with materials exhibiting low electron-phonon coupling and thin structures, such as 2-D materials or quantum dots, could provide a rich new area for electron emitters and integrated photonic devices.

        Speaker: Rehan Kapadia (University of Southern California)
      • 10:15
        Electron Emission and Gas Breakdown: Unification of Theory from Schrodinger’s Equation to Paschen’s Law 15m

        The continued miniaturization of electronic devices for applications including micro- and nano-electromechanical systems (MEMS and NEMS, respectively) and microplasmas requires a thorough understanding of electron emission behavior at these length scales for various pressures. Paschen’s law (PL) governs classical gas breakdown, but fails for microscale gaps, where field emission (FE) drives breakdown. Reducing the gap size below microscale further necessitates understanding the transitions from FE-driven breakdown to space-charge limited emission (SCLE) with Mott-Gurney (MG) at pressure, Child-Langmuir (CL) at vacuum, and quantum behavior at nanoscale. While piecewise connections of these breakdown and emission mechanisms have been studied, a complete analysis connecting all mechanisms remains incomplete.
        This study aims to fill this gap by nondimensionalizing the governing equations with a consistent set of scaling parameters to obtain a single, material-dependent parameter retained in PL. Thus, this study provides a universal (material-independent) set of equations characterizing electron emission behavior from quantum scales up to, but not including, the traditional PL. The universality of the dimensionless equations highlights the underlying physics driving each mechanism and demonstrates the respective regions of importance. These dimensionless equations can predict the specific emission characteristics for a given set of experimental parameters and provide insight into device design.

        This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-18-1-0218 and the by the Office of Naval Research under Grant No. N00014-17-1-2702. A. M. D. gratefully acknowledges funding from a Purdue Doctoral Fellowship. A. M. L. gratefully acknowledges funding from a graduate scholarship from the Directed Energy Professional Society and a fellowship from the Purdue Research Foundation.

        Speaker: Amanda Loveless (Purdue University)
      • 10:30
        Microscale Gas breakdown voltage dependence on electrode surface 15m

        Surface structure, such as surface roughness and electrode geometry, can modify field enhancement and work function, which subsequently modifies field emission. This can significantly change gas breakdown voltage for microscale gaps at atmospheric pressure [1], where field emission drives breakdown rather than Paschen’s law. This presentation uses a tungsten pin anode and a copper plate cathode polished to three different surface roughnesses to characterize the impact of surface roughness and subsequent cathode damage on breakdown voltage for interelectrode gaps of 1, 5, and 10 microns at atmospheric pressure. Changing the surface roughness did not cause a statistically significant change in breakdown voltage; however, it played a critical role in breakdown voltage and cathode conditions for repeated breakdown events. After treatment, the cathode contained small craters from 3 to 50 microns deep. The breakdown voltage for these subsequent breakdown events agrees with theoretical predictions for an effective gap distance equal to the sum of the interelectrode gap distance and the crater depth. These effective gap distances are sufficient to exceed the Meek criterion for streamer discharge, indicating potential breakdown mechanism transition for a single interelectrode gap distance. The implications of the impacts of electrode surface structure on breakdown before and after multiple breakdown events on microdevice operation will be discussed.
        [1] S. Dyanko, A. M. Loveless, and A. L. Garner, “Sensitivity of modeled microscale gas breakdown voltage due to parametric variation,” Phys. Plasmas, vol. 25, 2018, Art. no. 103505.
        This material is based upon work supported by the Office of Naval Research under Grant No. N00014-17-1-2702. A. M. L. gratefully acknowledges funding from a graduate scholarship from the Directed Energy Professional Society and a fellowship from the Purdue Research Foundation.

        Speaker: Russell Brayfield (Purdue University)
      • 10:45
        Quantum effects in electron emission from nanodiamond 15m

        Synthetic granular ultra-nano-crystalline diamond with high n-type conductivity via nitrogen doping ((N)UNCD) has emerged as a high efficiency photo- or field-emission source in that it has high internal and external quantum efficiency in near UV/visible or low turn-on field, respectively. It is widely anticipated that graphitic grain boundaries (surrounding diamond grains) are behind the high efficiency of (N)UNCD. Grain boundary effect hypotheses rest upon the fact that (N)UNCD emission efficiency can be largely “tuned” through the diamond-to-graphite ratio typically quantified by optical and x-ray spectroscopy techniques.
        In addition to high efficiency, we have experimentally observed that (N)UNCD demonstrates some unconventional behaviors that attend electron emission. These are 1) output current saturation [1,2] and 2) light emission [3] during field emission, and 3) intrinsic emittance/mean transverse electron energy independent of the excess photon energy during photoemission [4]. In this talk, we will summarize our recent experimental and theoretical work that further corroborate the critical role of defect grain boundary states on electron emission from (N)UNCD. Specifically, we will outline the role of
        1) Density of grain boundary states induced inside diamond fundamental band gap and electron transport properties on the Fowler-Nordheim and saturation regimes of field emission;
        2) Electron effective mass on light generation and spectrum during field emission;
        3) Ground state photoemission from spatially confined grain boundaries on transverse photoelectron momentum (intrinsic emittance).
        We will also outline how the models implying quantum effects associated with graphitic grain-boundary-promoted electron emission can simultaneously account for high efficiency and unconventional behavior of (N)UNCD in a non-contradictory way.

        [1] ACS Appl. Mater. Interfaces 9, 33229 (2017)
        [2] arXiv:1812.05726 (2018)
        [3] arXiv:1811.04186 (2018)
        [4] arXiv:1812.00323 (2018), Appl. Phys. Lett., accepted (2019)

        Speaker: Prof. Sergey Baryshev (Department of Electrical and Computer Engineering, Michigan State University)
      • 11:00
        Particle Emission Investigation from an Anode Liquid Surface of Electrolyte in Atmospheric Pressure DC Glow 15m

        Self-organization patterns observed on anode liquid surfaces in atmospheric pressure DC glow discharge represents both a mysterious and beautiful plasma physics phenomenon. The mechanisms underlying self-organization of plasmas in this context is still poorly understood. In this study, luminous particle emission from the liquid anode under self-organization condition has been observed. These particles have been collected in flight using witness plates. The particle impacts have the form of splats suggesting that they are molten. The splats were observed to have a great deal of structure including evidence of nano-precipitation. These resulting splats were examined using a scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDX) diagnostics. In particular, the size range of molten particle droplets was theoretical estimated by converting from the size of impact splats. Furthermore, high-speed camera analysis was used to map the 2D trajectories of these particles in order to analyze both the emission force and the drag experienced by the particle during flight. This yields insight into mechanisms of emission. A thermal effect such as localized heating and evaporation is one potential mechanism driving the emission of particles that may be formed in the liquid. Here we examine the local temperature of the liquid water at the emission zone just below the surface. The actually temperature of this region is not well known and thus provides insight not only into the mechanisms of emission but also potentially the underlying processes driving the self-organization formation itself.

        Speaker: Yao Kovach (University of Michigan)
      • 11:15

        Contact resistance and current transport are important to nano scale electrical contacts, such as those based on metal-insulator-metal (MIM) thin junctions, carbon nanotube (CNT) networks, and novel two-dimensional (2D) materials. Current tunneling and contact resistance across such contacts greatly influence the device properties and performance. Current crowding effects in these contacts can lead to localized overheating and the formation of thermal hot spots. To improve reliability and lifetime of the device, it is crucial to engineer these contact structures to mitigate the local heating. In this study, we propose a method to design nanoscale electrical contacts with controlled current distribution and contact resistance via engineered spatially varying contact layer properties and geometry. A lumped circuit transmission line model (TLM) [1] is used to get self-consistent analysis of contact resistivity, current and voltage distribution across tunneling contacts formed between similar/dissimilar contacting members separated by a thin insulating gap with varying thickness. It is found the nonhomogeneous current and voltage distribution in parallel tunneling contacts can be reduced by varying the specific contact resistivity along the contact length. This specific contact resistivity is either predefined (for ohmic contacts), or calculated from the local tunneling current in case of insulating tunneling layer [2].

        1. S. Banerjee, J. Luginsland, and P. Zhang, “Two dimensional tunneling resistance transmission line model for nanoscale parallel electrical contacts”, under review, 2019.
        2. P. Zhang, "Scaling for quantum tunneling current in nano- and subnano-scale plasmonic junctions", Sci. Rep., 5, 9826 (2015).

        This work was supported by AFOSR YIP Award No. FA9550-18-1-0061.

        Speaker: Sneha Banerjee (Michigan State University)
      • 11:30
        Theoretical investigation of the magnetic asymmetry effect by using a lumped element model 15m

        Latest experimental results have proven the existence of the magnetic asymmetry effect (MAE) in radio frequency capacitively coupled magnetron discharges [1]. Like the electrical asymmetry effect, the MAE allows to control the symmetry of a capacitive discharge. This contribution presents a lumped element model for such an RF driven magnetron which is an extension of a previously published lumped circuit description of unmagnetized RF discharges [2]. As its predecessor, the model represents the discharge by separate bulk and sheath zones which communicate via Kirchhoff relations. The extension accounts for the presence of a magnetized region with reduced electric conductivity [3,4]. The model provides short computation time and may be used for the purpose of model based control.
        Similar to the experiment [1] a nearly geometric symmetric discharge is set up with the model. Due to a variation of the applied magnetic field strength, the sheath and bias voltage become significantly affected. The theoretical model and the experimental results show very good qualitative agreement. The main goal of this work is to theoretically investigate the influence of the MAE to capacitively coupled discharges and the non-linear electron resonance heating.

        [1] M Oberberg et al., PSST 27, 105018 (2018)
        [2] T. Mussenbrock et al., PSST 16, 377385 (2007)
        [3] D. Bohm, The characteristics of Electrical Discharges in magnetic fields (1949)
        [4] S.M. Rossnagel, H.R. Kaufman, Journal of Vacuum Science and Technology A 5, 88 (1987)

        Speaker: Mr Dennis Engel (Ruhr University Bochum)
    • 09:45 11:45
      2.6 Non-Fusion Microwave Systems and 2.7 Microwave Plasma Interaction I Seminole D/E (Double Tree at the Entrance to Universal Orlando)

      Seminole D/E

      Double Tree at the Entrance to Universal Orlando

      This is a combined session

      Conveners: Sarita Prasad (UNM), John Leopold (Technion)
      • 09:45
        Practical Tunable Electrically Small Antenna Design for Transportable Ionospheric Heating 15m

        A tunable electrically small antenna (ESA) designed for transportable ionospheric heating (TIH) is presented as a practical implementation of a previously presented, basic design. Traditionally, Ionospheric Heating is achieved using large multi-element arrays of dipoles or similar antennas. One such array, the HAARP IRI, occupies 0.14 km2 in Gakona, Alaska with 180 crossed dipole elements. The presented research is aimed at eventually reducing the footprint of the array such that it becomes transportable enabling the study of ionospheric effects at any accessible location on the globe, including previously unstudied equatorial regions where the Earth’s magnetic field is nearly parallel to the ground. Utilizing a vertically oriented capacitive gap in a capacitively loaded loop (CLL) coupled to a small loop antenna (SLA) driven element, the design covers a range of frequencies suitable for TIH – 3 to 10 MHz (30-100 MHz in 1/10th scale) – with greater than 80% efficiency across the range, whilst being a fraction of the size of an equivalent dipole. A natural match to a 50 Ω source negates the need for any lossy matching networks and allows for a more compact and efficient transmitter system. The CLL, comprised of two quarter-cylinder shells and two capacitor plates, are hinged to allow for tuning of the CLL capacitance thereby tuning the resonant frequency of the antenna. Additionally, tuning of the coupling between SLA and CLL is added to maintain the good port matching and gain with decreasing frequency throughout the tuning range. An electro-mechanical tuning system implemented on the prototype antenna with stepper motor driven components enables continuous angle tuning of 0-16° included and 90-14° with respect to the ground plane for CLL and SLA, respectively. A gain of approximately 5 dBi is observed in simulation with a similar measured result.

        Speaker: Mr Benedikt Esser (Texas Tech University)
      • 10:00

        The behavior of the breakdown in air at RF frequencies for different gap lengths has been studied numerically at atmospheric pressure. The focus here is on gap lengths in the 1–5 mm range. A numerical analysis based on Monte Carlo calculations as discussed previously [1] is applied, with explicit inclusion of photon processes. The simulation results are compared with experimental data obtained within our group on RF breakdown in air at atmospheric pressure. The inclusion of photon-assisted charge growth through the photoemission process, is shown to serve as a delayed but continuous sources of electrons. This works to effectively lower the breakdown threshold, especially in geometries consisting of large area electrodes separated by short gaps, much smaller than the electrode areal dimensions.

        The simulated predictions match the breakdown data quite well for the tested gap lengths. In addition, frequency-dependent breakdown fields are also obtained through the Monte Carlo calculations. A general U-shaped characteristic with frequency results. These trends as well as other features of the RF breakdown, with the important role of photons, will be presented and discussed.

        [1]. H. Nguyen, J. Mankowski, J. C. Dickens, A. A. Neuber, and R. P. Joshi, "Model Predictions for Atmospheric Air Breakdown by Radio-Frequency Excitation in Large Gaps," Physics of Plasmas, vol. 24, 073505 (2017).

        • This work was supported by an AFOSR Grant # FA9550-18-1-0062 on ″Multipactor and Breakdown Susceptibility and Mitigation in Space-Based RF Systems″, and by Grant #A19-0103-001.
        Speaker: Benedikt Esser (Texas Tech University)
      • 10:30

        A high average power, pulsed RF source operating at 2.85 GHz is described for use in multipactor research. Four state of the art GaN HEMTs from Cree/Wolfspeed capable of 700 W in long pulse mode (~ 100 μs) each are combined for a total maximum power output of 2.8 kW with a rated output of 2 kW total. A low phase noise, free running 55 MHz modulation capable VCO is used as the RF source, buffered and amplified by a LNA. Generation of a 100 μs drive pulse is accomplished via a TTL switch with a rise time of 25 ns and a typical switching time of 35 ns driven by a pulse/delay generator. The pulse is further amplified by a microwave amplifier providing the majority of gain in the system, 45 dB, and the 6 W required for the final stage. A CGHV31500F GaN HEMT amplifier is used to amplify the signal to the required 50.5 dBm (~112 W) with split feeding of the final output stage consisting of four additional parallel connected GaN HEMTs. Each of the four has 12.5 dB of gain, providing 500 W for a combined output of 2 kW. An in-line attenuator provides the operator output power control in the range of approximately 42 to 64 dBm (16 to 2,800 W). A custom control PCB was designed to control, bias, and sequence the various parts of the source. Custom power splitters and directional couplers capable of high power were designed for the system. The amplifier enables multi-carrier mode amplification as well as modulated output amplitudes. A ring resonator is used to study the multipactor phenomenon in a WR284 waveguide section, increasing the effective power to approximately 40 kW, with a high impedance section where the phenomenon is observed within the tapered impedance transformer.

        Speaker: Benedikt Esser (Texas Tech University)
      • 10:45

        Multipactor is known to jeopardize the performance of vacuum electronic devices and high-power microwave systems. Though most treatments have focused on the secondary emission from incident primary electrons, the overall process is quite complicated. It involves not just the role of primary electrons, but also the effects of incident ions that might produce secondary electrons and/or cause heating of the surface layer due to the inelastic collisions as they enter the target electrode. Here we assess the secondary electron yield (SEY) as a function of the incident energy and angle of primary electrons. In addition, the potential for secondary electron emission by incident ions is also probed. Monte Carlo simulations are used along with the Furman-Pivi [1] formulation for electron-initiated SEY. Helium is used as a simple example ion. Finally, temperature increases produced at and near the electrode surface due to ion impact is also modeled based on Molecular Dynamic simulations. The values form the basis for temperature driven out-gassing from the surface, a process that will also be treated with discussion of results.

        [1]. M. A. Furman and M. T. F. Pivi, “Probabilistic model for the simulation of secondary electron emission,” Phys. Rev. Accel. Beams, vol. 5, no. 12, p. 124404, 2002.

        • This work was supported by an AFOSR Grant # FA9550-18-1-0062 on ″Multipactor and Breakdown Susceptibility and Mitigation in Space-Based RF Systems″, and the Office of Naval Research (N00014-18-1-2382).

        Authors: Hieu K. A. Nguyen, Xiaoli Qiu, Joy Acharjee, John Mankowski, James C. Dickens, Andreas A. Neuber and Ravi P. Joshi

        Speaker: Mr Hieu Nguyen (Texas Tech University)
      • 11:00
        Design and Implementation of an Ultra-wideband Multipactor Test Cell 15m

        Multipactor is cascade avalanche growth of free electrons in RF components under vacuum. In two-surface multipactor, a free electron gets accelerated by electric field and impacts a facing surface with enough energy to liberate secondary electrons. Cascade evolution of these electrons under certain conditions can load and short circuit the RF power.
        We will discuss the design and implementation of a test cell to study multipactor susceptibility and suppression in two-surface RF components. The cell was designed with a gradual transition from a coaxial line to a planar microstrip line. This allows for multipactor research over a wideband frequency range from DC to 1.5 GHz. A centered transmission line was designed, minimizing the disturbance of the fields and resulting in a low reflection of less than -30 dB over the entire frequency range. While the ending coaxial lines are fixed, a planar multipactor center section is replaceable and can stand above the ground plane with an adjustable gap distance. This platform facilitates the multipactor test with a wide range of gap dimensions, frequencies, surface treatments, and geometrical modifications.
        The above-mentioned transmission line is situated in an ultra-high-vacuum chamber that simulates the low-pressure environment in space-based applications. A chain of vacuum pumps including scroll pump, turbo, and ion pumps bring the chamber pressure down to 10$\times $e-8 Torr. The vacuum chamber has multiple arms which makes it possible to seed and collect electrons concurrently. Electron seeding is carried out using the photoelectric process. An ultraviolet LED creates UV light with a wavelength of 265 nm whose photon has enough energy to free electrons out of copper. The LED’s light is transferred into the vacuum chamber through fiber patch cords and focused onto the multipactor section.
        Work supported by Air Force Office of Scientific Research MURI grant FA9550-18-1-0062.

        Speaker: Dr Mirhamed Mirmozafari (University of Wisconsin-Madison)
      • 11:15
        Map-Based Multipactor Theory for Two-Carrier Operation 30m

        Multipactor is a vacuum discharge based on secondary electron emission that plagues microwaves devices, accelerator structures, and space-borne systems [1]. A novel theory based on principles from nonlinear dynamics and chaos theory has been introduced, in which all possible modes are recovered with no a priori assumptions on the electron trajectories [2-3]. The new methodology systematically applies iterative maps to identify multipacting boundaries more reliably and comprehensively than existing models. It does so by globally analyzing the structure of dynamical space, resulting in bifurcation diagrams that predict susceptibility to multipactor over a wide range of parameters.

        This model is generalized to multi-carrier operation, as found in modern space-communication devices [4]. These systems, where several radio-frequency (RF) carriers coexist, are especially challenging to analyze with conventional methods. As a result, little theoretical study on this area has been done [5-6]. Since the map-based theory rapidly scans vast areas of parameter space with no a priori assumptions, it is ideally suited to analyze this problem. This is illustrated for the lowest-order system, namely two RF carriers. Validation is conducted by scanning, in both the theory and the simulation, parameters such as the field strengths and geometry dimensions. The resulting multipactor growth rates are then compared, validating the accuracy of theoretical predictions. The effects of the second RF carrier on multipactor suppression are presented.

        [1] J. R. M. Vaughan, IEEE Trans. Electron Devices 35, 1172 (1988).
        [2] R.A. Kishek, Phys. Plasmas 20, 056702 (2013).
        [3] M. Siddiqi and R.A. Kishek, Proc. MULCOPIM 2017.
        [4] T.P. Graves, “Standard/Handbook for Multipactor Breakdown Prevention in Spacecraft
        Components”, Aerospace Report No. TOR-2014-02198 (2014).
        [5] S. Anza, M. Mattes, C. Vicente, J. Gil, D. Raboso, V.E. Boria, and B. Gimeno, Phys. Plasmas
        18 032105 (2011).
        [6] A. Iqbal, J. Verboncoeur, and P. Zhang, Phys. Plasmas 25, 043501 (2018).

        Speaker: Mr Moiz Siddiqi (University of Maryland, College Park )
    • 09:45 11:45
      4.2 Particle Acceleration with Laser and Beams Gold Coast I/II (Double Tree at the Entrance to Universal Orlando)

      Gold Coast I/II

      Double Tree at the Entrance to Universal Orlando

      Convener: Jens Osterhoff (DESY)
      • 09:45
        Progress of beam driven plasma acceleration at FLASHForward 30m

        The FLASHForward experiment at DESY is a beam line build for beam-driven plasma-wakefield acceleration. The drive beams, supplied by the linac of the free-electron laser FLASH, have energies of up to 1.25GeV, a typical charge of 400 pC, emittance of a few mm mrad and a pulse duration down to 50fs. The extensive experimental programme consists of various experiments among which three are considered core experiments, focussing on internally (X-1) and externally (X-2) injected witness bunch acceleration as well as studies on high repetition rate and high average power operation (X-3). The latter is facilitated by the unique capability of extracting multiple electron bunches from the superconducting linac at a variable temporal spacing down to 333 ns. Here the present status of the facility and its capabilities as well as the status of current and future experiments will be detailed.

        Speaker: Dr Lucas Schaper (DESY)
      • 10:15
        Time-dependent behavior of capillary discharge devices for plasma-wakefield acceleration 15m

        The future (and potential limitations) of compact particle accelerator technology depends on the ability to characterize and manipulate the conditions in plasma discharge devices, such as plasma targets and active plasma lenses (APLs).

        For many high energy physics applications and novel radiation sources, high repetition rates are required. For example, the FLASHForward [1] experiment at DESY aims to use beam-driven plasma-wakefield acceleration (PWFA) to produce GeV electron beams of sufficient quality to allow for free-electron laser gain, and plans to investigate the efficacy of PWFA at repetition rates up to the MHz level. The plasma-forming discharge causes an increase in temperature and pressure and an expansion of the plasma, and the time required for the plasma conditions to relax to a state that does not affect the formation of subsequent wakefields places a limit on the repetition rate [2].

        APLs are gas-filled capillary discharge devices that can provide strong radially-symmetric focusing fields in an extremely compact size. Within the capillary an electron temperature profile develops via competition between the current heating and heat lost to capillary wall. A non-uniform radial temperature profile results in a non-linear radial magnetic field profile contributing to emittance growth, and the temporal development of this phenomena is critical to the operation of aberration-free APLs [3,4].

        In this study, we investigate the heating and subsequent cooling phases of plasma capillaries after the initiation of a current discharge, to comment on the operation of high-repetition rate PWFA and aberration-free APLs.

        [1] A. Aschikin et al., Nucl. Instr. Meth. Phys. Res. A, 806, 175 (2016)
        [2] A. J. Gonsalves et al., J. Appl. Phys., 119, 033302 (2016)
        [3] C. A. Lindstrøm et al., Phys. Rev. Lett., 121, 194801 (2018)
        [4] J. van Tilborg et al., Phys. Rev. Accel. Beams, 20, 032803 (2017)

        Speaker: Gregory Boyle (DESY)
      • 10:30
        Proton Driven Plasma Wakefield Acceleration: AWAKE at CERN - Concept, Experiment and Latest Results 15m

        The Advanced Wakefiled Experiment (AWAKE) at CERN is a proof-of-principle experiment for the concept of a proton-driven plasma-wakefield accelerator. In AWAKE, electrons are externally injected in wakefields driven by the 400$\,$GeV proton beam of the Super Proton Synchrotron (SPS) and accelerated over 10 meters of plasma up to energies in the GeV range.
        We have shown that in plasma the initially long proton bunch is subject to the Seeded Self-Modulation (SSM) process. By self-modulation of the beam density, the proton bunch is transformed into a train of micro-bunches, which resonantly drives wakefields. Low energy electrons ($\sim$ 18$\,$MeV) externally injected into the wakefields reached energies up to 2$\,$GeV.
        The physics of SSM, the experiments and a sample of experimental results obtained so far will be presented.
        Furthermore, we show first results on the appearance of the Hosing Instability (HI), another transverse beam-plasma instability with a growth rate similar to that of the Self-Modulation Instability (SMI). The HI is caused by a small displacement of the proton bunch density distribution with respect to the bunch propagation axis. When the Self-Modulation process is not seeded, the hosing grows starting from noise and can overcome the SMI, leading to a break-up of the micro-bunch structure. In a plasma wakefield accelerator, this would drastically reduce the useful acceleration length. Even though, the HI is not a limitation for AWAKE as it mainly appears at lower plasma densities than the one optimum for acceleration, a better understanding of the physical processes is needed.

        Speaker: Mr Mathias Hüther (Max-Planck-Institut fur Physik (DE))
      • 11:00
        Study of the effects of laser pulse intensity modulations on the plasma oscillations and electron energy gain in the bubble regime. 15m

        The study was evaluated on a hydrogen Z-pinch plasma guiding channel generated by a fast discharge in a capillary with diameter of 3 mm and length of 50 mm. By using the 1D MHD model a radial distribution of the electron density profile was computed which capable for guiding the high intensity TEM$_{00}$ mode CO$_2$ laser pulse with input spot size of 150 $\mu$m and peak intensity of 10$^{18}$ W/cm$^2$. Intensity modulation of the laser pulse was obtained by inverse Fourier transform after wave optics computations performed on the plasma guiding channel in the frequency space. For demonstrating the effects of intensity modulations on the plasma oscillations and electron energy gain in the bubble regime a density perturbation and a Particle-In-Cell (PIC) model was used, respectively. The former model provides exact value of plasma frequency and its wavelength at each point of domain, so that it was used to compare the discrete plasma frequency with the continuous one. In order to minimize the difference between two plasma frequencies a coupling factor was introduced. The PIC simulations showed that during the propagation time the electrons gain their energy in cascaded way and this process is in sync with the intensity modulations.

        This study was supported by the Human Resource Development Operational Program (contract EFOP-3.6.2-16-2017-00005).

        Speaker: Anatoliy Shapolov (Institute of Physics, University of Pecs)
      • 11:15
        Experimental demonstration of a laser proton accelerator with accurate beam control through image-relaying transport 15m

        Laser proton accelerator has been considered as one promising candidate for the future compact and low-cost radiotherapy system for malignant tumors. A Compact LAser Plasma Accelerator (CLAPA) has been built at Peking University, which can reliably generate and transport MeV energy protons with designed charge, spot and energy spread on to the irradiation platform. The transverse geometric emittance of laser accelerated proton beam entering the beam line has been measured using the quadruple triplet scan technique, showing a level of a few mm•mrad, which is comparable with the one from conventional accelerator. The energy accuracy of the laser accelerator is tested with a foil shielding method and is better than 3%. With the accurate beam control, Spread-out Bragg peak (SOBP), is demonstrated, for the first time, based on laser accelerated proton beams. Although the energy is low, it proves the ability of laser accelerator and takes the first step toward the future proton cancer therapy. As the next step, a full functional beam therapeutic bean line based on PW laser proton accelerator is proposed, aiming to effectively transport laser accelerated proton beam up to 250 MeV with 10-20% energy spread

        Speaker: Dr Chen Lin (Peking University )
    • 09:45 11:45
      6.2 High-Pressure and Thermal Plasma Processing Gold Coast III/IV (Double Tree at the Entrance to Universal Orlando)

      Gold Coast III/IV

      Double Tree at the Entrance to Universal Orlando

      Convener: Paul Rumbach (University of Notre Dame)
      • 09:45
        Optical and electrical diagnostic of surface arcs 15m

        In many plasma based applications, surface flashover cannot be avoided. Repetitive exposure of such surface flashover (also termed as an arc), can deteorate the surface properties of the material. Depending upon the arc locations, and arc frequency ultimately it may reduce the functional life of the material. Hence developing an understanding of such arcing phenomenon on various substrate’s surface [i.e. insulator, conductor or semiconductor] is essential. Electric discharges or arcing prediction by simulations being still a technical challenge. Thus it is very important to capture the arc parameters such as arc locations precisely. This information helps to understand the reason behind arc initiation that helps in developing suitable arc mitigation techniques. Small duration arc events can be captured by using an advanced camera integrated with a trigger circuit but this is a cost intensive solution. Further surface flashover is a statistically happening event hence arc location and time cannot be predicted in advance. Therefore in various situations, this makes it difficult to integrate trigger circuits with advanced camera. In this paper a study of arc location on the satellite solar panel surface has been performed. We demonstrate the ability to precisely capture the ESD events and arc locations by using an infrared camera and an indigenously developed LabVIEW based automated test facility.

        Speaker: Prof. Suryakant Gupta (Institute for plasma research)
      • 10:00
        Characteristics of negative-polarity DC superimposed nanosecond pulsed discharge and its applications 15m

        Non-thermal plasma generated by pulsed discharge is expected to efficiently treat combustion exhaust gases such as nitrogen oxide (NOx) and sulfur oxide (SOx) due to its high chemical activity. Nanosecond pulsed discharge which has voltage rise time and fall time of 2ns, pulse width 5 ns and peak value of 60 kV, has been developed by our group. Nanosecond pulsed discharge mainly consists of streamer discharge phase, so that heat loss which caused by glow discharge is less, and plasma impedance is kept almost constant during the streamer discharge phase. Therefore, impedance matching between pulsed power supply and discharge load is possible. Applications on ozone generation and NO treatment using nanosecond pulsed discharge are reported with high energy efficiency compared to other discharge methods. However, the discharge mode transit to arc discharge phase sometimes. Also, for industrial applications, the plasma processing capacity leaves room to improve. It has also been reported that negative polarity nanosecond pulse discharges give better results depending on the plasma processing applications. In this study, negative polarity DC superimposed nanosecond pulsed discharge is suggested in order to improve the better performance of the nanosecond discharge plasma. Results of ozone generation and NO treatment using negative polarity DC superimposed nanosecond pulsed discharge have also been introduced.

        Speaker: Hirofumi Yamashita (Graduate School of Science and Technology, Kumamoto University - Japan)
      • 10:15
        Quantification of OH radicals generated by nanosecond pulsed discharge plasma 15m

        In conventional water treatment, waste water is purified by a biological method. However, this method has some problems, such as dissipation of time, high cost for treatment, necessity of large facilities, and existence of some pollutants that are not easily to be decomposed. Therefore, the development of advanced water treatment technologies that can solve those problems is required.
        In recent years, discharge plasmas have been used as effective method for purifying environmentally polluted water. Some researchers reported that the species which have high oxidation potential, such as OH radical, O radical, O3 and etc., are produced by pulsed discharge plasmas generated in water surface. Among them, OH radicals have the highest oxidizing power.
        In this work, OH radicals generated by the nanosecond pulsed discharge was evaluated using a chemical prove method. In this method, terephthalic acid (TA) was used as an OH radical scavenger and the given fluorescence of the resulted 2-hydroxyterephthalic acid (HTA) was measured. However, it can be estimated that the HTA in the reaction process could be destroyed by discharge afterwards. Furthermore, there is no report of OH radical measurement based on the amount of HTA destruction. This means, the accuracy of OH radical evaluation needs room to improve. Therefore, we measured the amount of HTA destruction by nanosecond pulsed discharge, and successfully examined the destruction amount of HTA. As the result, the generated OH radicals with consideration of HTA destruction is successfully evaluated.

        Speaker: Kiyotaka Okada (Graduate School of Science and Technology, Kumamoto University - Japan)
      • 10:45
        Single-step Synthesis of Molybdenum Carbide Nanoparticles by Wire Explosion Process 15m

        Molybdenum carbide (MoC) is used extensively in many industrial applications especially as catalysis replacing the expensive noble metals. Nano sizing of the material provides high surface area for reaction. Many processes are used to synthesize MoC nanoparticles (NPs). Those techniques involve multiple steps and a specific choice of precursors with long preparation time. In the present work, MoC NPs were synthesized by adopting wire explosion process (WEP), in single step. WEP utilizes the joule heating of wire by injecting high magnitude pulsed current obtained by discharging high energy capacitor. In the process, the wire sublimates to vapour/plasma, reacts with ambient and gets cooled down in ms time, to yield oxide, nitride, carbide NPs depending on the ambient used. To control the phase and morphology of NPs, two parameters are defined in WEP: energy ratio, K (ratio of energy supplied to wire and sublimation energy of wire) and pressure, P of ambient gas.
        We propose the synthesis of MoC NPs with Mo wire as starting material and to carryout explosion in the methane gas medium, which acts as carburizing medium, as well as coolant, to bring down the local temperature rise to a value lower than the melting point of the material. XRD, TEM, SEM and XPS were used to characterize the synthesized NPs. Pure MoC was synthesized for K = 5.8 and P = 170 kPa. Carburization is more for high K/pressure. For low pressure case, one has to provide more K to get complete carburization. XPS confirms the formation of MoC. Spherical NPs were obtained with least mean particle size of 20 nm. Particle size decreases with increase in K and/or decrease in P. Formation mechanism of metal carbide NPs by WEP, will be discussed based on thermodynamical aspects.

        Speaker: Mr Prem Ranjan (Department of Electrical Engineering, IIT Madras, Chennai, 600036 India)
      • 11:00

        It has been suggested that the movement factors of cathode spot are the electromagnetic force and force caused by pressure gradient. However, the quantitative force values causing movement has not been disclosed. In this paper, the movement factors of vacuum arc clarified that either electromagnetic force or force caused by pressure gradient is $9.98\times 10^4$ ~ $6.87\times 10^5$ N/m$^3$ . The parameters of calculation were the external magnetic flux density and amount of metal vapor. In this simulation, the ion current was calculated from the behavior of ions in order to analyze the retrograde motion in the vacuum arc. The ion current is larger than the electron current in the cathode spot because the number density of ions is large in cathode spot. In addition, the direction of ion current is the cathode to anode caused by the pressure gradient. For these reasons, the phenomenon of retrograde motion increases with increasing the electromagnetic force and amount of metal vapor. Moreover, the two cathode spots were analyzed. The parameter is the distance between two cathode spots in order to elucidate that the electromagnetic force or force of pressure gradient dominates. As a result, one cathode spot was moved by the force of pressure gradient with the other cathode spot, but one cathode spot was not moved by the electromagnetic force with the other cathode spot. Thus, the force of pressure gradient is dominant. Therefore, the 3D electromagnetic thermal fluid simulation was developed in order to elucidate the movement factors of cathode spot in the vacuum arc.

        Speaker: Mr Yusuke Nemoto (Tokyo City University)
      • 11:15

        The atmospheric pressure arc deflects and becomes unstable in the case of strong cross wind. For example, the weld defects such as the lack of penetration and blow hole caused by the contamination of nitrogen in weld pool are caused by arc deflection. It is necessary to elucidate the contamination process of nitrogen under consideration of flow field derived from cross wind in order to prevent the weld defects. The observation of nitrogen contamination process caused by the cross wind has been researched when the gas between the cathode and anode is covered by the shielding gas. However, the nitrogen contamination process has not been elucidated when arc is not generated between the electrodes. The measurement of flow field and nitrogen concentration distribution in arc is difficult because the mass density difference and strong radiation derived from the temperature increment of arc occur. For this reason, it is required to elucidate the nitrogen contamination process into the arc caused by cross wind using the numerical analysis of arc. The increment of shielding gas concentration caused by the magnetic pinch force near cathode has been researched using the numerical analysis. However, it is suggested that the nitrogen is contaminated by the arc in case of the high lateral gas flow velocity because the flow velocity to the direction of arc center increases.
        In this paper, the analysis of nitrogen contamination process into the arc affected by the lateral gas flow velocity in atmospheric pressure was elucidated. As a result, the nitrogen concentration near anode increases with increasing the lateral gas flow velocity. This is because the flow velocity to direction of arc center near cathode increased with the lateral gas. Therefore, the flow velocity near cathode plays an important role for the nitrogen contamination process into the arc.

        Speaker: Yoshifumi Maeda (Tokyo City University)
    • 09:45 11:45
      6.5 Biological and Medical Applications II Seminole C (Double Tree at the Entrance to Universal Orlando)

      Seminole C

      Double Tree at the Entrance to Universal Orlando

      Convener: Andrei Khomenko (Purdue University)
      • 09:45
        The cell activation phenomena in the cold atmospheric plasma cancer treatment 15m

        Cold atmospheric plasma (CAP) showed a promising application in cancer treatment through dozens of demonstrations. The activation phenomenon of the CAP-treated cancer cells is a new concept in plasma medicine. In plasma medicine, a basic concern is the role of CAP during the treatment. Our recent experimental evidence demonstrated that CAP plays at least two important roles during the cancer treatment in vitro. The first role is providing abundant reactive species in the extracellular environment such as in medium. The second role is the CAP-triggered activation of the cancer cells, which may be a unique feature of CAP treatment. Despite the CAP-triggered activation will not cause noticeable cytotoxicity on the cancer cells, such activation drastically decreases the threshold of these cancer cells to the cytotoxicity of reactive species particularly ROS. A quick sensitization and slow desensitization are two features of such activation. The activation started since the 2nd second in CAP treatment. In contrast, a full de-sensitization process of the activated cancer cells last 5 hours after a CAP treatment. This discovery drastically changes our previous understanding of the anti-cancer mechanism of CAP treatment. The activation phenomenon during the direct CAP treatment explains the stronger anti-cancer effect of a direct CAP treatment compared with an indirect CAP treatment based on the CAP-treated solutions. Recently, we demonstrated that the flow rate of helium, the discharge voltage and the discharge frequency can affect the activation state of the CAP jet-treated cancer cells. Among the three basic operational parameters, a medium discharge voltage (3.78 kV) can cause the strongest activation effect. In addition, we recently also demonstrated that a nanosecond-pulsed magnetic field generator (NMF) could sensitize the melanoma cell line B16-F10 to the cytotoxicity of a ROS, H2O2. The NMF treatment alone did not noticeably inhibit the growth of melanoma cells.

        Speaker: Dr Dayun Yan (The George Washington University)
      • 10:00
        Mathematical Modeling of Tumor Growth and Response to Electrochemotherapy 15m

        Electrochemotherapy, the application of electric pulses (EPs) in combination with chemotherapeutic injection, drastically increases the effectiveness of cancer therapy. Such combinations enhance the ability of tumor cells to absorb chemotherapy drugs by permeabilizing cell membranes; however, optimizing this synergy for various EP parameters and drugs is poorly understood. This presentation investigates the response of immunogenic avascular tumors to chemotherapy and EPs individually and to electrochemotherapy to quantify the synergy between treatments. We modify a previously derived mathematical model [1] to specifically quantify the synergy for cisplatin and bleomycin compared for different EP conditions. Development of a 3-D artificial tumor model to extend this mathematical model and parameter assessment independent of physiological complications, such as the immune system, will be discussed.

        Speaker: Ms Jennifer Firehammer (Purdue University)
      • 10:15
        Single cell laser mediated molecule delivery – infrared laser based microinjection 15m

        Single cell microinjection is a powerful technique used to introduce exogenous material into cells and to extract and transfer material between cells. Traditional microinjection is a purely physical mechanism that does not require other compounds, amplifies the physical effects of the injected substances, provides precise volume and timing control for intracellular delivery, requires less material, and facilitates experimental consistency by allowing the untreated cells to serve as the control. However, microinjection is extremely labor intensive and expensive.

        This presentation outlines the development of a new laser-based single cell injection method that is inexpensive, user-friendly, rapid, user-friendly, independent of operator skill, and minimized consumables. While previous studies have demonstrated promising results for single cell laser injection using femtosecond lasers, mostly at 800 nm, these offerings are expensive (the laser alone costs ~$100k) and do not always achieve the users’ expectations. On the other hand, 1550 nm fs lasers are almost an order of magnitude less expensive than 800 nm lasers. We previously showed that a 1550 nm wavelength laser permeabilized multiple cells because the greater water absorption induced greater membrane temperature gradients, which enhance membrane permeabilization [2]. This presentation extends this work by targeting a single cell of interest with a 1550 nm commercial femtosecond laser. We summarize the hardware, control software, their integration, innovative beam alignment methods, and preliminary results for propidium iodide and plasmid delivery.

        [1] A. L. Garner, V. B. Neculaes, M. Deminsky, et. al, “Plasma membrane temperature gradients and multiple cell permeabilization induced by low peak power density femtosecond lasers," Biochem. Biophys. Rep., vol. 5, pp. 168-174, 2016.
        [2] J. Song, A. L. Garner, and R. P. Joshi, "Molecular dynamics assessment of the role of thermal gradients created by electromagnetic fields on cell membrane electroporation," Phys. Rev. Appl., vol. 7, 2017, Art. no. 024003.

        Speaker: Allen Garner (Purdue University)
      • 10:30
        Electrochemotherapy Enhances the Curcumin Effect on TNBC Cells in a Dosage and Energy Dependent Manner 15m

        Compared to other breast cancer phenotypes, triple negative breast cancer (TNBC) has a much lower five-year survival rate (30% compared to 66%) because it is refractive to standard chemotherapy since it lacks three main receptors. Thus, TNBC requires alternative treatment modalities. This motivates our study of treating TNB with electrochemotherapy (ECT) using curcumin, the yellow pigment of turmeric, which is loaded with anticancer phytochemicals with minimal side effects and lower expense than traditional chemotherapeutics.

        We treat MDA-MB-231 cells, and aggressive, human TNBC cell line, with 10μM or 25μM curcumin exposed to four, ten or twenty 100 μs electric pulses (EP) of 1200 V/cm to establish a correlation between the applied energy and cell death. The 24 h cell viability results show that combining EPs with curcumin effectively targets TNBCs in a curcumin dosage and EP energy density dependent manner. The viability for 10μM and 25μM curcumin alone samples did not differ significantly from control (100%). The viability was 32% and 43% for 20 and 10 EPs, respectively with no curcumin added; however, the viability was 17% and 85% for 10 and 4 pulses, respectively, with 25 μM curcumin. The highest cell death (5% viability) was achieved for 20 pulses with 25 μM curcumin. These results highlight the synergy of EP and curcumin against aggressive TNBC cells and the potential of using EP energy density to tune cellular responses. The low cost and natural herbal anticancer properties of curcumin could make this therapy an attractive alternative for TNBC treatment. Further potential tuning using a multi-electrode system was assessed using finite-element simulations. Such evaluations allow us to assess the field intensity and profile in the vicinity of a tumor target to help predict parameters for ultimate clinical applications. The implications of these results on guiding future in vivo work will be discussed.

        Speaker: Allen Garner (Purdue University)
      • 10:45

        Increased risks due to outbreaks of foodborne viruses and bacteria give us strong motive for the development of a viable non-thermal technology for the disinfection of foods and their contact surfaces. Plasma discharges occurring in the presence of oxygen and nitrogen generate an abundance of reactive oxygen and nitrogen species (RONS) at close to ambient temperatures. With minimal requirements for addition of any chemicals, disinfection using atmospheric pressure air plasmas could be a sustainable and green non-thermal technology.

        This work attempts to study the dependence of the biocidal activity of plasma sources on surface humidity of the sample used. Cold plasmas at atmospheric pressure were employed for in vitro treatment of pathogens spiked on stainless steel surfaces. The discharge effluent was used to treat the samples at both dry and wet surfaces.

        Plasma treatment was ineffective against dry samples. Similar humidity effect has been found in food science where lipid oxidation rates are triggered only beyond a threshold level of water activity. While wet virus samples were much more susceptible to plasma treatment, the effect was strongly dependent on the amount of water on the surface. An analysis of the transport of long-lived reactive species into aqueous phase, eventually responsible for microbial inactivation, will be presented. The present findings suggest that the control of sample surface humidity is crucial for effective and reproducible plasma-based disinfection.

        Acknowledgement: This work is partly supported by USDA, National Institute of Food and Agriculture (2017-67017-26172) and DOE, Office of Fusion Energy Sciences (DE-SC0016053).

        Speaker: Peter Bruggeman (Department of Mechanical Engineering, University of Minnesota)
      • 11:15
        Comparison of Plasma Sporicide Using Different Power Sources in Atmospheric-Air 15m

        Hao Wang, Liyang Zhang, Haiyun Luo * and Xinxin Wang
        Department of Electrical Engineering, Tsinghua University Beijing 100084, China
        *Email: lhy@tsinghua.edu.cn

        The sterilization of spores (sporicide) is a stubborn problem, since the firm exterior and dipicolinic acid (DPA) provide remarkable resistance of spores. Conventional methods, such as heat, UV, chemical sporicide, are usually hard to achieve sterilization effect. We used DBD pulsed plasma in air at atmospheric pressure to treat the Bacillus subtilis spores on biological indicator. The results suggest that the plasma treatment could achieve sterilization within 70s. To research the main factors of plasma sporicide, we compare the killing effects with plasmas produced by a pulse power source and an AC power source, and the result indicates that the pulse power source works far better. The transmission electron microscope (TEM) of two experiment groups show that the spore surfaces are more severely damaged in pulse power source. Analyzing the breakdown voltage and the spectrum, we infer that the higher electron field and higher electron energy by pulse power source are the key factors to destroy the exterior and DPA of bacteria spore and then kill protoplasm. This experiment prove that the pulse power source plasma is a better choice to sterilize the spores. This work is supported by the National Key Research and Development Program under contract 2017YFC1200404.

        Speaker: Dr Hao Wang (Tsinghua University)
      • 11:30
        A novel device enhanced the active antimicrobial components in the plasma treated solution 15m

        The plasma have shown a wide application prospect in sterilization, such as the medical device disinfection,tooth whitening and fruit preservation. Among these plasma generator devices, the surface dielectric barrier discharge plasma have been widely researched. This kinds of plasma device have a feature that the plasma generated area is bigger than plasma jet. But, as the the active species is short-lived and the transfer distance is extreme short, Neither the directly reached RONS nor the plasma-liquid reaction regenerated RONS generally accumulated in the surface layer of liquids. So in this experiment, a magnetic stirring apparatus was designed and utilized to enhance the solution streaming when the solution was treated by plasma.
        By adjusting the rotational speed, we treated several group of solution. The short-lived ·OH and long-lived H2O2 were measured. Apart this the ORP and pH were detected, which can indicate the electrochemical properties. Analyzed these parameters, we can find the streaming of solution influence the amount of RONS in the solution. Besides, we choose yeast as a model cell to study the specific difference under the streaming or not. The yeast after treated were examined via colony forming unit (CFU) count, and further verified by LIVE/DEAD staining and scanning electron microscope (SEM).

        Speaker: Hangbo Xu (zhengzhou university)
    • 09:45 11:45
      7.2 High Current and High Power Pulsers I Space Coast I-III (Double Tree at the Entrance to Universal Orlando)

      Space Coast I-III

      Double Tree at the Entrance to Universal Orlando

      Convener: Mr Mark Savage (Sandia National Laboratories)
      • 09:45

        We recently continued the pioneering work done by Chris Rose and Kalpac Dighe at Los Alamos National Laboratory. (“Multiple-Pulse High-Voltage Diode Isolation Testing for a Linear Accelerator (LIA)” B. Trent McCuistian, Dale Dalmas, Kalpak Dighe, Chris Rose, Manolito Sanchez, Robert Sedillo, J. Martin Taccetti, in Proceeding of the 2017 Pulsed Power Conference at Brighton, England, June 2017). We did not use Blumleins (240kV) but 30 Ohm cable pulsers. Hence, our setup was limited to 100kV maximum testing across the diode cartridges. Therefore, we were forced to test only diode cartridges of ~ 40% the scale of that of LANL. Our research was mainly concentrated on the physics of semiconductor diodes and especially on measuring the reverse recovery times and currents. In addition, we explored the effect of the reverse bias pulses on a diode still under reverse recovery times. We utilized our Component Test Stand facility (CTS) (“Testing High Voltage (200kV) DC cable and feed-through designs in rep-rated modes” Michael G. Mazarakis, Mark L. Kiefer, Joshua J. Leckbee, Del. H. Anderson, Frank L. Wilkins, Robert J. Obregon, in Proceeding in Proceedings of the 2017 Pulsed Power Conference at Brighton, England, June 2017.) modified to power two diode cartridges connected in parallel to a common load (CTS-II). Although our set up could deliver two separate pulses per diode assembly, in this study, for simplicity’s sake, we utilized only one pulse per diode. The cartridges were composed of 6 to 12 stages each and having three to five high power 10kV diodes.
        * Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

      • 10:15
        Field-Circuit Coupling Simulation of Petawatt-class Z-Pinch Accelerator 15m

        Most of the previous studies of petawatt-class Z-pinch accelerator are performing the circuit simulation based on a full circuit model. This model may cause non-ignorable error because it is under the assumption of TEM modes transmission along the MRTLs. In this paper, a method for field-circuit coupling simulation of petawatt-class Z-pinch accelerator was developed, which considered the non-TEM modes in the MRTLs. A 3-D electromagnetic simulation of the MRTLs was conducted and MRTLs’ equivalent circuit was created based on the scattering transfer parameters drawn from the electromagnetic simulation. By inserting the MRTLs’ equivalent circuit into the pulsed generators and Z-pinch load, a field-circuit coupling model of the whole petawatt-class Z-pinch accelerator was obtained. This method was used in the simulation of Z800 accelerator, a petawatt-class Z-pinch accelerator, and the load was a wire-array for Z-pinch. Compared to the previous circuit model, the load current obtained with the field-circuit coupling model was lower and thus the load-implosion time was longer. Then we compared the load current of accelerators using exponential, hyperbolic and linear MRTLs respectively and recommended the linear one. The relation between the energy transmission efficiency and load parameters were also investigated.

        Speaker: QUAN ZHOU
      • 10:30
        Compact Marx Generator to Drive a Low-Impedance MILO 15m

        A low-impedance MILO is being developed at Texas Tech University, and a compact Marx generator was designed to drive it. The target design goals of the Marx are an output voltage greater than 500 kV and an output current greater than 40 kA. Risetime needs to be sub 150 ns and the pulsewidth must be greater than 100 ns. These performance goals were determined from PIC simulation of the MILO such that an RF efficiency (>10%) and RF peak power (> 1 GW) can be achieved.

        Tests using smaller 3 and 4 stage Marx generators with the same topology as the final design were used to determine a per-stage inductance of approximately 120 nH. From this derived inductance, multiple configurations were simulated to decide upon the ideal design for the desired performance goals. From these simulations, an 18-stage Marx with 2 capacitors per stage was chosen as the most optimal design, and from simulations into a 12 Ohm load a number of the criteria can be met with this configuration.

        The simulated peak voltage and current are 570 kV and 48 kA, respectively, while pulse risetime and pulsewidth are 170 ns and 540 ns, respectively. The designed Marx is being experimentally validated to confirm the findings of the simulation, firing into an approximately 12 Ohm water load to represent the low-impedance MILO that is being designed.

        Speaker: Tyler Buntin (Texas Tech University)
      • 10:45
        Large Scale System Using Pulsed Electric Fields as an Invasive Fish Barrier 15m

        Invasive species, plants and animals introduced to ecosystems without natural predators or controls, are a global problem. The Asian Carp has invaded the Mississippi River Basin in the USA, and now threatens the Great Lakes. To prevent migration of this large invasive species into the Great Lakes, the US Army Corp of Engineers has built and operated two demonstration barriers in the Chicago Sanitary and Ship Canal since 2002. These barriers use pulsed electric fields to block Asian Carp from moving through the Canal, the primary connection between the Basin and Lake Michigan.
        Diversified Technologies, Inc. was recently awarded a subcontract from exp Federal, under a prime contract from the US Army Corps of Engineers, to build the large pulsers required for a permanent barrier on the Canal.
        This barrier uses bi-polar pulses driving an array of electrodes to create an electric field across the entire Canal. This barrier field will be sufficient to deter the Asian carp and even smaller fish from swimming upstream to Lake Michigan. This field must operate continuously, even in the presence of barges and ships. Two pulsers are planned, with the first in construction now. Each pulser includes:
        • 4.5 MW, 4 kV DC power supply, with voltage regulation
        • 4 MJ capacitor bank, which stores energy for the pulses
        • Solid-state pulse switches, which produce currents up to 30 kA with a frequency of up to 100 Hz, and pulsewidths of 1 – 1,000 milliseconds
        • Mechanical output reversing switch, allowing the pulse polarity on the electrodes to be reversed
        This paper will detail the design and intended operation of this pulser, which will be the largest known PEF system in the world when completed.
        This effort is funded under US Army Corps of Engineers contract W912P6-18-C-0021 with exp Federal.

        Speaker: Mr Michael Kempkes (Diversified Technologies, Inc.)
      • 11:00
        Analysis of triggering behaviour of Marx generators by using Spice simulations 15m

        The basic operation of a Marx generator is well known and simple: capacitors are charged in parallel through high impedances and discharged in series, thus multiplying the output voltage compared to the charging voltage. As a basic explanation, in a Marx generator using spark gap switches, triggering the first stage is sufficient to double the voltage on the second stage’s switch and so on. All the switches are then switched on in an avalanche mode. However, the behaviour is often more complex. The parasitic impedances of the geometry play an important role for the creation of overvoltages. This can make the design and the development of Marx generators quite challenging, especially when aiming for good reproducibility and precise timing.
        To achieve the best performances, various triggering techniques have been developed, but unfortunately, there is no “best practice” technique that can be applied systematically. For each new design, the engineers must establish the best way to obtain the required performances. If the initial choice of the triggering scheme does not achieve the expected performances, a lot of time could be spent experimenting to optimize the triggering scheme. This study presents SPICE simulation methods that could be helpful to compare different triggering schemes and their effects on the generator erection time, jitter and reliability.
        By using a spark gap model, the simulations presented give a thorough understanding of the benefits and drawbacks of the various available triggering schemes. These simulations could facilitate the optimization of the erection time and the delay by adjusting triggering methods. Secondly, by coupling LTspice and Python, a more complete approach is presented to take into account the statistical behaviour of the closing time of the switches and its effects on the jitter and the complete generator’s operation efficiency.

        Speaker: Benjamin Lassalle (ITHPP)
      • 11:15
        Characterization of Nano-second Pulsed Power Generator Synchronizing Double Inductive Energy Storage Circuits with Semiconductor Opening Switch 15m

        As a new method to enhance nanosecond pulsed power, aiming improvement of cold plasma applications, we designed a type of circuit that is amplified by synchronization of double simple inductive energy storage (IES) circuits with a semiconductor opening switch (SOS) diode. Secondary circuits of simple IES circuits which consist of capacitors, a pulse transformer, MOS-gated thyristors, and a SOS, were connected in parallel and in series, and power amplification has been succeeded at low repetition rate by synchronization of reverse currents through the SOS diodes in two circuits. However, there are some problems, e.g., synchronization deviation by variation of load and repletion rate, difficulty of circuit adjustment for synchronization, and low efficiency. Aiming to those improvement, characterization of the pulsed power generator was carried out by spice simulation and experimental circuit estimation. Our presentation details current and energy transfer path in the circuit including parasitic components, which were obtained from the circuit characterization.

        Speaker: Taichi Sugai (Nagaoka University of Technology)
      • 11:30
        MHD Modeling of Shock Physics Experiments with the PHELIX Portable High Magnetic Field Driver 15m

        The PHELIX portable pulsed power driver has recently completed a set of experiments examining the response of granular material to convergent shock loading. Here a nearly 4 MA peak current is delivered to a Z-pinch load with a quarter wave cycle time of ~3 us. This produces B ~ 0.30 MG field at the surface of a ~3 cm diameter, 1 mm thick, 3 cm tall Al liner. The liner is accelerated to ~800 km/s before shock impacting a target cylinder filled with fine-grain CeO$_2$ powder. Design and analysis simulations are performed with 2D MHD Lagrangian/ALE code to predict the liner performance and material response. Computational results are compared to the PHELIX Faraday rotation measurements for load current as well as proton radiographic imaging of the evolution of the density profile in the CeO$_2$.

        Speaker: Christopher Rousculp (Los Alamos National Laboratory)
    • 11:45 13:00
      Lunch Break 1h 15m
    • 13:00 14:30
      Poster - Microwave Generation and Plasma Interactions and Pulsed Power Switches and Components Universal Center ()

      Universal Center

      Conveners: Jason Sanders (Transient Plasma Systems, Inc.), Joel Ennis (NWL), Jose Rossi (National Institute for Space Research)
      • 13:00
        2P01 - Modeling a compact A6 relativistic magnetron operating with permanent magnets 1h 30m

        We present simulation results which demonstrate that a relativistic magnetron can be operated with permanent magnets. Using permanent magnets makes the magnetron a more compact device. In a recent paper (Leopold et al., IEEE-Trans. Plasma Sci., vol. 44, no. 8, pp 1375-85, 2016) we showed that the power balance in an A6 single radial output magnetron with its longitudinal slots covered by anode caps, is the result of a complex process involving the applied voltage and the distribution of the current between the axial leakage and magnetron currents. The axial magnetic field which is usually considered to be fixed and uniform in the interaction volume is an important parameter. The parameter space for pulse shortening and mode competition to occur has also been clarified. Replacing Helmholtz coils with permanent magnets is not though straightforward. It is possible to create sufficiently high axial magnetic fields by inserting magnets in the six vanes of an A6 magnetron, in the cathode or both. It is though difficult to use long enough permanent magnets for their edges to be far enough from the interaction region. On the other hand if the edges are too close, then the balance between the axial and magnetron currents is affected. Therefore it becomes more difficult to optimize the performance of such a magnetron.

        Speaker: Dr John Leopold (Physics Department, Technion, Israel Institute of Technology)
      • 13:00
        2P02 - Modeling the wakefield excitation by a 28 GHz microwave pulse in a plasma filled waveguide 1h 30m

        A simple 1D model of the propagation of an ultra-short (≤ 1ns), TM01 mode, 28 GHz, ~1 GW, microwave pulse produced by an SRBWO (Super Radiant Backward Wave Oscillator) in a ~10$^{10}$ cm$^{-3}$ density plasma shows that a wakefield develops as a result of the radial ponderomotive and the Lorentz force. This is a scaled-down equivalent of a laser wakefield experiment with more manageable parameters. The model shows that for best results the waveguide radius needs to be such that the Lorentz and ponderomotive forces balance in a particular way. We simulate the system by the 3D PIC LSP code and confirm this model. Moreover, we simulate the experimental waveguide which has a slotted wall. These slots are required to be wide enough so that the plasma produced at larger radii penetrates the waveguide filling it uniformly, large enough to allow diagnostics, and sufficiently small, so that the microwave radiation is contained.

        Speaker: J.G. Leopold (Physics Department, Technion, Israel Institute of Technology)
      • 13:00
        2P03 - Effects of the Mesh Anode Transparency on the Operation Characteristics of the Virtual Cathode Oscillator 1h 30m

        An axial virtual cathode oscillator is experimentally analyzed depending on the transparency of the mesh anode. The axial virtual cathode oscillator is operated using a 140J/170kV Marx generator. A stainless steel cathode and stainless steel mesh anodes with different transparency are used as a high power microwave generating diode. The gap distance of the virtual cathode diode is 4 mm. To analyze the operation characteristics depending on the mesh anode transparency, the output power, voltage, and current are measured.

        Speaker: Mr Se-Hoon Kim (Hanyang University)
      • 13:00
        2P04 - Development and testing of the 190 GHz dual mode OAM gyrotron with axial output 1h 30m

        We presented the design of a 190 GHz dual mode OAM gyrotron with axial output configuration for a prototype experiment of Orbital Angular Momentum (OAM) communication. A mode pair of second harmonic modes ($\rm TE_{8,3}$/$\rm TE_{11,2}$) is excited at 28/35 kV, 5A electron beam input in the presence of a uniform magnetic field of 3.56 T. It incorporates a perturbed cavity with two sinusoidal perturbations to excite higher order axial modes with the suppression of spurious fundamental modes. Cavity simulation has been performed by in-house developed code “UNIST Gyrotron Design Tool (UGDT)”. It suggests the generation of the ~30 kW power in both the modes at their respective operating voltages. Switching between these modes is to be carried out by tuning the applied cathode voltage from 33.5 kV to 36 kV. Moreover, these mode-pair is directly radiated into free space from a raised cosine taper which is placed after collector to reduce its divergence. It incorporates a quartz RF window with a thickness of 4.87 mm to achieve more than 90% transmission for both the modes. The experimental testing of the gyrotron is currently in progress and we expect to present its detailed performance analysis in conference.

        Speaker: Ashwini Sawant (UNIST)
      • 13:00
        2P05 - Fast-Wave and Slow-Wave Interactions in the Rippled-Field Magnetron 1h 30m

        The rippled-field magnetron is a compact millimeter wave source developed by Bekefi [1]. This source is driven by a rotating electron stream. The electron stream moves through an azimuthally periodic wiggler magnetic field oriented transversely to the flow and a uniform axial magnetic field. The advantages of this circular device compared to linear devices is that the beam circulates continuously, resulting in a long effective interaction. The anode-cathode gap is part of the magnetic wiggler interaction region.

        The rippled magnetic field can be achieved by permanent magnet or wires carrying current (electromagnet). The rippled-field magnetron uses samarium-cobalt bar magnets positioned behind grounded stainless steel cylinders and held in place using a grooved aluminum holder. In this work, the magnet bars are replaced by azimuthally periodic longitudinal strips carrying high current. A combination of strips connected to the cathode and anode are considered. They carry currents in opposite directions in order to form the desired magnetic wiggler fields. This setup is similar to the inter-digital magnetron (Mitron) which raises the possibility of electrons interacting with Hartree harmonics. The slow-wave growth is similar to magnetron interaction which can be achieved even for a moderate relativistic beam. On the other hand, the fast-wave growth is a free-electron laser type of instability which requires MeV or higher electron beam energies.

        1. G. Bekefi, “Rippled-Field Magnetron,” Appl. Phys. Lett., vol. 40, 578, 1982.

        • Work supported by DARPA Grant #N66001-16-1-4042
        Speakers: Artem Kuskov, Ms Stacie Hernandez (University of New Mexico)
      • 13:00
        2P06 - Frequency tunable X-band Relativistic Backward Wave Oscillator 1h 30m

        Relativistic tubes are generally used for High Power ElectroMagnetic (HPEM) applications. Most of these tubes radiate high level electromagnetic fields but operate at a fixed frequency. Nevertheless, in most cases, a variable frequency is useful if not required. In a precedent study, CEA worked on a very compact HPEM source named CLAIRE. The used tube was an optimized X-band sub-gigawatt relativistic resonant Backward Wave Oscillator (BWO) using low-level magnetic field. A new BWO design (based on the previous one) has been achieved with frequency tunable capability. This tube is cautiously designed to be compatible with the CLAIRE generator and to provide at least 500 MHz frequency range. Pros and cons of mechanical and electrical tunability are firstly evaluated. Particles In Cell Simulations (PIC) were carried out and revealed that an optimized mechanical solution provides the desired performances. Desired tunable frequency range is obtained by changing the distance “D” between the resonant reflector and the Slow Wave Structure (SWS). Finally, a prototype is realized. The distance “D” is mechanically actuated by moving the resonant reflector of few millimeters. A particular attention has been paid on the realization for two reasons. Firstly, operating in X-band implies high mechanical accuracy in order to achieve great performances. Secondly, it’s necessary to maintain good vacuum state while mechanically moving the resonant reflector. This paper presents the design, numerical PIC simulations, and first experiments.

        Speaker: jean-christophe diot (CEA)
      • 13:00
        2P07 - Examination of stability against beam parameters in a Ku band helix TWT 1h 30m

        A Ku band tape helix TWT with a center frequency of 13.25 GHz which was chosen for the RF driver present in our lab for a future prototype, was modeled and simulated using CST MWS and CST PS. Connectors for the RF input and output were also modeled in two different types. Firstly, a 50 Ohm coaxial connector was modeled and directly coupled to the helix and secondly an impedance matching section starting with a 50 Ohm and gradually increasing until coupling section to the helix was modeled. Beam parameters such as voltage, current and radius were swept while the other two were held constant. Aim of the study was to investigate the beam parameters for optimum operation and effects of matching were also studied in simulations. It was observed that a system that breaks down due to oscillations could, in some cases, be recovered just by proper matching.

        Speaker: Prof. Lutfi Oksuz (Suleyman Demirel University)
      • 13:00
        2P08 - Metamaterial Based RF Source 1h 30m

        We present our current progress towards the development of a novel RF source
        utilizing a low-loss, dispersion engineered, artificial EM material, based upon
        a complementary split ring resonator that supports both forward and
        backward wave propagation. The frequency dispersive media was designed
        numerically to yield specific constitutive parameters, determined using a
        Nicholson Ross Wier based retrieval approach. The media is designed to
        reduce EM wave propagation to approximately 0.2 c, facilitating interaction with a 20 keV electron beam.
        To explore the beam - wave the interaction a cylindrical waveguide loaded with the artificial material is considered and modeled using
        the FDTD-PIC simulation software MAGIC. Where a 20 keV, 0.5 A electron beam
        acted upon by a 0.3 T external magnetic field propagates on axis, to excite
        a wave in the media. The simulation results demonstrate Beam-Wave energy transfer, producing a narrow band EM wave at 8.45 GHz. Results of
        the frequency spectrum and the power generated are presented, along with
        eigenmode simulation results for both the empty and the loaded system. FEM simulations show the artificial material is capable of supporting a 1 KW
        continuous wave propagating through the media.

        Speaker: Simon Foulkes (University of Huddersfield)
      • 13:00
        2P09 - Cold Test Validation of Metamaterial Based Rectangular Slow Wave Structure for High Power Backward-Wave Oscillators 1h 30m

        In this paper, novel S band metamaterial based rectangular slow-wave structure (RSWS) is proposed for high power backward-wave oscillator (BWO). The circular waveguide is loaded with R-SWS that shows negative permittivity and permeability. Since RSWS shows the negative permittivity and permeability which is the key characteristics of metamaterial, it can work below the cut-off frequency. A high frequency characteristic of SWS is analyzed using CST Studio Suite. Dispersion diagram of unit cell is observed with Eigen Mode Solver which is dedicated to simulation of closed resonant structure. Interaction impedance of the unit cell is also analyzed for future work. Since the beam-wave interaction occurs with TM mode, axial and absolute electric field is observed in the simulation. Slow wave structure composed of 8 unit cell is fabricated and measured in Network Analyzer. Cold-test measurement validates the TM mode propagation and dispersion diagram of RSWS. Hot test simulation is also achieved using CST Particle Studio. It is achieved for the 450 kV, 100 A annular electron beam under the 2 T analytical magnetic field with 10.7 MW peak power and % 23.8 peak efficiency at 2.49 GHz

        Speaker: Ms Doğancan Eser (Middle East Technical University)
      • 13:00
        2P10 - Simulation of an Industrial Magnetron Using Cathode Modulation 1h 30m

        Magnetrons can be phase-locked using external systems. Previous 2-D PIC simulations of a rising sun magnetron[1] have shown that phase-locking is possible using modulated electron injection to control the spoke formation. An experimental setup using Gated Field Emission Arrays (GFEAs) for the modulated electron injection offers a potential solution to this problem by permitting the injection of electrons into the interaction space. Current work focusses on extending previous simulation results into 3-D. A commercially available industrial cooker magnetron (the L3 CWM-75kW) has been successfully simulated by using the 3-D PIC code VSim under the magnetron’s typical operating conditions (18kV, 5A, 1900G, 896-929MHz). The simulation generated results that are consistent with known experimental results in terms of power and frequency. Cavity oscillation starts within 100ns using 20ns of RF priming at half of the typical power. Preliminary results have shown that modulated electron injection has a significant effect on the working mechanisms of the magnetron in terms of spoke formation and start-up time, acting as a form of “cathode priming.” It has been experimentally shown by L3 technologies that this magnetron is capable of operating at ~9kV, 150 mA, and 900G. Future work will focus on the simulation of the magnetron under the aforementioned low-voltage-low-current conditions as well as further exploration of the effect of modulated electron injection on start-up and phase.
        This work is supported by the Air Force Office of Scientific Research under award #FA9550-16-1-0083. Geometry drawings, the magnetron hardware, and experimental low-voltage-low-current parameters were generously provided by L3 Technologies. Technical support of the VSim code was provided by Tech-X Corporation

        1. S. Fernandez-Gutierrez, J. Browning, M. Lin, D. N. Smith, and J. Watrous. “Phase-Control of a Rising Sun Magnetron Using a Modulated, Addressable, Current Source.” Journal of Vacuum Science & Technology, vol. B 33, pp. 031203. 2015.
        Speaker: Mr Andong Yue (Boise State University)
      • 13:00
        2P11 - NLTL Frequency Chirp through Dynamic Bias of Inductor Cores 1h 30m

        Nonlinear transmission lines (NLTL) have demonstrated the ability to convert a low-frequency video pulse into a narrowband RF packet whose frequency may vary from pulse to pulse. Synchronous wave magnetic NLTLs achieve this via a DC current bias prior to the launch of a video pulse which sets the subsequent shock velocity and thus the RF frequency. This paper explores the idea that a dynamic (time-varying) bias waveform can yield NLTL RF output with a chirp frequency characteristic by varying the shock velocity along the length of the line. A low power NLTL was utilized to prototype the concept using 1 kV – 3 kV input drive, 0 – 150 V bias waveform, and 0 – 1 A DC bias current. The center frequency output of the low power NLTL ranges from 120 MHz – 260 MHz, which varies with bias current and drive voltage, and has RF pulse widths between 50 ns and 300 ns. The optimization of the input waveform required to produce a chirp output waveform and experimental characterization of various dynamic bias waveforms on a low power test bed are described.

        Speaker: Emily Schrock (Sandia National Laboratories)
      • 13:00
        2P12 - The Influence of Magnetic Field Profile on the Downstream Electrons and the Output Mode of MDO 1h 30m

        Magnetron with Diffraction Output (MDO) possesses advantages like direct couple to axial output waveguide; hence it has higher output extraction efficiency. Like traditional magnetron, MDO also requires an axial magnetic field to force the explosive field emitted electrons to go into a spiral motion and form resonant fork, and unlike low power traditional magnetron where the B field is formed by permanent magnet, in high power pulsed relativistic magnetron the B field is usually formed by a pair of Helmholtz Coils that provides near constant Z axis B field if the radius and distance between the coils are large enough. Due to the nature of the pulsed power system that drives such high power MDO, the electrons tends to drift towards the output horn and the magnetic field profile in the horn region affects how the electrons propagate in this region. Using Particle-In-Cell (PIC) simulation, we found if the magnetic field is setup as a unit function, then the downstream electrons continue to drift in a straight line. If the magnetic field gradually decreases as in a real Helmholtz Coil, then the downstream electrons drift outwards in the vanes of the MDO. Due to the influence of the downstream electron positions, the output mode is slightly changed at the output horn end.

        Speaker: Prof. Shen Shou Max Chung (Department of Electrical Engineering, National Penghu University )
      • 13:00
        2P13 - Hybrid Kinetic-Fluid Simulations of a Ku-band MILO 1h 30m

        There has been tremendous progress in the modeling and simulation of high-power microwave devices, especially in the lower frequencies, such as L-band (1-2GHz) and S-band (2-4GHz). Here we look at significantly higher frequency, 14GHz, while still studying a tube with GW-class power levels [Tao Jiang et. al. Phys. Plasmas 2015]. The choice of this Ku-band allows both high frequency and high power density (due to the smaller dimensions of the slow wave structure) to be investigated. Additionally, the tube under study is a magnetically insulated line oscillator, where the self-magnetic field from the intense relativistic electron beam population is sufficient to insulate the transmission of current across the vacuum gap. In this way, the Ku-band MILO offers physics and power densities approaching those seen in magnetically insulated transmission lines (MITL), a critical pulsed power technology for high-energy density physics (HEDP). We report on the application of fully electromagnetic particle-in-cell (VSim) and fluid models (USim) in modeling electron flow in the device. Additionally, due to the intense power loading experienced in this device, we also investigate various plasma production models that introduce ion space charge into the device. We use the combination of both kinetic and fluid simulations to study the interaction of electron and ion populations in a fully electromagnetic environment, as well as use the Ku-band MILO as a test bed for active and automated transition between PIC and fluid models during individual run to produce a hybrid model of the plasma physics in this device.

        Speaker: Peter Stoltz (Tech-X Corporation)
      • 13:00
        2P14 - W-band 2D Periodic Lattice Oscillator 1h 30m

        Two dimensional (2D) periodic surface lattices PSLs have been used successfully in both fast-wave sources [1] and in slow-wave sources [2-4]. Numerical simulation codes have been used to design an electron beam driven W-band millimeter-wave source, in which a cylindrical two dimensional (2D) periodic surface lattice (PSL) forms an over-sized mode-selective cavity. The 2D PSL consists of shallow periodic cosinusoidal perturbations in both the azimuthal and axial directions on the inner wall of a cylindrical waveguide. Electrochemical deposition of copper on a cylindrical aluminum former with the aluminum subsequently removed by dissolving in strong alkali solution was used to construct the 2D PSL. Analytical theory and numerical PIC simulations have been used to design the W-band oscillator that has been constructed. The ratio of the diameter of the cylindrical cross-section of the structure to the operating wavelength is ~5. The performance of oscillator will be compared with the predictions of the numerical simulations.

        Support from AFOSR under awards FA8555-13-1-2132 and FA9550-17-1-0095 is gratefully acknowledged.

        [1] N. S. Ginzburg, N. Y. Peskov, A. S. Sergeev, et al. , “Theory of free-electron maser with two-dimensional feedback driven by an annular electron beam”, J. Appl. Phys., vol. 92, pp. 1619-1629, Aug. 2002.
        [2] A. W. Cross, I. V. Konoplev, A. D. R. Phelps, and K. Ronald, “Studies of surface two-dimensional photonic band-gap structures”, J. Appl. Phys., vol. 93, pp. 2208-2218, Feb. 2003.
        [3] N. S. Ginzburg, E. V. Ilyakov, et al., “Theoretical and experimental studies of relativistic oversized Ka-band surface-wave oscillator based on 2D periodical corrugated structure”, Phys. Rev. Accel. Beams, vol. 21, (8), art. no. 080701, Aug. 2018.
        [4] A.J, MacLachlan, C.W. Robertson, I.V. Konoplev, A.W. Cross, A.D.R. Phelps and K. Ronald, ‘Resonant Excitation of Volume and Surface Fields on Complex Electrodynamic Surfaces’, accepted for Phys. Rev. Appl., Feb 2019.

        Speaker: Dr Colin Whyte (University of Strathclyde)
      • 13:00
        2P15 - Operation of a Gyromagnetic Line with Magnetic Axial Bias 1h 30m

        A growing interest has been rising around the use of Gyromagnetic Nonlinear Transmission Lines (GNLTLs) for Radiofrequency (RF) generation since recent results that were published has demonstrated great prospects for this end. The interest on this type of transmission line comes from the high RF conversion efficiency around 20.0% according to some works already elaborated, what shows a great capability of operates in a frequency range with considerable value, between 300.0 MHz and 6.0 GHz. The focus of this type of research is for the development of systems with greater reliability, efficiency and of course cheaper than those that already exist. Several authors used different approaches to study the gyromagnetic effect in order to understand the electron magnetic dipole precession movement of the ferromagnetic material, responsible for compress the pulse oscillation. The model proposed and studied here to analyze the GNLTL has a coaxial structure using NiZn ferrite beads distributed in a 20-cm coaxial line, for low and high voltage operation. Different measurements are compared in order to check the influence of the voltage injected onto the input, as well as the influence of the medium in which the coaxial line is and the use of a solenoid to create an axial magnetic bias. This work aimed the oscillation generated at the output caused by the presence of a magnetic field and by the changes in the system setup. For this, we analyzed the GNLTL behavior according to the results obtained from experimental tests, in order to observe the frequency response when the axial bias is present. It is expected that the results presented here will be useful as a basis to develop a system capable of generating RF for the use in space and mobile defense platforms.

        Work funded by AFOSR under contract number FA9550-18-1-0111.

        Speakers: Fernanda Yamasaki (INPE), Jose Rossi (National Institute for Space Research), Edl Schamiloglu (University of New Mexico)
      • 13:00
        2P16 - Simulations of a W-Band Circular TWT 1h 30m

        We are exploring the amplification of W-band electromagnetic radiation using a dielectric-loaded traveling wave tube (TWT) by employing several particle–in–cell (PIC) codes. We are seeking to replicate recent results obtained by a Naval Research Laboratory’s (NRL’s) dielectric–loaded TWT design [1] consisting of a solid circular electron beam (26 kV, 100 mA and 0.185 mm beam radius) surrounded with dielectric material, εr=13.5, and coupled to a TM01 electromagnetic wave at a frequency of 94 GHz. NRL used a finite-difference-time- domain (FDTD) formulation in a 2–D cylindrical coordinate system to perform the dielectric–loaded TWT simulations. In our case, we have opted for PIC simulations comparing three different software tools–a 3–D Cartesian coordinate system ‘FDTD–PIC method–based MAGIC’, ‘CST Electromagnetic and Multiphysics Simulation Studio Suite’, and ‘Improved Concurrent Electromagnetic Particle–In–Cell (ICEPIC)’. An earlier structure similar to that published by NRL, but at K-band and using a sheet beam and planar dielectric material, has been studied and confirmed by Los Alamos National Laboratory (LANL). We are seeking to confirm the results obtained by LANL at K-band using PIC simulations in a 3-D Cartesian system. Results from MAGIC and CST simulation will be presented.

        1. J. P. Calame and A. M. Cook, “Design and large-signal modeling of W-band dielectric TWT,” IEEE Trans. Plasma Sci., vol. 45, 2820–2834 (2017).
        Speaker: Khandakar Nusrat Islam (University of New Mexico)
      • 13:00
        2P17 - Pulsed RF Signal Irradiation Using a Low Voltage NLTL Coupled to a DRG Antenna* 1h 30m

        Nonlinear Transmission Lines (NLTLs) has been used for RF generation with great success. Possible applications of NLTLs as an RF generator include aerospace radars, telecommunications, battlefield communication disruption, etc. The RF pulses generated by the NLTLs can be radiated using antennas connected to the output of the lines. Also, there has been a paucity in the literature considering experimental results on the extraction and radiation of the RF signals from the NLTL output. This work reports the results obtained with a low voltage lumped capacitive NLTL in which oscillations of about 230 MHz were produced and radiated using a Double-Ridged Guide (DRG) antenna. The RF signal from the NLTL output was extracted using a high-pass filter decoupling circuit. The pulsed RF signal measured on a resistive load connected to the line output was evaluated in time and frequency domains as well as the signals obtained from the DRG transmitting and receiving antennas. A SPICE line model has been implemented showing a good agreement between the simulation and experimental results.
        *Work supported by SOARD/AFOSR under contract no. FA9550-18-1-0111.

        Speakers: Dr Jose O. Rossi (National Institute for Space Research), Edl Schamiloglu (University of New Mexico)
      • 13:00
        2P18 - E-band Overmoded Relativistic Backward Wave Oscillator 1h 30m

        An E-band relativistic backward wave oscillator (RBWO) is proposed to generate megawatts of power. An overmoded rectangular slow wave structure (SWS) is chosen and combined with a relativistic hollow electron beam to increase the interaction impedance and avoid RF breakdown. To overcome the frequency limits of conventional fundamental mode version, a higher order mode is selected as the operating mode by using a mode selection technique. To demonstrate its capability, the RBWO based on the axisymmetric SWS has been designed and simulated using the particle-in-cell codes CST and MAGIC. The Gaussian output mode is obtained from the operating TM03 mode through a corrugated waveguide mode converter.
        * Work at the University of New Mexico is supported through DARPA Grant #N66001-16-1-4042.

        Speakers: Liangjie Bi (University of New Mexico,University of Electronic Science and Technology of China), Ahmed Elfrgani (University of New Mexico)
      • 13:00

        We present results of 3D ICEPIC simulations of a relativistic X-band (9.9 GHz) twistron based on a design reported in the literature. Here we report on progress made in our hot test simulations of this device. Full 3D ICEPIC simulations were made using the supercomputers of the DoD Supercomputing Resource Centers (DSRCs). Our hot test simulations used a 373 kV, 6.5 kA annular beam with 13.75 mm inner radius, 15.25 mm outer radius, focused with a 0.7 T axial magnetic field; the ICEPIC cell size used for our simulations were typically 0.25 mm. The 0.57 GW output RF power leaves the twistron as a TM01 mode via a downstream cylindrical waveguide. We have made some improvements to the twistron including adding a downstream beam catcher after the slow wave structure.

        Speaker: Dr Paul Gensheimer (AFRL/RDH)
      • 13:00
        2P20 - Simulations of Surface Inhomogeneities in Field Emission 1h 30m

        Surface inhomogeneities can have a large impact on field emission, because it is strongly dependent on the local value of the surface electric field and work function of the metal. Within a given area the bulk of the emitted current may stem from a protrusion where the field is enhanced, or from a site where the work function is lower than in the surroundings. In systems that are large these effects tend to be obscured due to the large number of emitter sites, but for small systems there may be large variation in the performance because each aberration is a significant contributor to the total current emitted. Here we report on simulations done using our molecular dynamics code1,2 to simulate the emission from a planar cathode with non-uniform work function on the surface. The distribution, number of inhomogeneities and size are studied and how they affect the beamlet. This is done by calculating the beam emittance and electric current through the system during the simulation.

        This work was supported by a grant form the Icelandic Research Fund under grant number 174127-053.

        1. K. Torfason, A. Valfells, A. Manolescu, “Molecular dynamics simulations of field emission from a planar nanodiode”, Phys. Plasmas 22, 033109 (2015).
        2. K. Torfason, A. Valfells, A. Manolescu, “Molecular dynamics simulations of field emission from a prolate spheroidal tip”, Phys. Plasmas 23, 123119 (2016).
        Speaker: Dr Kristinn Torfason (Reykjavik University)
      • 13:00
        2P21 - Beam-Current Loss in Emittance-Dominated High-Frequency Tubes 1h 30m

        The next generation high frequency tubes will face significant challenge in focusing the beam into small aperture for beam-wave interaction. The efficiency of such tubes will depend largely on available beam energy that will interact with electromagnetic wave as the beam is transported. Unlike low frequency tubes, emittance is emerging as a major concern since beam loss at the wall or at the surface of the slow wave structures are expected to increase appreciably as the frequency increases. To determine the beam loss for a certain pipe size, typically numerical analysis of particle simulation is conducted which is often expensive and tedious. In addition, there has not been any concrete analysis demonstrating how the current profile evolves as the beam is transported in such tubes. In this paper, we apply the beam physics developed for linacs to high-frequency tubes for the first time. We provide necessary theoretical tools to determine the fundamental limit of the beam pipe sizes for a desired limit of beam interception. Specifically, the effect of both space charge and emittance are incorporated into iterative solution of equilibrium distributions of charge densities in the presence of a uniform focusing axial magnetic field. The effect of phase-space rotation and evolution of beam current is demonstrated through the calculation of beam divergence and maximum excursion of particles. Hence, the numerical solutions and tools provided here are complete analysis and can be used to determine the beam pipe size for any beam emittance. The theoretical formulation and results are expected to be particularly useful for devices operating from mm-wave to sub-THz frequency regimes.

        Speaker: Muhammed Zuboraj (Los Alamos National Laboratory)
      • 13:00
        2P24 - Hybrid Quantum-Hydrodynamics/Kinetics Model for Dense Plasma Mixtures 1h 30m

        Fusion energy promises nearly unlimited, clean energy. One approach to fusion energy production is by means of inertial confinement fusion experiments where a fuel boundary exists (e.g., fuel-liner or fuel-ablator). Unfortunately, in the presence of a wide variety of energy loss mechanisms, obtaining a net gain in energy remains a challenge. The mixing of cooler materials into hot regions can spoil the production of fusion energy. Two ways that cooling occurs is from the mixing of two ion species, or by conduction from the electron species. An existing kinetic model for studying the mixing of ions, is the multi-component BGK (McBGK) equation which describes the ionic heat transfer. One way to add the effects of heat conduction from the electrons is by solving a kinetic equation which is not a computationally tractable approach due to the considerable difference in timescales for the electron and ion species. Instead, hydrodynamic equations of motion for the electron species are derived directly from the McBGK equation and are used to determine how the electrons transfer heat to the ion species. We plan to use our model to aid in the design and interpretation of experiments at Sandia National Laboratories that are being performed on the Z Machine, a large pulsed-power facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND Number: SAND2019-1736 A

        Speaker: Mr Lucas J. Stanek (Michigan State University and Sandia National Laboratories)
      • 13:00
        2P26 - Plasma Simulation and Modeling of Pseudospark Discharge for High Density and Energetic Electron Beam Generation 1h 30m

        Generation of high density and energetic electron beams of short duration are important in growing areas such as the generation of extreme ultraviolet/X-ray radiation, microwaves, THz radiation and for biomedical and radiography applications [1-2]. A pseudospark discharge (PSD) has the ability to produce the combined highest current density (>108A/m2) and brightness (~1012Am−2rad−2) electron beams with fast current rise times (dI/dt ~1011 A/s) [2]. Analysis of the PSD has been carried out for the generation of high density and energetic electron beams from single to multi-gap PSD configurations using plasma simulation codes OOPIC-PRO and COMSOL. The generated e-beams are strongly influenced by the gas pressures (20-80 Pa), electrode apertures (2-6 mm), number of gaps (1-4), trigger energy (1-4 kV) and applied voltages, etc. The generated e-beam currents decrease with the increase in electrode apertures while increase with increase in gas pressures. Detailed consideration is required in choosing suitable trigger energy for operation at higher gas pressures and lower cathode apertures in a multi-gap PSD arrangement [3-5]. It is found that there is a decrease in the breakdown voltage for increasing gas pressures and electrode apertures [3-4]. It has been found that potential distributions in the PSD source is very much responsible for confinement of the plasma and generation of high density and energetic e-beams of different peak currents and sizes.

        [1] D. Bowes, et al. Nucl. Inst. Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, vol.335, pp. 74–77, 2014.
        [2] A. W. Cross et al., J. Phys. D, Appl. Phys., vol.40, no.7, pp. 1953–1956, 2007.
        [3] Varun et al., IEEE Trans. Electron Devices, vol.65, no.4 pp. 1542-1549, 2018.
        [4] Varun, et al., IEEE Trans. Electron Devices, vol.65, no.10, pp. 4607–4613, 2018.
        [5] Varun et al., IEEE Trans. on Plasma Sci., vol.46, no.6, pp. 2003-2008, 2018.

        Speaker: Varun . (CSIR- CEERI, Pilani, India and AcSIR, Ghaziabad, India)
      • 13:00
        2P27 - PIC-DSMC numerical grid heating in collisional plasmas: Application to streamer discharge simulations 1h 30m

        Numerical heating due to the mesh size being larger than the Debye length is well understood for collisionless PIC simulations [1]. However, the importance of grid heating in collisional, partially ionized plasmas such as streamer discharges is less understood. In these plasma regimes the artificial heating of the plasma can, at least theoretically, be mitigated by collisional energy transfer to the dense background gas. On the other hand, elastic energy transfer is extremely inefficient and by inaccurately increasing the electron temperature both the electronic excitation and ionization rates will increase, potentially leading to significant error in the plasma evolution and the streamer channel density and temperature. To some extent, whether one cares about the numerical error introduced depends on the quantity of interest. Specifically, while the density and temperature of the streamer channel may not affect the streamer velocity or branching, it would most likely change the current carried through the channel. In the present work we investigate how numerical heating in collisional plasmas affect various quantities such as the electron energy distribution function and net ionization coefficient for several cases across a range of mesh sizes. The cases include a 0D, extremely low E/n plasma representing the streamer channel, a 1D fixed-field “electron avalanche” case representing the streamer tip region, and a 2D simulation of a streamer.

        [1] C.K. Birdsall and A.B. Langdon, Plasma Physics via Computer Simulation (2005).

        • Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
        Speaker: Chris Moore (Sandia National Labs)
      • 13:00
        2P28 - Dispersion Engineering for O and M-Types High Power Microwave Sources 1h 30m

        The design and the development of High-Power Microwave (HPM) sources today relies heavily on Particle-In-Cell (PIC) codes, which allow the source concepts to be virtually prototyped and optimized prior to being built experimentally. The current state of source development consists of developing the geometry of the structure, extracting the dispersion relation from the eigenmodes, evaluating the dispersion properties, and finally adjusting the geometry to obtain the desired wave dispersion [1]. O-type devices and amplifiers have an advantage in having axial symmetry, which can be simulated with a single slow wave structure and periodic boundary conditions [2-3]. The dispersion relation for M-type devices, however, cannot be constructed in the same way due to the entire period being 2π. The UNM HPM group found a methodology that can ease the extraction of the dispersion curve from M-type devices, such as the magnetron or Mitron (inter-digital magnetron), which can be a novel method for dispersion engineering of cross-field devices.

        This work explores a methodology for deriving the dispersion relations for O- and M-type devices and attempts to simplify the process of dispersion engineering from the required dispersion characteristics to the corresponding geometry.

        1. E. Schamiloglu, “Dispersion Engineering for High Power Microwave Amplifiers,” in Proceedings of the 2012 EAPPC-Beams Conference, Karlsruhe, Germany, Sept., 2012.
        2. S.C. Yurt, Design of an O-Type Metamaterial Slow Wave Structure for High Power Microwave Generation (Ph.D. Dissertation, University of New Mexico, Albuquerque, NM, 2017).
        3. R.K. Singh, “Cold Analysis for Dispersion, Attenuation and RF Efficiency Characteristics of a Gyrotron Cavity,” World Academy of Science, Engineering and Technology, International Journal of Electrical, Computer, Energetic, Electronic and Communication Engineering, vol. 6, 1233-1238, 2013.
        Speaker: Artem Kuskov (University of New Mexico)
      • 13:00
        2P29 - Fast A-Stable Implicit Scheme and Scalable Software MOLTN For Electromagnetics 1h 30m

        Maxwell’s equations based vector potential formulation of electromagnetism is widely used in classical and quantum physics. However, in PIC simulations, the community has primarily focused on Maxwell’s equations in first-order form, exploiting the explicit Yee method for the fields with the Boris push for the particles. However, a challenge with this approach is geometry. The community has embraced CUT cells as a solution, however, both particle weighting and field updates on cut cells are problematic and the approximations used together can lead to instabilities, and the fields introduce a stability issue in terms of restrictive CFL. In this work, we are developing a new approach to overcome these issues, based vector potential formulation under the Lorenzo gauge. The scheme is based on the MOLT which first discretizes the operator in time and then solves the resulting boundary value problem using a kernel method. It is fast (O(N)), linear-time, high-order in space and A-Stable to all orders in time. ADI scheme is used for the extension to multi-dimensions, with each line solved independently. High-order is achieved using successive convolution to correct for the splitting error. It avoids the use of matrices, eliminating the main bottleneck in scaling implicit methods. An embedded boundary method is employed to deal with complex geometries, and It does not suffer from small time step limitations. The eventual goal is to combine this method with a novel particle method for the simulations of plasma. So far, the consistency and performance of the scheme are evaluated for EM wave propagation and scattering using different shaped objects including curved boundaries, and the introduction of true point sources that demonstrate how we will look to handle particles. We are developing an open-source code MOLTN which is hardware-independent, scalable software tool, using multi-node MPI, multi-core OpenMP, and GPU Cuda implementation.

        Speaker: Mathialakan Thavappiragsam (michigan state university)
      • 13:00
        2P30 - Electrostatic Finite Element Numerical Modeling of Spark Gap and Related Accelerator Structures 1h 30m

        L3 Applied Technologies is developing series pulsed forming water transmission lines for Los Alamos National Laboratory. The Series Pulse-Line Integrated Test Stand (SPLITS) consists of a set of four, 5.5 ohm coaxial water pulse forming lines in series. Each water line is capable of producing a -300 kV pulse when driving a matched resistive load.

        As part of this effort, the University of New Mexico is carrying out a 2d and 3d Finite Element Electrostatic modeling of the main spark gap switch design in support of advanced laser triggered switch resistivity studies. This paper presents results of 2d electrostatic modeling of the main spark gap geometries using the FEM tool ESTAT. 3 dimensional models of these structures using FEM code HiPhi will also be shown as well as some preliminary electrodynamic time domain models for selected geometries.

        Speaker: Ms Rena Berdine (University of New Mexico)
      • 13:00
        2P31 - Modeling of gas recirculation effects in nanosecond-pulsed high-frequency discharges 1h 30m

        Gas recirculation effects have been modeled following a nanosecond-pulse high-voltage discharge across a pin-to-pin air gap, where subsequent pulses in a burst have been found to couple very efficiently if the repetitive pulses occur above a critical frequency. Previous data from the same pulse discharge system indicated that the inter-electrode gas temperature increased rapidly following the first pulse up to several thousand Kelvin, but the inter-electrode region was rapidly cooling by 4 $\mu$s after the first pulse, presumably due to recirculating fresh gas. For this system, repetitive pulses at frequencies of 20 kHz and above exhibited strong thermal coupling, indicating that fresh gas recirculation does not cause a de-coupling of the following discharge until about 50 $\mu$s after the first pulse. The model developed is a computational fluid dynamics code that simulates the pulse energy deposited by the initial 10 ns arc into the inter-electrode gas and then computes the evolution of the resulting gas density and temperature profile after the pulse. The grid is set up to be two-dimensional and axially symmetric about the inter-electrode axis with the shape of the electrodes accurately represented. The simulated density profiles as a function of time are compared to experimental measurements which used Rayleigh laser scattering to determine the gas density along several different radial lines through the inter-electrode space. The Rayleigh scattering technique employed a pulsed 532 nm laser and gated intensified CCD camera that allowed both temporal and spatial resolution of the gas density after the pulse discharge.

        Speaker: Asher Straubing (University of Dayton Research Institute)
      • 13:00
        2P32 - Study of two-surface multipactor susceptibility using Monte Carlo simulation 1h 30m

        Multipactor is a nonlinear phenomenon driven by an rf electric field in which secondary electron emission from metallic or dielectric surfaces, leads to an exponential growth of charge. It is harmful to satellite communications and microwave systems [1]. Here, we apply Monte Carlo (MC) simulation [2] to study the multipactor susceptibility in a gap between two parallel metallic electrodes. For a given fD (f is the frequency of rf field, D is the gap distance between the two surface), we scan the average secondary electron yield (SEY) for a range of magnitude of the input microwave voltage V using MC simulation, to obtain the multipactor susceptibility diagram in the V-fD plane. The results are obtained for secondary emission processes with SEY based on Vaughan’s model [3], with fixed emission energy and normal emission angle, and with random initial energy and angle following a preassigned distribution. For both cases, the MC results are different from the analytical theory [2,3]. Analysis of the electron trajectories reveals that the deviation from the analytical theory is due to the presence of mixed multipactor mode.

        1. Special sessions on Multipactor, I and II, ICOPS, Denver, CO, June 2018.
        2. R. A. Kishek, Y. Y. Lau, L. K. Ang, A. Valfells, and R. M. Gilgenbach, Phys. Plasmas 5, 2120 (1998).
        3. J. R. M .Vaughan, IEEE Trans. Electron Devices 35, 1172 (1988).

        Work supported by AFOSR MURI Grant No. FA9550-18-1-0062.

        Speaker: Mr Zizhuo Huang (Michigan State University)
      • 13:00
        2P33 - High Power Radio Frequency Pulse Shaping For a 1.5MW S Band Magnetron Source 1h 30m

        Verus Research designed and modeled a novel plasma switch assembly to optimize the mechanical and electrical characteristics required to create an efficient and reliable family of switches for megawatt sources. There is a need in the high-power community for a fast rise-time High Power Radio Frequency (HPRF) pulse shaping tool to augment slow risetime HPRF sources, such as magnetrons, to support physics and engineering tests. We developed an efficient High-Power Pulse Shaping (HPPS) capability that can be incorporated into an existing source without the need to procure a different source or pulsed power system, providing test facilities with a tunable, cost-efficient capability. Our augmented capability is used for applications requiring adjustable pulse width while achieving a fast risetime and maintaining pulse repetition frequency; applications include electronics testing, antenna testing, model verification of RF coupling, among others. The use of a high-power circulator combined with HPPS isolates the HPRF source from reflected power created during the pulse shaping process. We discuss optimization of the HPPS design with emphasis on maximizing the ratio of the shaped, output-pulse, peak power to the input peak power. A rise and fall time of less than 10 ns was observed during initial testing with a pulse width of less than 100 ns. We present results from extensive parametric modeling of S-band configurations utilizing a WR284 waveguide, examining the effect of materials properties, gap spacing, and dielectric strength.

        Speaker: Michael Butcher (Verus Research)
      • 13:00
        2P34 - Feasibility Study of Guiding High Power Microwave with Laser Created Plasma Ring Channels or Photonic Crystals in Air 1h 30m

        High Power Microwave (HPM) is a proven effective mean to suppress electronic system or disable Unmanned Aerial Vehicle (UAV) because the power level can be as high as GW, however, due to basic limitation from antenna theory, the power per unit area quickly decline when the distance to target increases, therefore difficult to extend the effective range to more than a few km. Elaborately designed phased array can ameliorate this situation but comes with its own tradeoffs. Recently substantial progress has been made in High Energy Laser (HEL) such that 50-100 KW DC or pulsed lasers are possible, and it is also used in destroying UAV but with heating mechanism. It seems there is a possibility to combine these two directed energy technologies together to form a new weapon. A ring plasma channel with different reflective index can be formed in air with DC or pulsed HEL, and the HPM can be confined inside the plasma channel to travel to a greater distance without as high attenuation as in open air. Also possible is several plasma photonic-crystal structures can be formed by HEL to trap the HPM inside such structures to transfer the HPM to a longer distance. Each approach comes with its own tradeoffs in total energy efficiency and performance. This HEL outside, HPM inside directed energy weapon can have both the heating mechanism of the HEL and the breakdown mechanism of HPM and maybe described as “Photon Torpedo”. Simulations are used to estimate the field confinement effect of such Photon Torpedo.

        Speaker: Prof. Shen Shou Max Chung (Department of Electrical Engineering, National Penghu University, Penghu, Taiwan, R.O.C.)
      • 13:00
        2P35 - Investigation into the Propagation of Electron Beams of Different Shapes through Gas-Filled Space Using PIC Simulations 1h 30m

        The propagation of moderately high energy (10-100 keV) electron beams through gas-filled tubes has been being long studied for various potential applications, such as microwave generation, EUV/X-ray radiation and surface modification [1-3]. However, it appears that not much attention has been paid to understand the mechanism as to how the e-beam cross-sectional shape affects the breakdown of a gas by beam electron impact ionization and how the self-focusing of e-beam by ion channel as well as eventual formation of instabilities under certain conditions takes place [3].This paper attempts to develop the understanding of such a mechanism by making a comparative investigation into the elecron beam propagation of solid cylindrical and annular electron beams through a gas-filled tube, using PIC simulation, under typical operating pressures (5-50 Pa), beam energies (10-50 keV) and beam currents (10-100 A). Analytical formulation of space-charge limiting current for different beam shapes along with the spatial and temporal evolution of beam envelope and cross-section is presented. It has been found that the accumulation of ion channel triggers instabilities deteriorating the beam quality, which happens much earlier in a solid cylindrical beam than in an annular beam. This has been quantitatively inferred based on the dependene of self-focusing behavior, controlled by the space-charge potential and charge-neutralization factor, on beam shapes. Several results investigating the role of beam and plasma parameters in the electron beam propagation through a gas-filled space have also been preented. It is worth extending the scope of the present simulation to study an e-beam penetrating through such a gas-filled space for beam-plasma convective instability in a beam-plasma amplifier.

        [1] Varun et al., IEEE Trans. Plasma Sci., vol.46, no.6, pp.2003-2008, 2018.
        [2] N. Kumar, et al., Appl. Phys. Lett., 111, 213502, 2017.
        [3] U. N. Pal, et al., IEEE Trans. Plasma Sci., vol.45, no.12, pp.3195-3201, 2017.

        Speaker: Udit Narayan Pal (CSIR-Central Electronics Engineering Research Institute, Pilani, India)
      • 13:00
        2P36 - Electron Temperature and Density Measurements of Plasma Generated at the Focus of a CW Microwave Beam 1h 30m

        An experimental setup to study plasma generated at the focus of a continuous-wave (CW) microwave beam was designed and constructed at the Air Force Research Laboratory at Kirtland AFB, NM. Experimental studies of free space plasma are of interest because they can help validate and improve theoretical models, such as the Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC) code. Free space plasma does not interact with bounding wall surfaces, which help prevent non-ideal effects, such as contamination and secondary electron emissions, from influencing the experimental results. In our experimental setup, free space plasma is generated by a multi-kW, 4.7 GHz CW microwave system at pressures ranging from 100 to 200 mTorr. A precision mass flow system controls the composition of the gas used to generate the plasma. Gas pressure, gas composition (a mixture of Ar, N2, and O2), and the power of the microwave beam are varied to study their effects on the stability, uniformity, and parameters of the plasma. Invasive and non-invasive plasma diagnostic methods were implemented to measure the electron temperature and density of the plasma. In addition, simulations of the plasma generated in our experiment were conducted with GlobalKin, a zero-dimensional global-kinetics model, using estimates of the total power absorbed by the plasma generated under different conditions. The results from the experiments and simulations conducted to date will be presented.

        Speaker: Adrian Lopez (Air Force Research Laboratory)
      • 13:00
        2P37 - Multipactor in Coaxial Transmission Lines 1h 30m

        Despite decades of research, understanding and prediction of multipactor in complex geometries remains predominantly empirical. This results in large safety margins in RF design, driving up production and deployment costs to prevent costly operating disruptions or complete device failure.
        As part of a Multi-University Research Initiative (MURI) led by Michigan State University, the University of Michigan is investigating multipactor discharges in coaxial geometry. The asymmetric field intensity of coaxial geometry results in different transit times for electrons born on the inner or outer surface of the coaxial line, altering the resonant conditions typically observed in parallel plate multipactor. Minimal experimental data on coaxial multipactor exists in the public domain [1,2]. The published data are for a limited set of materials, and for frequency-gap products, fd, below 3 GHz-mm.
        To characterize and mitigate multipactor, we have built a coaxial test chamber comprised of OFHC copper tubing with a replaceable test region. The inner conductor is increased in diameter in the test region using ¼-wave step transformers to allow a range of gap distances, d, to be explored. Coupled with frequencies, f, ranging from 0.9 to 2.835 GHz, this test setup will acquire multipactor susceptibility information for fd > 3 GHz-mm, validating concurrent analytic and computational work. Initial coaxial multipactor susceptibility data will be presented.

        [1] R. Woo, “Multipacting Discharges between Coaxial Electrodes,” Journal of Applied Physics, vol. 39, no. 3, pp. 1528–1533, Feb. 1968.
        [2] T. P. Graves, “Experimental investigation of electron multipactor discharges at very high frequency,” Ph.D. Thesis, Massachusetts Institute of Technology, 2006.

        Speaker: Dr Nicholas M. Jordan (University of Michigan)
      • 13:00
        2P39 - Suppressing single-surface multipactor discharges using non-sinusoidal electric field 1h 30m

        Multipactor discharge is a major concern in a multitude of electromagnetic devices, often requiring suppression to properly operate devices and avoid damage. The factors affecting multipactor discharges are mainly from the dielectric window properties and the field distribution. Modifying the window geometry including periodic grooves on the window surface in rectangular or triangular shape, and applying an external dc electric field pointing into the dielectric window or an external dc magnetic field parallel to the surface (perpendicular to the tangential rf field) were discovered to effectively reduce multipactor in the previous studies [1-4]. In our work, both particle-in-cell (PIC) and Monte-Carlo simulations demonstrate that applying a temporal Gaussian-type electric field can suppress single surface-multipactor discharge. Decreasing the half peak width of the Gaussian electric field can reduce the time-averaged multipactor intensity by an order of magnitude at fixed input power. PIC simulation reveals the underlying physical mechanism by examining the electron
        impact energy and angle distribution, as well as the dynamic secondary electron yield (SEY). At smaller half peak width, more electrons striking the surface have energies below the first crossover energy of the SEY curve, and a small fraction of electrons have energies higher than the second crossover of the SEY curve with fixed input power, giving rise to weaker secondary electrons emission and a weaker multipactor discharge.
        Acknowledgment: This work is supported by AFOSR MURI Grant No.FA 9550-18-1-0062.
        [1] A. Neuber, G. Edmiston, and J. Krile et al, IEEE Trans. Magn. 43, 496 (2007).
        [2] G. Edmiston, A. Neuber, and H. Krompholz et al, J. Appl. Phys. 103, 063303 (2008).
        [3] C. Chang, G. Liu, and C. Tang et al, Appl. Phys. Lett. 96, 111502 (2010).
        [4] A. Valfells, L. K. Ang, and Y. Y. La et al, Phys. Plasmas 7, 750 (2000).

        Speaker: Dr Deqi Wen (Michigan State University)
      • 13:00
        2P40 - Linear plasma experiment for non-linear microwave interaction experiments 1h 30m

        As a non-linear medium, plasma can exhibit diverse dynamics when excited by multiple EM waves. Electromagnetic waves are vital to the introduction of energy in laser plasma interactions and the heating of magnetically confined fusion reactors. In laser plasma applications Raman coupling via a Langmuir oscillation or Brillouin scattering mediated by ion-acoustic waves are of interest. Signals with normalised intensities approaching those used in some recent laser plasma interactions can be generated using powerful and flexible microwave amplifiers, interacting in relatively tenuous, cool and accessible plasma. Other multi-wave interactions are interesting for magnetic confinement fusion plasmas, for example beat-wave interactions between two microwave signals coupling to cyclotron motion of the ions and electrons or the lower hybrid oscillations may be useful in heating the plasmas or for driving currents.

        A linear plasma experiment is being built to test such multifrequency microwave interaction in plasma, based on prior research on geophysical cyclotron wave emission and propagation [1,2]. The main section of the plasma will be magnetised at up to 0.05T, with the plasma created by an RF helicon source to generate a dense, large, cool plasma with a high ionisation fraction. A range of frequency-flexible sources will provide microwave beams to enable multi-wave coupling experiments. The paper will present progress on this apparatus and experiments.

        The authors gratefully acknowledge support from the EPSRC, MBDA UK Ltd and TMD Technologies Ltd.

        [1] Ronald K., Speirs D.C., McConville S.L., Phelps A.D.R., Robertson C.W., Whyte C.G., He W., Gillespie K.M., Cross A.W., Bingham R., 2008, Phys. Plasmas, 15, art.056503

        [2] Speirs, D.C., Bingham, R., Cairns, R.A., Vorgul, I., Kellett, B.J., Phelps, A.D.R., Ronald, K, 2014, Phys. Rev. Lett., 113, art 155002

        Speaker: colin whyte (University of Strathclyde)
      • 13:00
        2P44 - Perspectives of Supercritical Fluids for Switching Applications 1h 30m

        Fast and repetitive switching in high-power circuits is a challenging task where the ultimate solutions still have to be found. Areas of application are power switches in high-voltage networks and heavy duty switches for pulsed power applications.

        Supercritical switch media
        We propose a new approach: the use of supercritical fluids as switching medium. Supercritical fluids have insulation strength and thermal properties like liquids and fluidity, self-healing and absence of bubbles like gases. These properties are very beneficial of power switching, and in particular allow very high breakdown voltages (thus compact switches) and very fast recov-ery behaviour (thus repetitive switches). We will pre-sent the concept of a supercritical switch, and data of breakdown behaviour of a prototype supercritical switch. In addition, a model for calculating the re-covery time will be presented, supported by experimental data on the recovery behaviour of supercritical nitrogen.

        Speaker: Prof. Guus Pemen (Eindhoven University of Technology)
      • 13:00
        2P45 - High performance triggering transformer for stack of series connected thyristors 1h 30m

        Large Hadron Collider (LHC) - the world biggest and highest energy proton accelerator/collider is built on Switzerland/France border at ~100 m underground. Its circumference is 27 km and it will accelerate up to 4e14 protons per beam to a peak energy of 7 TeV. Under these conditions energy of each beam will be 360 MJ. Safe dumping of the beam with such energy is crucial for the safety of the LHC.
        LHC beam dumping system (LBDS) consists of 30 extraction and 20 dilution generators delivering altogether ~1MA. Extraction generator operates at up to 29 kV and delivers up to 18 kA peak current with pulse duration of 91 us. It employs 2 parallel stacks of 10 series connected fast thyristors with 80 kA rating. Thyristor commutation speed depends on the triggering pulse performance with recommend triggering current peak value 2 kA with slew rate of 5kA/us. Triggering is ensured by a triggering transformer (TT) with a primary driven by two triggering generators operating at 3.5 kV and with multiple floating secondaries individually supplying each thyristors within the stack. Presently used TT (custom designed) limits the triggering current to 500 A peak with slew rate of 400A/us. Ongoing upgrade of LHC calls for increasing of the whole LBDS reliability including triggering system. Main modifications target reduction of TT and cabling inductances. The new TT has fully coaxial design with common primary and ten single turn secondaries with independent magnetic circuits. HV triggering cables are made of 12 parallel twisted pairs with common shielding. Significantly reduced total stray inductance resulted in more than 3x higher peak current and 10x higher dI/dt (1.8kA, 4kA/us respectively) with the same trigger generator voltage. The whole thyristors stack turn-on delay and rise time were reduced by more than 200 ns with proportionally reduced turn-on losses.

        Speaker: Viliam Senaj (CERN)
      • 13:00
        2P46 - Data acquisition system for HEH monitor 1h 30m

        Reliable operation of the Large Hadron Collider Beam Dumping System (LBDS) is vital for the machine safety. The role of the LBDS is to extract two counter rotating beams and to dispose of them safely on their respective 8 m long graphite dump blocks. The LBDS consists of 50 pulse generators located in the LHC galleries: each generator is connected to a kicker magnet by, 30 m long, coaxial cables. The pulse generators contain High Voltage (HV) semiconductors, which are susceptible to Single Event Burnout (SEB) - a catastrophic phenomenon for HV semiconductors - due to High Energy Hadrons (HEH). In order to better assess the HEH flux and impact, the development of an HEH monitor, based on the SEB phenomenon in HV Si diodes, is ongoing. This will improve the accuracy of SEB related failure rate estimates and will help to guide mitigation measures. A low cost acquisition system for the HEH monitor was developed. The acquisition system is based on a micro-controller continuously checking new events in up to 16 independent channels each with up to 30 HV diodes per channel: the number of channels and diodes per channel are adaptable according to the sensitivity required. To avoid false counts and coupling effects an adjustable ‘dead-time’ filter is used for each channel. Periodic ‘alive’ signals and HEH events related information, such as total number of counts, HV value, temperature and total current consumption measurements are both sent to a database, via Ethernet or Wi-Fi connection, and stored in an internal USB or SD card as a backup. In addition, the system allows remote control of the sensitivity of the monitor by modifying the voltage applied to the HV diodes. The whole system has already experienced several irradiation campaigns without any malfunction.

        Speaker: David Cabrerizo Pastor (CERN)
      • 13:00
        2P47 - Development and Switching Characterization Study of Hot Cathode Thyratron for Pulse Modulator Applications in Linear Accelerator 1h 30m

        High power plasma switches, such as, hot and cold cathode thyratrons have always been the key components of pulsed power systems including pulse modulators, linear accelerators, synchrotron sources, crowbar circuits, cargo scanning systems, sterilization, etc. These switches are classified by their unique pulse characteristics and performances. The unique features of hot cathode thyratron are long lifetime, higher efficiency, moderate rate of current rise (1010A/s), better jitter and high stability. The paper has represented the recent technological efforts made by CSIR-CEERI, India for the design, development and switching characterization of high power 35kV/3KA hot cathode thyratrons for their potential applications in pulse modulators in linear accelerator. The proposed thyratron is a multi-gridded geometry which mainly consists of oxide coated cathode, reservoir/getter, pre-ionize grid, control grid, cathode and anode. The characterization and emission study of the oxide coated cathode in the pre-ionize grid assembly have been performed at different operating conditions in a suitable diode assembly. This has assured for the proper and uniform emission of the electron from the cathode. Deuterium gas has been used in place of hydrogen to improve the hold-off voltage characteristics. The parts and assemblies of the pre-ionizing grid, control grid and anode are designed and fabricated in such a way to have low misfire and quenching free switching at the designed and operating parameters. The switching characterization of the thyratron have been performed for its maximum peak parameters (35kV, 3kA), pulse width of ~5 µs at different operating conditions. The switching performances have been optimized for the range of different parameters, such as, pressure, cathode current/voltage, reservoir current/voltage, etc. It is showing less than 5 ns jitter which makes it suitable of above mentioned applications. The design, fabrication, processing, development and characterization issues of the thyratron plasma switch have also been presented.

        Speaker: Udit Narayan Pal (CSIR-Central Electronics Engineering Research Institute, Pilani, India)
      • 13:00

        Pseudospark discharge are widely used in high power switches, intense electron beam generation and extreme ultra violet light source. Current quenching is one of the most important problems that seriously hamper the applications of the pseudospark discharge, which is characterized by the appearance of sudden current interruption and inductive voltage spikes at the same time. In this paper, current quenching phenomena are studied experimentally by testing the pseudospark discharge device under various conditions. It is observed that current quenching may occur at the starting, rising, peak, descending and zero-crossing phases of the current waveforms. Four main patterns are summarized from the testing results. The first kind occurs at the transition of current from hollow cathode discharge phase to high current conduction phase; the second kind is the oscillation superimposed on the whole current rising edge; the third kind is the temporary extinction of current at zero crossing; and the last one occurs in the high current conduction phase. Previous studies have focused on quenching correlated with the transition two discharge phases at current rising edge. Its mechanism is mostly believed to be the ion depletion near the cathode surface, while it might need amendment for the other quenching patterns. The details related to specific pattern are discussed. The results in this study could provide further understanding for current quenching.

        Speaker: Jiaqi Yan (Xi'an Jiaotong University)
      • 13:00
        2P49 - Performance of 20-kV, 20-A Silicon Carbide High-Voltage Modules 1h 30m

        This work describes laboratory measurements of recently-fabricated, state-of-the-art Silicon Carbide (SiC) Insulated-Gate Bipolar Transistors (IGBTs) developed for medium-voltage converters and pulsed-power applications. These next-generation, monolithic devices, have an active area of 0.3 cm2, a drift region of 160 µm, and are rated for 20-kV and 20 A. The IGBTs were co-packaged with SiC JBS/PiN anti-parallel diodes in a half-bridge configuration and utilize Al2O3/SiN substrates for improved thermal performance. In this paper the characteristics of the modules are presented with a focus on switching and conduction losses, power dissipation, burst-mode, and continuous operation. This work supports the U.S. Army Research Laboratory mission to push existing state-of-the-art SiC IGBT technology beyond its current state, to drive innovation in device process, fabrication, design, and packaging, to steer design of SiC IGBTs for pulsed power and low-duty-cycle continuous power Army applications, and demonstrate latest SiC technology with small scale prototypes.

        Speaker: Miguel Hinojosa (Army Research Laboratory)
      • 13:00
        2P50 - Surface Passivation of GaAs Photoconductive Semiconductor Switches with Silicon Resin 1h 30m

        The surface defect states of GaAs substrate may cause surface flashover which results in the immature breakdown of the GaAs based photoconductive semiconductor switch (PCSS) devices. It is also expected that the shorter carrier life time and the high surface recombination velocity at the GaAs surface have impact on the performance of GaAs based PCSS devices.
        In this study, the surface passivation effect on the performance of the lateral type 2-mm-gap GaAs PCSS is investigated. The 200-nm-thick SiNx grown by plasma enhanced chemical vapor deposition, and drop casted 1-cm-thick silicon resin have been considered as passivation materials on the GaAs PCSS surface. The operational characteristics of the GaAs PCSS were measured by using 1064-nm triggering laser exhibiting a nominal illumination optical energy of 135 μJ and an optical pulse width of 700 ps. It is shown that the surface passivation can increase the carrier lifetime and decrease the surface recombination velocity by reducing the density of the surface states at the GaAs surface. Thereby, the surface passivated PCSSs exhibit the higher pulse height and the longer pulse width compared to non-passivated devices. It is also noted that the surface passivation of the PCSS retards the onset of surface flashover of the PCSS leading to the higher voltage operation capability. The PCSS without surface passivation suffers from flashover at 2.4 kV and permanently fails after 200 times of operation due to the cracks formed on the surface. The PCSS passivated with silicon resin successfully operates without surface flashover up to the bias voltage of 4 kV. The output characteristics of the GaAs PCSS with and without passivation will be compared in terms of the onset voltage of surface flashover, the height and the width of the electrical pulses. This work was supported by KEPCO (R18XA06-79) and Korea Agency for Defense Development.

        Speaker: Mr Yong-Pyo Kim (Gwangju Institute of Science and Technology)
      • 13:00
        2P51 - Comparison of Lateral and Vertical Photoconductive Semiconductor Switches Fabricated on 4H-SiC 1h 30m

        Silicon carbide (SiC) based photoconductive semiconductor switches (PCSSs) are interesting because of their potential for higher voltage operation originating from the excellent material properties of SiC. However, the experimentally demonstrated performances of the SiC based PCSSs are still far inferior to those of the GaAs counterpart. To harness their potential inherited from the excellent material properties, more researches are required to improve the device performances.

        In this study, the lateral- and vertical-type 4H-SiC PCSSs are fabricated and their performances are compared. The 500-μm-thick, high purity semi-insulating 4H-SiC substrates are utilized to fabricate two different types of PCSS devices. The optoelectronic conversion characteristics of the two types of PCSS devices were measured by using 532-nm-wavelength triggering laser under the applied bias voltages up to 3 kV. The vertical-type PCSS outperforms the lateral type PCSS in various aspects. The vertical-type PCSS exhibits 12-times higher output voltage and 3-times wider full-width-half-maximum (FWHM) pulse width when compared with the lateral-type PCSS for same bias voltage and same irradiation conditions. The vertical PCSS enjoys longer optical path length leading to higher photocurrent. The current conducting channel is formed in the bulk of the vertical PCSS, but the dominant channel of the lateral-type PCSS is formed along the top surface of the device. The high recombination velocity at the SiC surface makes the electron-hole pairs generated close to the surface hard to be collected by the anode and cathode electrodes, which lowers the peak voltage and shortens the output pulse width. It is also noted that the vertical-type PCSS outperforms lateral-type PCSS in terms of high voltage operation capability. The vertical PCSS structures effectively suppress the effect of the imperfect SiC surface and achieves better performance compared with the lateral PCSS structures. This work was supported by KEPCO (R18XA06-79) and Korea Agency for Defense Development

        Speaker: Pyeunghwi Choi (GIST(Gwangju Institute of Science and Technology))
      • 13:00
        2P53 - The Influence of Electrode Profile on Repetition Performance of Corona-stabilized Switch 1h 30m

        Gas-discharge closing switches usually have more poor repetition performance than semiconducting switches. This is due to the recovery of gaseous insulation takes a lot of time, and can restrict the use of the gas switch in repetitive applications. The corona-stabilized switch is a potential plasma closing switch that has excellent PRF (pulse repetition frequency) capability. The electric field inhomogeneity of the switch is considered to have an important influence on the stabilization effect of corona plasma. In this paper, effects of electric field inhomogeneity on recovery rate and repetition performance of gaseous insulation are studied.
        A double-pulse power supply, which is capable of generating two continuous voltage pulses with adjustable voltage amplitude and interval time, is used. A corona-stabilized switch with a single rod-plane electrode and gaseous medium of SF6 is investigated. Recovery rate of gaseous insulation in switch is obtained by comparing the breakdown voltage of the first pulse to that of the second at different pulse intervals (from 20μs -1s). Several rod electrodes are tested and the results are compared. Repetition performance under different electric field inhomogeneity is investigated by a repetition rate pulse generator. Charging and breakdown waveforms under a continuous repetitive mode are recorded. The repetition performance of the switch can be indicated by divergence of breakdown voltage. The results are explained by calculating corona critical volume under different electrode profiles. Further, physical model of corona stabilization at negative impulse in SF6 are proposed to help one to better understand the breakdown and recovery characteristics of electric field inhomogeneity dependence, relates the spatial extension of the corona plasma in an inhomogeneous field gap to the inhomogeneity of electric field. The trend of breakdown voltage according with pulse number indicates the relationship between electric field inhomogeneity and cumulative effect that result from continuous repetitive gas-discharge discharge.

        Speakers: Longjie Li (Xi'an Jiaotong University), Mr Yongsheng Wang (Xi’an Jiaotong University)
      • 13:00
        2P54 - Polarity Effect of Repetitive Corona Stabilization Breakdown 1h 30m

        Gas-discharge closing switches have poor repetition performance, which is due to the gaseous insulation will decrease for a short time after the last breakdown , and therefore the gas switch will usually close at a much lower voltage than the initial breakdown voltage. The corona-stabilized switch is based on corona stabilization phenomenon that a space charge develops around the highly stressed electrode and prevents premature breakdown to take place. This means the switch can readily operate with high repetition rates at high operating voltages.
        Polarity effect in highly non-uniform electric field is a well-documented discharge feature. The electrode arrangement of the corona-stabilized switch results in a typical highly non-uniform electric field. In this electrode gap, ionization always begins from the rod electrode. But the drifts of space charge under positive and negative impulses have significant difference, which leads to different corona stabilization effects. And the difference is supposed to have a noticeable impact on corona stabilization breakdown. In this paper, polarity effect of repetitive corona stabilization breakdown is investigated. Double-pulse method is employed to investigate the insulation recovery rate between single rod and plane electrode. Recovery rate curves for hold-off voltage are obtained under both positive and negative pulses. Indexes for Repetition performance of corona stabilization breakdown, such as divergence of breakdown voltage, failure rate and cumulative effect, under different external impulse polarity are also investigated by a repetition rate pulse generator. Kinematic law of space charge in corona stabilization breakdown process is studied. The physical model of corona stabilization breakdown in gas switch is established under both positive and negative pulses.

        Speaker: Dr Longjie Li (School of Electrical Engineering, Xi’an Jiaotong University)
      • 13:00
        2P55 - On the performance of triggered closing switches deployed in high explosive pulsed power experiments* 1h 30m

        High explosive pulsed power experiments conducted by Lawrence Livermore National Laboratory employ several different triggered closing switches. Since experiments are single-shot events, failure in any one of these switches can be catastrophic to the experiment outcome. Thus, repeatability and reliability are key metrics in the assessment of closing switch performance. Presented in this paper are efforts to improve the performance of a triggered closing switch system used in a 450 kilojoule capacitor bank, which is used as a seed current source for magnetic flux compression generator experiments. The capacitor bank switch utilizes two different triggered closing switches: a commercial-off-the-shelf pressurized spark gap and a Livermore-designed solid dielectric puncture switch. Discussion of the commercial spark gap will focus on the results of an experimental investigation into switch reliability - specifically to determine the pre-fire probability. A campaign aimed at improving the repeatability of the solid dielectric puncture switch will be detailed, where almost a two-fold decrease in the average switch function time was observed by reducing the thickness of the solid dielectric. Data captured during the preparation and execution of high explosive pulsed power experiments will also be shown, encompassing the results of these improvement efforts.

        *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

        Speaker: Dr Andrew Young (Lawrence Livermore National Laboratory)
      • 13:00

        HERMES III is a 20-MeV linear induction accelerator that was constructed at Sandia National Laboratories in the late 1980’s and continues operation to this day. The accelerator utilizes 10 Marx banks for its initial energy storage and pulse formation. These Marx banks discharge their energy into 20 intermediate storage capacitors which, in turn, feed 80 pulse forming lines that further condition the pulse. Transmission line feeds from the pulse forming lines then deliver the electrical energy to 20 induction cavities arrayed along the axis of the machine to build the final output pulse along a central magnetically insulated transmission line (MITL). There are two triggering systems within the accelerator that work together in this energy discharge process. One simultaneously triggers the initial energy discharge of energy from each of the 10 Marx banks; the other staggers the triggering of the Rimfire gas switches following each intermediate storage capacitor to synchronize the energy delivery to the downstream cavities with the pulse already propagating along the MITL from the upstream cavities. Until recently, these triggering systems were the original systems dating back to the initial commissioning of the accelerator, however both have now been replaced with new and more modernized systems. Design details for both triggering systems will be presented, along with an overview of some of the initial operational data from the HERMES III accelerator using these new triggering systems.

        Speaker: Dr Chris Grabowski (Sandia National Laboratories)
      • 13:00
        2P57 - Silicon Carbide drift step recovery diode structures evaluated as >10kV nanosecond pulse power switches using Mixed-Mode simulation 1h 30m

        Mixed-Mode modeling, is a combination of SPICE circuit and finite element device physics, is used to evaluate SiC drift step recovery diode structures for optimal operation as opening switches for the generation of narrow high voltage pulses in a single loop resonant LC circuit. This comparison is made as a practical means to define device design elements and process steps prior to the fabrication of actual SiC devices.
        Parameter entitlements of single devices are made for diodes in the 2000 – 3000 voltage rating range, anticipating serial stacks of these devices to achieve > 10 KV peak nanosecond voltage pulses. Epitaxial structures, high voltage terminations, and carrier lifetime are explored as device variables, with resultant peak voltage, rates of rise, and voltage gain are calculated across a 50-ohm resistive load.
        Model results to date indicate for single devices with peak voltages of ~ 2600 volts, dV/dt (90 to 10% of peak) ~2100 volts/nanosecond are possible, pulse FWHM of 2 nanoseconds, and voltage gains (Vpeak/Vsupply) in the range of 12 to 17 can be achieved.
        Such devices have potential uses in a variety of pulse power applications e.g. ignition, cell membrane modifications, and environmental (pollution) control.

        Speaker: Mr Stephen Arthur (GE Global Research Center)
      • 13:00
        2P59 - A High-gain nanosecond pulse generator based on inductor energy storage and pulse forming line voltage superposition 1h 30m

        Pulsed gas discharge is an important means of generating low temperature plasma. Short pulses with fast frontier show superior performance in terms of increasing the active particle content, ionization coefficient and electron conversion rate due to its higher voltage rise rate. The common nanosecond pulse generator is based on capacitive energy storage. Compared with the nanosecond pulse generator based on capacitive energy storage, the inductive energy storage has outstanding advantages in energy storage density, miniaturization of the device, and less influence of loop inductance. However, the inductive energy storage also suffers from problems such as limitation of disconnect switch, uncontrollable outputs and waveform distortion.
        In this paper, the inductance unit in the transmission line is used as the energy storage inductance, and combined with the characteristics of the rectangular pulse output of the transmission line, and the modular voltage superposition is carried out by using the propagation delay of electromagnetic wave in the transmission line to achieve high-gain rectangular nanosecond pulse output.Then we expand the design of the terminal superposition structure, optimize the magnetic field distribution between the lines to reduce the waveform distortion, and output the nanosecond short pulse.Finally, the paper analyzes the load matching characteristics of the designed pulse generator and provides experimental support for the actual application of the generator.In this paper, the superposition experiment of 10-stage inductive energy storage modules was carried out. The experimental results show that the time-delay isolation method of transmission line can effectively isolate the pulse voltage at the front and rear. The volume of the 10-stage circuit module is 25 cm6 cm12 cm, rectangular waveform output, the charging voltage is DC 58 V, the voltage amplitude is 8.2 kV, the voltage gain is about 140 times, the pulse duration is 23 ns and the rise time is 8 ns.

        Speaker: Jianhao Ma (Chongqing University)
      • 13:00

        Cable is the main component of the network of power system. It is usually laid in the underground corridor and its function is to transmit and distribute electrical energy. As with further development of urban construction, cable usage is inevitablty getting higher and higher in hub substation with 110kV or higher voltage levels. Meanwhile, high voltage single-core XLPE insulated power cables will be widely used. In engineering, high voltage level and low voltage level cables share a tunnel or pipe jacking is very common, which can significantly increase the current carrying capacity of the cable. However, the induced voltage of the metal sheath of the single-core cable and the spatial temperature field will be significantly changed.

        In this paper, long-distance 330/110kV cable shared the same pipe jacking is taken as example to compare the two types of cross-interconnected methods. First, the appropriate segment length is determined based on the value of sheath induced voltage under steady state. Then, the sheath induced voltage of transient fault is analyzed, including many single-phase ground faults which occur in different places of the cable. Finally, the influence of cable length growth under steady state and transient conditions is discussed.

        It is found that the steady-state induced voltage of the sheath increases linearly with the increase of the length of the cross-interconnected small segment. No matter where the fault occurs, the transient induced voltage at the cross-interconnected point closest to the fault point is the highest. If the distance from the fault point increases, the induced voltage will gradually decrease. When fault occurs in the 330kV cables, the sheath induced voltage of the 110kV cables will have a great impact.

        Speaker: Mr Shuhan Liu (School of Electrical Engineering, Xi’an Jiaotong University)
      • 13:00
        2P62 - A comprehensive design procedure for high voltage pulse power transformers 1h 30m

        Typical pulsed power applications cover the field as e.g. collision and fusion experiments or the generation of high temperatures, as well as the generation of X-rays in medical applications [1]. In these applications, pulsed power modulators are used for generating highly accurate high voltage pulses with very fast rise and fall times and pulse widths from microseconds to milliseconds. In order to produce such very fast rising voltage pulses, pulse transformer based modulator systems are utilized. The rise and fall times can be directly adjusted by the leakage inductance and the stray capacitance of the transformer. Therefore, the proposed design method combines calculation of parasitics with isolation design and dynamic voltage distribution design within the windings.
        In the considered application, the required nominal pulse voltage amplitude is 44.2 kV with a pulse power of 4.42 MW, a pulse length of 5 us and a maximal rise time of 1us. In this paper, a comprehensive design procedure for high voltage pulse power transformers is presented. The procedure is based on the finite element method (FEM) and contains an electrical model, a magnetical model, a thermal model of the transformer and a procedure for the isolation design. In addition, to avoid over voltages within the winding, a model for the dynamic voltage distribution is included in the approach as well. For validation of the models and the design procedure, a prototype has been built and is tested under full load conditions. There, the main focus is on evaluating the parasitics, which are crucial for the shape of the output voltage pulse. Further, the isolation design will be proofed by high voltage impulse tests.

        [1]…D.A.Gerber, "Ultra-Precise Short-Pulse Modulator for a 50 MW RF Output Klystron for Free-Electron Lasers," Ph.D. dissertation, ETH Zürich, 2015

        Speaker: Dr Michael Jaritz (University of Applied Scienes Rapperswil)
      • 13:00
        2P65 - Design of A Long Pulse High Energy Water Transmission Line to Drive HPM Sources 1h 30m

        The University of New Mexico has designed and built a 10-stage high energy Marx generator with a pulse length of 1.5 Microseconds. This Marx generator has the capability to drive low impedance HPM loads such as the Magnetically Insulated Line Oscillators as well as HEDP loads like a DPF. A transmission line coupling the Marx to the load is the typical energy transfer mechanism for such a system. However, these loads require both fast rise times as well as relatively flat top peaks as both coupling of electromagnetic fields to cavities and plasma density during stagnation are sensitive to the onset of peak fields and to peak voltage variations. Additionally, both loads are sensitive to capacitive coupling of the Marx charge voltage to the load-leading to premature plasma production and possibly leading to instabilities in the run down phase of DPF devices as well as early onset of neutral desorption in HPM sources. All of these problems must be addressed by careful transmission line design. To this end, this presentation discusses time- and frequency-domain finite element electromagnetic numerical simulations of a 2.5-meter-long, high dielectric constant (81) water transmission line with a spark gap peaking switch, as well as with an additional pre-pulse peaking switch.

        Speaker: Dr Salvador Portillo (University of New Mexico - Electrical and Computer Engineering Department)
      • 13:00
        2P66 - A 1 MV Tesla pulsed transformer 1h 30m

        A 1 MV transformer, based on Tesla technique, was designed and most of its components were already manufactured. The paper will present the calculations performed for obtaining an optimum design for the major components of the arrangement, such as the windings and the 1 MV switch connecting the load.

        Speaker: Mr Matthew Woodyard (Loughborough Univesrity)
      • 13:00
        2P68 - Design of a Dielectric Compression Bushing for Compact, High-Voltage Applications 1h 30m

        High voltage insulation methods that can easily be assembled and disassembled are of use on compact pulsed power systems. High voltage isolation systems are a key element for reliable pulsed power operation. Verus Research’s Dense Plasma Focus (DPF) system, under development as a radiation test source for the U.S. Army, requires an electrical-mechanical interface that can withhold a minimum of 100 kV peak voltage in air, during nominal operation over a small distance. A solid dielectric compression washer was used to make a critical seal providing high voltage isolation. A layered design constructed of thin Kapton film was fabricated to provide a long tracking path and sufficient dielectric strength with minimal inductance to prevent failure of the bulk material and transfer electrical stress upon the high voltage compression region. A test apparatus was designed and fabricated to test the failure point, and to identify the failure mechanisms. Multiple materials, such as silicone and urethane, as well as different compression concepts were tested to failure using the test apparatus and a 1 stage MARX to generate a high voltage pulse up to 200 kV for the test. Results from electrostatic modeling and empirical testing of the high voltage designs are presented here, as well as findings for the leading breakdown failure mechanism.
        This work is performed under contract through the U.S. Army’s Program Executive Office for Simulation, Training, and Instrumentation (PEO STRI) and funded by the Test Resource Management Center’s Test and Evaluation/Science and Technology (TRMC T&E/S&T).

        Speaker: Dr Michael Butcher (Verus Research)
      • 13:00
        2P69 - A sequential characterization method for the insulation evaluation of the rod-plane gap under repetitive frequency nanosecond pulses in high-pressure nitrogen 1h 30m

        The high voltage repetitive frequency nanosecond pulse (RFNP) generator is an important equipment in many industrial and scientific applications, including the plasma-assisted combustion and plasma-surface interaction. The gas-insulated gap, one of the typical insulation infrastructures, is required to withstand RFNP. It is usually accepted that the insulation capacity of the gas gap under RFNP is much lower than that under single pulse due to the accumulation of metastable species, electrons and positive/negative ions. However, little research has been devoted to systematically characterize the dynamic evolution of the insulation capacity under RFNPs in high-pressure gas. In this paper, a sequential characterization method and typical influential parameters were investigated. Every voltage and emission light intensity in a sequence were acquired in the sequential acquisition mode to achieve the pulse sequence resolution.

        Under positive RFNP, the number of pulses before breakdown Ni from 0.1 to 0.4 MPa was obtained with the same voltage working coefficient. It was found that Ni dramatically and nonlinearly depended on the pulse repetitive frequency (PRF). The emission intensity of corona discharge under positive RFNP was found in the discrete mode in a sequence and the repetitive frequency of corona discharge was closely related with the PRF. On the contrary, under negative RFNP, the corona discharges in a sequence were in the continuous mode. The latter average emission intensity of corona discharges was lower than that of the first corona discharge. Meanwhile, the inception time of following corona discharges decreased with the PRF.

        The chemistry and diffusion dynamics during the pulse-off period was found to be vital for the discharge characteristics under RFNP. The proposed sequential characterization method illustrated advantages for the complete description of insulation strength evolution of the rod-plane gap under RFNP in nitrogen and was beneficial for understanding the space charge behaviors.

        Speaker: Mr Zheng Zhao (Xi'an Jiaotong University)
      • 13:00
        2P70 - Experimental approach of the dielectric strength of a vacuum insulator 1h 30m

        One of the main limiting factors in the design of high-power vacuum systems is a surface flashover occurring over the insulator surface between two conducting regions separated by a high-voltage gap. To decrease the probability of this phenomenon, traditional approaches include an increase in the area of insulators, screening tricks of triple junctions and limitation techniques of the secondary electron multiplication.
        The design of multi-pulse systems such as multi-MeV, multi-kA induction injector needs to consider radial vacuum insulator stacks which can withstand multiple (two or more) mechanical impulses and multiple electric stresses coming from the generation of multiple high-power pulses. Those high-voltage components are generally designed from J.-C. Martin and Bluhm laws describing the breakdown probability as a function of the electric field, surface concerned and time of exposure.
        We take the opportunity of having a dual-pulse high-voltage generator to manage an experimental study on a dedicated setup in order to improve our understanding breakdown phenomena in such geometries and to verify the validity of Martin’s law for a two times stressed insulator. The experimental setup is a vacuum chamber in which one can test different dielectric materials compressed between different electrode shapes and stressed by two 250kV-70ns high voltage pulses. Main results of the first campaign will be presented.

        Speaker: Baptiste Cadilhon (CEA)
      • 13:00

        With the increasing usage of sulfur hexafluoride, the disadvantages of its high GWP are becoming increasingly prominent. Therefore, looking for alternatives to SF6 gas to promote gas-insulated technology to an environmentally friendly direction has important engineering and practical value. The mixture green gas C4F7N/CO2 which is most potential to subscribe SF6 arouses the concern of the researchers worldwide. However, recent researches focus on the breakdown performance of gas gap and the flashover performance in this mixture gas is rarely studied.
        In this paper, the surface flashover experiment platform is built and PR equation and Antoine equation are used to amend the mixing method and calculates the liquefaction temperature. The influence of creepage distance on power frequency flashover voltage under uniform electrical filed is studied. The gas pressure of the mixture is 0.1MPa、0.3MPa and 0.5MPa while the concertation of the C4F7N is 0%、5%、9% and 13%.
        Results show that the surface flashover voltages of the pure CO2 will be promoted up to 2 times when adding 5% C4F7N to it. With the increase of the creepage distance, the dielectric strength of the mixture gas is going to decrease under the same pressure and C4F7N concentration. When gas pressure rises, the downward trend exacerbates with the increase of the creepage distance. What’s more, Increasing the C4F7N concentration has no obvious effect on increasing the surface flashover voltage.

        Speaker: Zhongbo Zheng (Xi’an Jiaotong University)
      • 13:00

        SF6/N2 gas mixtures has the huge potential. However, due to the inevitable insulation defects in high voltage apparatus, partial discharge is caused, and then the mixture is decomposed, which ultimately endangers the safe operation of the equipment. The study on the decomposition of SF6/N2 mixtures in corona discharge is of guiding significance to the timely detection of early failure of equipment and the insulation of diagnostic equipment.
        In this paper, the defect model of electrode corona discharge was designed, and the experimental platform was set up. The variation of discharge during SF6/N2 mixtures decomposition was studied by using impulse current method. By changing the applied voltage, gas pressure, gap distance, water content, and mixing ratio respectively, the effects of these factors on the discharge energy, discharge quantity and decomposition products of SF6/N2 mixtures were studied. The experimental results show that the SF6/N2 mixtures decomposition produces include NF3, SOF2 and SO2F2 under the defect of the corona discharge of the pin-to-plate electrode. The production rate of NF3 is low while the output of SOF2 is more than SO2F2. The production of decomposition products also increases along with the discharge time increasing, the applied voltage increasing, the pin-to-plate electrode gap distance reducing, the SF6/N2 gas pressure and the proportion of SF6 reducing. The ratio and the total yield of SF6/N2 mixtures decomposition products can be used as the characteristic parameters to distinguish the spark discharge and corona discharge of the pin-to-plate electrode.

        Speaker: Ms Jiayin Yan (Xi'an Jiaotong University)
      • 13:00

        SF6, an insulated gas with high dielectric strength, is widely used in the power system. However, SF6 is one of the six limited greenhouse gases specified with a GWP of 23500. Therefore, it is necessary to assess a candidate as alternatives to SF6 in electrical equipment.

        In this paper, a new insulating media C5F10O with a GWP value of only 1 and a dielectric strength twice that of SF6 is studied. The discharge characteristics of C5F10O mixed with N2, CO2, and Air in different pressure, ratio are discussed in uniform field under AC voltage. An experimental platform for the discharge was designed, which consists of a 300 KV AC generator, divider, and a test chamber, where the Rogowski electrodes with the distance of 10mm are mounted. The breakdown experiments of 95%CO2/N2/Air mixed with 5% C5 at 0.1-0.5 MPa were carried out respectively. The experiments were repeated at least five times for each test condition. The results show that the insulation performance can be greatly improved by adding a small amount of C5 gas to CO2/N2/Air. Among them, C5 mixed with Air has the best insulation performance, followed by the mixture with N2 and CO2.The insulation strength of 5% C5 /95% Air at 0.5MPa can reach 78% of SF6 in the same experimental conditions. In addition, the breakdown voltage of 2%C5, 5%C5 and 8%C5 mixed with Air at 0.1-0.5 MPa were measured. The results reveal that the higher the mixing ratio of C5, the higher the discharge voltage of mixed gases is. At 0.5MPa, the insulation strength of 8%C5/92%Air is approximately 87% that of SF6.

        Therefore, the mixtures of C5 and Air are expected to replace SF6 in terms of electrical strength. However, the liquefaction temperature of gases and the toxicity of products after decomposition need to be considered comprehensively.

        Speaker: Jiayin Yan (Xi'an Jiaotong University)
      • 13:00

        In recent years, C5F10O has become one of the most promising alternatives to SF6.Not only because it fundamentally solves the issue of greenhouse effect of SF6 with the GWP of 1, but also, thanks to its high content of fluoride, the insulating strength of C5 is twice as good as SF6.However, the liquefaction temperature of C5 is 27℃ under normal pressure, so the most crucial challenge for it is the liquefaction case for high voltage application.
        In this paper, N2 is added to C5 to reduce its liquefaction temperature. Refer to the practical application, the scheme of C5 mixed gas instead of SF6 is explored combining with liquefaction temperature and the concentration of C5.Firstly, the saturated vapor pressure of C5/N2 gas mixture at different temperatures and molar concentrations was calculated by using Antoine equation and gas-liquid equilibrium law. The results reveal that 2%C5/98%N2, 5%C5/95%N2and 8%C5/92%N2 liquefied at 0.7MPa, 0.3MPa and 0.2MPa under the lowest operating temperature of GIS( -15℃)respectively. And then, the breakdown voltage of mixture with different concentration in critical conditions is measured under AC voltage. Furthermore, the LM algorithm is used to fit the function with gas pressure P and molar ratio K. It demonstrates that under the saturated vapor pressure, the insulation strength of 2%C5, 5% C5,8%C5 gas mixture can reach up to 78%, 56% and 43% of SF6 at 0.5MPa.Therefore, increasing the pressure is a more effective way to improve the insulation strength of C5 mixture than ratio.

        In summary, the paper indicates the mixture of 2%C5/98%N2 at 0.7MPa is expected to replace SF6 at 0.5MPa in GIS, which provides an important reference for the substitution of C5 in high voltage application.

        Speaker: Jiaqi Yan (Xi'an Jiaotong University)
      • 13:00
        2P77 - Investigation of impulsive breakdown of interfaces formed by ester insulating liquids and solid dielectrics 1h 30m

        As naphthenic mineral oils are classified as a Class 1 water hazard, both the power and pulsed power industries are actively investigating suitable replacement liquid insulation. Natural and synthetic ester liquids present a possible alternative to these naphthenic mineral oils, primarily due to their comparable dielectric properties. Furthermore, ester liquids offer a number of additional benefits over conventional naphthenic oils such as improved biodegradability, reduced toxicity, increased flash point and the ability to absorb large amounts of moisture, as a consequence of the higher saturation point of ester liquids. For these reasons, significant research efforts have focused on the suitability of esters in the replacement of naphthenic mineral oils. However, most published research has examined ester liquids as the insulating medium within bulk insulating systems, with little known of the performance of liquid-solid interfaces formed between esters and solid polymers used in practical high voltage power and pulsed power systems.
        This paper will present and discuss the breakdown performance of liquid-solid interfaces formed by MIDEL 7131 synthetic ester, FR3 natural ester and different solid dielectric materials, Nylon 66, Perspex and PVDF. These interfaces will be stressed with standard lightning impulse voltages of both positive and negative polarity, following the IEC 60897 methodology. This standard uses a point sphere geometry generating a highly divergent field. Key breakdown characteristics, such as breakdown voltage and time to breakdown will be obtained and compared with those for liquid-solid interfaces formed between the same chosen solid dielectric materials and a naphthenic mineral oil. The results of the study will provide data for designers and operators of power and pulsed power systems, helping to determine whether naphthenic oils can be directly replaced with esters in existing high voltage designs, and informing the clearances in new designs.

        Speaker: Mr Chris Williamson (Strathclyde University)
      • 13:00
        2P80 - Insulator Technologies to Achieve Maximum Electric Field Holdoff 1h 30m

        In large machines, such as accelerators and high power microwave systems, it is common to implement pulsed power technology. Pulsed power attempts to deliver large amounts of power in a short amount of time. This is done by generating high voltage and delivering that energy to the desired load quickly through switches. To ensure that the energy is delivered to the desired load it is necessary to use insulators to separate high voltage from ground. The insulators function is crucial in the success or failure of the system and because of this, much research has been done in the materials, geometries, and sizes of insulators. A common mean of failure for these insulators is surface flashover. Surface flashover occurs when the electric field becomes strong enough to accelerate electrons along the surface of the insulator to a point where an arc is created between high voltage and ground. The machine is therefore limited to the amount of voltage it can holdoff and the amount of power it can deliver. By making modifications to the insulator, improvements in electric field holdoff has been documented. This paper attempts to analyze the different methods used to increase the electric field holdoff to improve the function of the system.

        Speaker: Mr Cameron Harjes (UNM)
      • 13:00
        2P81 - Impact on electrodes during plasma decomposition of carbon dioxide 1h 30m

        A reactor being developed and instrumented for plasma decomposition of carbon dioxide contains a pin-to-plane microdischarge, with a stainless steel pin and aluminum electrode. The degradation of the aluminum electrode over testing time is an unwanted effect of this particular system. A predictive model of degradation of the current electrode is being developed to relate the system parameters and treatment time with degradation of the electrode. Other aspects of the set-up are also being studied based on this phenomenon, including energy losses from the system, which can detract from the overall efficiency of the process of plasma decomposition of carbon dioxide. A test electrode of aluminum is arranged with a demarcated grid of test sections. Then, plasma discharges are applied at the centers of these grids within each area of approximately 2 mm x 2mm. Scans of these areas are taken using a three-dimensional optical profiler for non-contact measurement and characterization of micro- and nano-scale features of the aluminum surface. It should be noted that the instrumentation utilized provides up to 0.15 nm vertical precision. Hence, a predictive model can be developed with the purpose of determining how long the discharge gap length can remain within a reasonable range to sustain the plasma discharge across the electrodes. Toward the goal of engineering a plasma system which can be consistently deployed to decompose carbon dioxide, considerations on longevity of the electrodes and/or necessary maintenance can be a useful step in scaling these systems and preparing them for more widespread use. Results will include impact of the microdischarge on the electrodes during typical treatment times of plasma decomposition of carbon dioxide.

        Speaker: Dr Kamau Wright (University of Hartford)
    • 14:30 15:30
      Plenary Tues PM - Uri Shumlak Seminole Ballroom ()

      Seminole Ballroom

      Convener: Stuart Jackson (US Naval Research Laboratory)
      • 14:30
        Sustained Fusion Reactions from a Sheared-Flow-Stabilized Z Pinch 1h

        The Z-pinch plasma configuration is one of the earliest magnetic confinement concepts. It has a simple cylindrical geometry and an equilibrium characterized by radial force balance in which plasma pressure is confined by an axial (Z) current. The force balance indicates that fusion conditions can be achieved through plasma compression simply by increasing the current; however, virulent pressure-driven instabilities quickly destroy the equilibrium and obfuscate the path to fusion for the traditional Z pinch. More recently, introducing a sheared axial flow to the plasma was theorized to stabilize the Z pinch. Closely coupled with computational studies, a series of Z-pinch experiments (ZaP and ZaP-HD) at the University of Washington were used to test the theory of sheared-flow stabilization. Diagnostic measurements of the plasma equilibrium and stability confirmed that in the presence of a sufficiently large flow-shear, gross Z-pinch instabilities were mitigated, and radial force balance was achieved. Z-pinch plasmas of 50, 100, and 126-cm lengths were held stable for durations much longer than predicted for a static plasma, i.e. thousands of growth times. Adiabatic scaling relations combined with single-fluid and two-fluid simulations facilitated the theoretical understanding of stabilization which enabled increasing plasma parameters. Flow-shear stabilization was demonstrated to be effective even when a 50-cm long plasma column was compressed to small radii (3 mm), producing increases in magnetic field (8.5 T), density (2e17 /cc), and electron temperature (1 keV) as predicted by adiabatic scaling relations. The collaborative FuZE (Fusion Z-pinch Experiment) project between UW and LLNL scaled the sheared-flow-stabilized Z pinch to fusion conditions, and showed that fusion neutrons are produced in a 50-cm long Z-pinch plasma generated with a deuterium and hydrogen gas mixture. A 30-cm long neutron production volume was temporally and spatially resolved. Sustained neutron production was observed for durations up to 8 microseconds during which the plasma was stable, and the current was sufficiently high to compress the plasma to fusion conditions. The neutron production was demonstrated to be consistent with a thermonuclear fusion process since it was not associated with MHD instabilities and beam-target effects were found to be negligible. Likewise, the neutron yield scaled with the square of the deuterium concentration and agreed with the thermonuclear yield calculated from the measured plasma parameters. Experimental observations generally agree with theoretical and computational predictions, indicating that sheared flows can indeed stabilize and sustain a Z-pinch equilibrium and offering a potential path to achieve even higher performing plasmas.

        Speaker: Uri Shumlak (University of Washington)
    • 15:30 16:00
      PM Break 30m
    • 16:00 17:30
      1.2 Computational Plasma Physics II Seminole A/B ()

      Seminole A/B

      Convener: YANGYANG FU (Michigan State University)
      • 16:00
        Macroparticle combination algorithm for plasma PIC simulation 15m

        The efficient simulation of plasmas via the particle-in-cell (PIC) algorithm requires that the macroparticle density be isotropic, while the physical density can vary significantly in space and time. This isotropic macroparticle density ensures good statistics in reactions and charge deposition while maintaining ideal computational load balance. Especially in the case of cascade ionization, when the physical density of electrons locally grows exponentially in time, the macroparticle density must be managed so that it does not also increase. This is accomplished through macroparticle recombination, where multiple macroparticles are combined into fewer macroparticles. We present a novel algorithm for preserving the momentum, energy, flux, charge, and phase space distribution of the original particles during this recombination process. Conservation of these quantities is important when the distribution function of the particles is non-Maxwellian, and especially when it is multi-modal. This new algorithm will be discussed in detail, and plasma simulations benefitting from its unique conservation properties will be shown. These simulations will include a simple example of crossing beams, as well as a complex simulation of plasma generation in the C100 SRF cavity for in situ cleaning. In both of these cases, there are discrete populations of electrons with different temperatures and mean velocities that must be preserved to accurately model the system.

        Speaker: Jarrod Leddy (Tech-X Corporation)
      • 16:15
        Benchmarking the Kinetic Global Model framework (KGMf): EEDF evaluations in low-temperature argon plasmas* 15m

        Global (volume-averaged) models present valuable tools in predicting macroscopic plasma behavior and giving the ability to evaluate the importance of individual reactions in plasmas, which further helps identify the key reactions for spatial-dependent simulations[1]. The Kinetic Global Model framework (KGMf) was extended and coupled with a Boltzmann equation solver, BOLOS[2] (using two-term spherical approximation) and MultiBolt[3] (multi-term spherical approximation), to self-consistently compute electron energy distribution function (EEDF). By capturing the temporal evolution of EEDF, the KGMf enables fidelity of the results even for dynamic systems at a cost of a higher computational complexity. Adaptive EEDF evaluations are imperative to preserve the advantage of the global model while maintaining the accuracy of the solutions. Using the low-temperature argon plasma chemistry at high pressure, we compared different methods of controlling the EEDF evaluation frequency depending on changes of plasma parameters, e.g. electron density or electron temperature. The impact of individual parameters on the temporal evolution of discharge parameters is presented in terms of selected parameter values and computational time. The results are also compared to the simulation results obtained by ZDPlasKin with BOLSIG+[4].

        [1] G. M. Parsey, Ph.D. thesis, Michigan State University, (2017).
        [2] A. Luque, https://pypi.python.org/pypi/bolos, (2004).
        [3] J. Stephens, J. Phys. D: Appl. Phys. 51, 125203 (2018).
        [4] G. J. M. Hagelaar and L. C. Pitchford, Plasma Sources Sci. Technol. 14, 722-733 (2005).

        *This work supported by the DOE Plasma Science Center Grant No. DE-SC0001939.

        Speaker: Janez Krek (Michigan State University, CMSE)
      • 16:30
        Advanced Implicit and Hybrid Techniques for the Simulation of High Density Volumetric and Electrode Plasmas 30m

        Recent advances in implicit and hybrid techniques have demonstrated that finite-difference-time-domain particle-in-cell (PIC) simulation codes can effectively model volumetric and electrode plasmas at high density. Energy-conserving implicit kinetic algorithms greatly relax the spatial Debye length and temporal plasma frequency constraints allowing for larger simulations volumes and times. A new implicit technique based on the Magnetic Implicit algorithm for strongly magnetized plasma permits more accurate orbit calculations even with for cyclotron frequency-time step product much greater than unity. PIC fluid techniques facilitate hybrid simulation and further accelerate the computational speed. These new capabilities allow for more accurate simulation of pulse-power accelerators, high power diodes, laser-plasma interactions, as well as magnetic and inertial confinement machines. In this paper, we explore PIC methodologies for kinetic and multi-fluid simulation. Hybrid techniques for blending the various PIC descriptions into a single integrated simulation will be discussed. Finally, we will present stressing practical examples using these techniques in the LSP simulation code.

        Speaker: Dr Dale Welch (Voss Scientific)
      • 17:00
        A multi-term spherical harmonic expansion of the Boltzmann equation for modeling low-temperature collisional plasmas 15m

        The Boltzmann equation describes the evolution of the electron and ion distributions over time through a six-dimensional phase space and is at heart of the plasma kinetic theory. For highly-collisional plasmas, scattering collisions keep the distribution function nearly isotropic in velocity space with small perturbations created by the hydrodynamic and electromagnetic forces. These plasmas are very common and include surface plasmas generated in pulsed power devices, plasma-filled microwave devices, air-breakdown in high-power microwave propagation, the plasmas generated by intense electron or ion beams, plasma medicine, electron-beam pumped excimer lasers, and hypersonic flows. For these plasmas, a spherical-harmonic expansion of the velocity-space distribution function is an effective technique for solving the Boltzmann equation. This talk will examine each of the terms in the Boltzmann equation in detail to derive conditions where a spherical harmonic expansion is useful. Expressions for the matrix elements in the expansion terms, which represent the projection of the various operators in the Boltzmann equation onto the spherical harmonics basis set, will be presented. The resulting multiple-term spherical-harmonic expansion makes no assumptions about either the direction of the E and B fields or the magnitude of the spatial gradients. Therefore, this expansion is appropriate for coupling with a Maxwell equation solver. When only the two lowest-order terms are kept, it is shown that the resulting equations are very similar in form to the continuity and force-balance fluid equations. Additional kinetic terms appear in the continuity-like equation which are related to collisions and to energy gains in the electric field that leads to Ohmic heating. Kinetic terms also appear in the force-balance-like equation. One related to collisions and another that is proportional to the derivative with respect to energy of the energy density.
        *This work is supported by the NRL base program.

        Speaker: S.B. Swanekamp (US Naval Research Laboratory)
      • 17:15
        The rigid-beam model as a test case for simulations of plasma generated by an intense electron beam 15m

        There has recently been a renewed interest at the Naval Research Laboratory (NRL) in better understanding the physics of the breakdown of air by a high-current, fast, pulsed electron beam. In order to simulate the breakdown of air that occurs under these conditions, new computational tools are being developed which will be able to accurately model the breakdown in the relevant parameter regimes. The rapid breakdown of air by an intense electron beam is a complex plasma physics problem. There are three main parts to this problem: the electromagnetic fields governed by Maxwell’s equations, the plasma and beam electron dynamics described by the Boltzmann equation, and the plasma-driven chemistry of the air modeled by coupled rate equations for all the chemical and ion species. In order to test various simplified models for the Boltzmann and plasma chemistry parts of the problem, we have developed a standardized approximation to Maxwell’s equations and the beam dynamics equations. Here we describe the resulting rigid- beam model, and show an example of how the rigid-beam model can couple to a Boltzmann solver. This rigid-beam model will allow us to more quickly develop and benchmark new models, which can then be validated against new data collected at NRL.
        * Supported by the Naval Research Laboratory Base Program

        Speaker: Steve Richardson (Naval Research Laboratory)
    • 16:00 17:30
      1.4 Partially Ionized Plasmas Gold Coast III/IV ()

      Gold Coast III/IV

      Convener: Tobin Munsat (University of Colorado)
      • 16:00
        Measurement of Photoionization Rates and Quenching Pressures 30m

        Many computational codes that involve transient plasmas, incorporate photoionization rates to describe the evolution of such phenomena as streamer development and propagation. At near atmospheric pressures, photoionization rates decrease significantly due to collisional quenching. The collisional quenching is the process where an excited molecule returns to the ground state non-radiatively at higher pressures. In air discharges, the nitrogen molecule is typically the excited molecule that is responsible for photoionizing oxygen, and oxygen is the quenching molecule. There are concerns that the currently widely-used photoionization models significantly overestimates the photoelectron production which is typically attributed to the quenching pressure of the radiative states. Progress on measurements of the photoionization rates and the onset of collisional quenching are reported along with progress in the model development suitable for inclusion in computational codes.

        The authors gratefully acknowledge support through from the Navy’s STTR program 14-16-C-1038

        Speaker: Justin K. Smith
      • 16:30
        Experiment on the propagation of relativistic pulsed electron beam in plasma 15m

        We present an experiment designed to investigate the propagation of a high-current relativistic pulsed electron beam (4MeV, 2kA, 60ns) in plasmas. This experiment takes place on an existing electron source at CEA-CESTA, and uses a plasma cell specially designed to this purpose.
        It consists of a ~1 meter long beam line section holding a glass tube in the path of the electron beam. We generate a DC glow discharge a few tens of cm long in the glass tube and we have estimated the electron density to be around $10^9 cm^{-3}$ with 3 different methods, namely electrostatic probes, microwave interferometry and an original diagnostic based on a capacitive coupling with the plasma. A coil for additional inductive heating of the plasma has also been developed to increase the plasma density.
        The plasma cell features some critical aspects such as plasma/vacuum windows or return current system, which deserve special care. We have particularly tested the resistance of different plasma/vacuum foils against the energy flux of the electron beam, as well as the mechanical and thermal stress due to the plasma discharge. In addition, the influence of the foils on the beam emittance and focusing has been numerically evaluated by PIC simulations with the LSP (large scale plasma) code. Finally, the preliminary results of the beam propagation in the plasma cell will be presented.

        Speaker: Thomas Lahens (CEA-CESTA, Le Barp, F-33116 France)
      • 16:45

        A model based on plasma propagation velocity has been recently developed to estimate the electron temperature ($Te$) of atmospheric pressure u-DBD plasma. In this work, we have extended this model to calculate Te for plasma generated with Ar gas. Plasma has been generated by input discharge voltage of 2.7 kV at a driving frequency of ≈ 45 kHz. A high-speed single-frame intensified charged coupled device (ICCD) has been used to observe the space and time-resolved discharge images and estimate the value of plasma propagation velocity ($ug$). The value of $ug$ for Ar plasma has been obtained in the range of 6.2x10$^3$ m/s. The electron temperature has been calculated for this plasma. The average electron temperature has been found to be about 1.18 eV and the average plasma density has been found to be about 3.62×10$^{14}$ cm$^{-3}$ for Ar plasma. Our results obtained with the modified convective-wave packet model can be a new contribution to plasma medicine.

        Speaker: Mr PRADOONG SUANPOOT (Maejo University Phrae Campus)
      • 17:00
        Overview and challenges of partially magnetized plasma modeling 15m

        The current status and challenges of the discharge plasma modeling capabilities in crossed field devices are reviewed. The plasma flows in such sources, e.g., Hall effect thrusters and magnetron discharge, are partially magnetized, i.e., electrons are magnetized while ions are not magnetized. The partially magnetized plasmas present unique and complex phenomena in that collisions with neutral atoms (a feature of low-temperature plasmas) and instabilities and turbulence (a feature of high-temperature plasmas) coexist. A discharge plasma model is constructed by choosing the numerical methods, e.g., fluid or kinetic, for individual plasma constituents, including neutral atoms, ions, and electrons. In this talk, various models are used to illustrate device-scale phenomena, including the breathing mode and azimuthally rotating spokes, as well as small-scale physics, such as the electron cyclotron drift instability. Additionally, the limitation of drift-diffusion approximation is discussed for magnetized electrons.

        Speaker: Kentaro Hara (Texas A&M University)
    • 16:00 17:45
      2.7 Microwave Plasma Interaction II Seminole D/E ()

      Seminole D/E

      Conveners: John Leopold (Technion), Prof. John Verboncoeur (Michigan State University)
      • 16:00
        Direct Detection of Multipactor in Waveguide Structures 15m

        An experimental setup was designed and implemented for the study of the multipactor effect in a rectangular WR 284 waveguide geometry under high vacuum. Operated at S-Band frequencies the multipactoring electrons are directly detected via an Electron Multiplier Tube (EMT). The custom fabricated test section consists of a copper coated steel structure transitioning to standard WR 284 waveguide dimensions via varying tapers. An RF transmitter with 2.8 kW peak power and a center frequency of 2.85 GHz is used to inject an RF signal into a traveling-wave resonator, to which the test section is integrated, pushing the traveling wave power to > 20 kW.
        Operating in the dominant TE$_{10}$ mode allowed the height of the waveguide to be changed without affecting the cutoff frequency, which was facilitated with an exponentially tapered impedance transformer. Multiple tapers have been constructed from OFHC copper with gap sizes conducive to developing first and third order multipactor (at 2 mm and 5.5 mm, respectively). With the reduction of the inner waveguide height from ~34 mm to 2 mm, an electric field greater than 2.7 kV/cm was achieved at 2.8 kW input power within the ring resonator. Simulations reveal a secondary electron yield of greater than one for copper for these conditions.
        For detecting multipactoring electrons, the EMT is aligned with a 1 mm circular aperture in the broadside surface of the waveguide enabling electron multiplication through the tube dynodes. Initial electron seeding is accomplished via UV light below 280 nm injected into the waveguide structure. The onset of multipactor in graphene coated copper versus pure copper surfaces is discussed. Clearly observable is the early emergence of the direct electron signal over any visible light emission that could be detected with a photomultiplier tube.

        Speaker: Zachary Shaw (Texas Tech University)
      • 16:15

        Multipactor [1] is a nonlinear phenomenon in which an electron avalanche driven by a high frequency rf field sustains itself by an exponential charge growth through secondary electron emission from surfaces. This work investigates the time dependent physics [2] of multipactor discharge on a single dielectric surface by a novel multiparticle Monte Carlo simulator [3] with adaptive time steps. This model is advantageous over previous models as it can estimate the particle flight durations exactly, offering better statistics than the single macroparticle model [4,5], yet less computationally costly than models with growing number of particles over time [2].

        The study shows that the presence of a second carrier frequency [6] of the rf electric field changes the saturation level and temporal oscillation pattern of the normal surface field. It is found that in the parameter regime of our investigation, the instantaneous normal surface field and the multipactor electron population remains at a lower value for a longer duration within an rf period for dual tone operation than for single tone operation.

        [1] P.T. Farnsworth, J. Frankl. Inst. 218, 411 (1934).
        [2] H.C. Kim and J.P. Verboncoeur, Phys. Plasmas 12, 123504 (2005).
        [3] A. Iqbal, J. Verboncoeur, and P. Zhang, Phys. Plasmas, 26, 024503 (2019).
        [4] R.A. Kishek and Y.Y. Lau, Phys. Rev. Lett. 80, 193 (1998).
        [5] P. Zhang, Y. Y. Lau, M. Franzi and R. M. Gilgenbach, Phys. Plasmas, 18, 053508 (2011).
        [6] A. Iqbal, J. Verboncoeur, and P. Zhang, Phys. Plasmas 25, 043501 (2018).

        Work supported by AFOSR MURI Grant No. FA9550-18-1-0062.

        Speaker: Asif Iqbal (Department of Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan 48824-1226, USA)
      • 16:30
        The Effects of Multipactor on the Quality of a Signal in a Transmission Line 15m

        Multipactor is a much studied AC discharge [1,2] that is harmful to microwave components. There is substantial current interest on this topic because of its threat to satellite communications [3]. In this paper, we present an analytical transmission line model to assess the effects of multipactor, should it happen, on the distortion of a signal. Both planar and coaxial transmission lines will be studied and compared. Extensions to complex, multi-tone signals will also be investigated. The I-Q plots (normalized error vector) for all of the cases considered will be presented to show the effects of multipactor.

        1. J. R. M. Vaughan, IEEE TED, Vol. 35, No. 7, 1988.
        2. R. A. Kishek et al., Physics of Plasmas 5, 2120 (1998).
        3. Special sessions on Multipactor, I and II, ICOPS, Denver, CO, June 2018.

        This work was supported by AFOSR MURI Grant No. FA9550-18-1-0062, by AFOSR Grant No. FA9550-15-1-0097, and by L3 Technologies Electron Devices Division.

        Speaker: Patrick Wong (Michigan State University)
      • 16:45

        Multipactor breakdown is a cascade phenomenon that occurs in RF and microwave systems. It has been observed in microwave tubes, RF windows, coupling structures, transmission lines, and in accelerator structures. Multipactor can cause loading of microwave cavities, localized heating and detuning of signals. These effects can ultimately lead to inefficient operation and possible destruction of the device. Disruption of such devices, particularly of space-borne systems, must be avoided due to the extreme cost incurred by unexpected device failures. The large safety margins necessary to ensure that multipactor will never occur add excess bulk and cost to the device. These safety margins are based on susceptibility measurements from historical experiments, of which few have been conducted for coaxial geometry [1,2].
        This work aims to investigate if simulations performed in CST Particle Studio can accurately predict the onset of multipactor in coaxial transmission lines. We use secondary electron emission data for copper with chemically cleaned surfaces [3]. Growth of multipactor has been explored by studying the evolution of the electron population after seeding with a few electrons. These simulations replicate the susceptibility data from Woo’s experiments [1]. Ultimately, these simulations will be used to guide the design of a coaxial test bed that will be used to validate new theories and test the efficacy of mitigation schemes.

        [1] R. Woo, “Multipacting Discharges between Coaxial Electrodes,” J. Appl. Phys., vol. 39, no. 3, pp. 1528–1533, Feb. 1968.
        [2] T. Graves, "Experimental Investigation of Electron Multipactor Discharges at Very High Frequencies", Ph.D, Massachussetts Inistitute of Technology, 2006.
        [3] I. Bojko, N. Hillert, Scheuerlein, "Influence of air exposures and thermal treatments on the secondary electron yield of copper", J. Vac. Sci. Technol. A Vac. Surf. Films, vol. 18, no. 3, pp. 972-979, May 2000.

        • Work supported by AFOSR MURI #FA9550-18- 1-0062.
        Speaker: Stephen V. Langellotti (University of Michigan)
      • 17:00
        Multipactor dynamics under obliquely incident rf electric field 15m

        It is well known that single-surface multipactor discharges have a negative effect the electromagnetic wave transmission in high power microwave devices. In this work, we examine the single surface-multipactor dynamics under obliquely incident rf electric field, such as occurs in TM waveguide modes, using particle-in-cell simulation. The results show that the oblique angle, θ, between the electric field and the dielectric surface has a strong effect on the multipactor discharge, and significantly reduced multipactor is found with increasing θ from 0 to 0.15π. In addition, in the base case θ = 0, the time-dependent electron number has two identical oscillations over one rf period. However, one of these two oscillations decreases in magnitude at θ = 0.05π and disappears at θ = 0.15π, because the perpendicular component of the rf electric field alternately reinforces and reduces the restoring field, increasing and decreasing the oscillation of the electron impact energy, respectively. In addition, the electrons are forced into a few branches in the phase space of velocity and position. Finally, we develop a simple dynamic model to investigate the multipactor suppression, and the susceptibility diagram shows the upper and lower boundaries get close, implying no multipactor develops at large oblique angles.
        Acknowledgment: This work is supported by AFOSR MURI Grant NO.FA 9550-18-1-0062

        Speaker: Dr De-Qi Wen (Michigan State University)
      • 17:15

        Vacuum electronic communication devices, such as the traveling wave tube (TWT) and backward wave oscillator (BWO), can at times experience degraded performance up to complete failure due to the multipactor effect. This effect is tied to the production and acceleration of secondary electrons due to electron impact and coupling to the electromagnetic energy within the tube. As part of a Multidisciplinary University Research Initiative (MURI) led by Michigan State University, the University of New Mexico is carrying out a study of the SEY contribution from various materials used in high power vacuum electronic devices. This presentation describes SEY data from electron bombardment in the low energy regime, from 10 eV to 1 keV, on Cu (baseline), Monel, Kovar, Invar, Al of various tempers, Fe (Cu Plated) as well as silver- and gold-plated samples. Angular resolved measurement data will be presented. In addition, different surface cleaning treatment protocols employed in this study will be described. Experimental results are compared with available simulations and previous data published in the literature.

        Speaker: Mr Talal Ahmed Malik
      • 17:30

        Currently, the total power, performance and lifetime of high-power RF devices, like vacuum electron devices, RF space systems, and accelerators, are severely limited by a phenomena known as multipactor. This occurs when the electromagnetic field is in resonance with secondary electron emission leading to a runaway avalanche of electrons in the device, resulting in degraded operation and potentially a corona discharge and the destruction of the device. An effective strategy for mitigating multipactor is to use in these devices materials that show a reduced secondary electron yield (SEY), however at present, ideal materials are not known and so currently there is much effort in identifying such low SEY materials. Here we present our efforts, as part of a Michigan State University led Multidisciplinary University Research Initiative (MURI) towards building a model by which the SEY of a material can be predicted based only on the atomic structure of the material, thereby allowing the in silico search for effective device materials. Details of this model, in which Density Functional Theory (DFT) is used to calculate the frequency dependent dielectric function of a copper metal surface, which is then used as input to Monte Carlo simulations for secondary electron emission, are presented and compared to experimental results.

        This work was supported by AFOSR MURI Grant FA9550-18-1-0309 through a subaward from Michigan State University.

        Speaker: Dr Ryan Johnson (University of New Mexico)
    • 16:00 17:30
      4.1 Fusion (Inertial, Magnetic and Alternate Concepts) Space Coast I-III ()

      Space Coast I-III

      Convener: Adam Sefkow
      • 16:00
        Radiative stabilization of the shock-driven interfacial instabilities in double-shell targets 15m

        Double-shell targets, as an additional design for demonstrating and exploring ignition, consist of two concentric shells, a low-Z outer-shell (ablator) surrounding a high-Z inner-shell (pusher) kept in place by a low-density foam. The hydrodynamic instabilities such as Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) are especially critical due to the existence of several interfaces, such as the pusher/foam interface with a high Atwood number. In the talk, we will discuss the hyrdodynamic stabilization of the pusher/foam and foam/ablator interfaces due to the radiative shock in the foam layer. When the ablatively-driven shock by the laser or radiation enters the low-density foam layer (tens of mg/cc) from the outer shell, the shock may become radiating as the radiative flux plays a role in the dynamics to stabilize the pusher/foam and foam/ablator interfaces. The preheat of the radiative shock promotes the outer and inner shells expanding towards the foam layer, and the move of the interface renders the effective reduction of the Atwood number at the interfaces, resulting in the hydrodynamic stabilization. 2D simulation by the radiation hydrodynamic codes is also performed to verify the stabilization due to the radiative shock for the double-shell targets.

        Speaker: Dr Zhensheng Dai (Institute of Applied Physics and Computational Mathematics)
      • 16:15
        Numerical Analysis of Direct-drive Golden Double-shell Implosion 15m

        The volumetric ignition which can substantially reduces the radiation loss requires low threshold temperature and low implosion velocity. Golden double-shell design is one of the volumetric ignition designs. The design consists of two concentric golden shells with the inner shell enclosing a volume filled with DT fuel. The thick inner golden shell can re-radiate the escaped radiation back to DT fuel to reduce the radiation loss. The outer shell consists of two layers: an Au layer covered with a CH layer. The CH layer which can efficiently absorb the energy of the incident laser beams acts as the ablated material to drive the vicinal Au layer as payload mass to fly inward like a rocket to collide with the inner shell to get velocity and pressure multiplication. In this report we numerically re-examined the physical processes of double-shell implosion. Numerous 1D multi-groups hydrodynamic simulations show that the velocity multiplications due to shells collision locate in range about 1~1.3 for mass ratio of two Au shells in 2~10 . Although increment of mass ratio of two Au shells does not increase the velocity multiplication, it increases the areal density of DT fuel in the highest compressed moment, thence increases the burning efficiency of DT fuel. Usage of high mass ratio of two Au shells is a way to increase the areal density of DT. The lower amount of DT fuel filled the higher implosion velocity required to get ignited. To get DT fuel efficiently burned the state ρRT>2g/cm^2 keV must be reached. 2D simulations are used to illustrate the development of non-uniformity of lasers illumination in the processes of implosion.

        Speaker: Dr Yan Xu
      • 16:30
        The preliminary experiment of driven pressure enhancement by hybrid drive on ShenGuang Laser facility 15m

        For the purpose towards to ignition based on Laser inertial confinement fusion(ICF), the traditional indirect drive scheme proposed to create an extremely high hot-spot pressure up to ~350Gbar at stagnation phase and makes it ignited. On the other hand, the high compression ratio implosion(Cr~35) design was necessary while the driven pressure was just around 100Mbar and limited by ~300eV radiation temperature or peak laser power (Phot- spot~ Cr3*Pdriven). And then, hydrodynamic instability become serious and makes ignition difficult.
        Hybrid drive that combined with indirect drive and direct drive was proposed few years ago[1]. The hybrid drive scheme can increasing driven pressure few times based on “snowplow” effect and make the implosion compression ratio decreased to 25 in numerical simulations and make the hybrid drive scheme become robust.
        The experiment of driven pressure enhancement was performed on ShenGuang Laser facility. The target was a half-cylindrical hohlraum with 1500micro length and 2500micro diameter. 20 laser beams as indirect-drive beam with 15TW/3ns were inject into hohlraum and create 200eV radiation. 4 laser beams as direct-drive beam with 4TW/2ns and 2ns delay were interaction on the sample that mounted on the bottom of half-hohlruam. The sample was consisted with three layers: CH as an ablator, Al as an preheating barrier and Quartz as a window. The diagnostic using VISAR for shock velocity history measurement. The experimental result show that the indirect-drive shock velocity was 43.6km/s, hybrid-drive enhanced shock velocity was 83.8km/s, respectively. The experimental result compared with simulation well. The driven pressure of hybrid drive is up to 150Mbar and 3.6times than 200eV radiation driven only.
        [1] X.T.He, et al. Phys.Plasmas 23,082706(2016)

        Speaker: Dr Ji Yan (Research Center of Laser Fusion)
      • 16:45

        The plasma with fusion parameters produced in tokamak, or at the interaction of powerful lasers with matter or z-pinch discharges deals with similar questions, mechanism of generation of high energy beams of charge particles similar to the plasma in solar flares1,2. The plasma generated in plasma focus discharges has some advantages in solving of this problem – convenient parameters for complementary interferometric, XUV, deuteron and neutron diagnostics with temporal, spatial and energy distribution. The presented experimental research of fusion DD reaction was provided on the dense plasma focus (DPF) device3 at the current above 1 MA and the total neutron yield at the level of 10E11. These diagnostics made possible to register the existence and evolution of organized structures and time and locality of acceleration of charge particles. The fast deuterons were recorded by pinhole camera and CR-39 detectors. They showed their origin in small regions in plasmoids and in rings with diameter above DPF dense column. We discuss also the distribution of internal magnetic fields, filamentary structure of the current, difference in acceleration of electrons and ions, and the possible model of the fast energy acceleration based on the magnetic reconnection, which can be inspiration as for tokamak and laser-fusion as for solar-flare communities.

        Speaker: Pavel Kubes
      • 17:00

        Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial fusion concept that is presently being studied on Sandia’s Z machine. The concept utilizes an axial magnetic field of order 10 T to reduce thermal conduction losses as a cm-scale beryllium can implodes and compresses fusion fuel, which was preheated to of order 100 eV with a few kJ from a TW-class laser. During the implosion, the magnetic field is amplified through magnetic flux compression to several thousand Tesla, and the fuel temperature increases to several keV.

        Scaling studies indicate that >100 kJ deuterium-tritium fusion yields are possible on the Z machine; however, this level of performance can only be reached by simultaneously scaling up the initial applied B-field, the energy coupled to the fuel by the laser, and the current driving the system. Using the original MagLIF platform, the input parameters were limited to 10 T, approximately 1 kJ of laser energy coupled, and 16 MA. Experiments conducted with these parameters resulted in a primary deuterium-deuterium neutron yield around 3x1012 and a burn-averaged ion temperature of 2.5 keV. Recent efforts focused on developing enhanced capabilities for the MagLIF platform have demonstrated peak load currents approaching 20 MA and initial applied B-fields exceeding 15 T. Combining these improvements with a change in laser preheat protocol led to a primary neutron yield exceeding 1013 and an ion temperature over 3 keV. Additional efforts to further increase the B-field to >20 T, the laser preheat to >2 kJ, and the current to >20 MA are underway.

        Speaker: Matthew Gomez
      • 17:15
        Generating an imploding rotating plasma in MagLIF targets 15m

        Axially rotating plasmas implode very differently than non-rotating ones since the kinetic energy delivered by the implosion is now shared between pressure and rotation, rather than pressure alone. We propose to study this approach numerically inside a MagLIF target using a cryo-DT fiber rather than a gas pre-fill. An external pulsed-power generator is used to turn the fiber into a plasma. When the plasma has filled completely the liner, it is imploded by a much larger driver, such as the Z-machine. The rotation is initiated via $J \times B$ forces when a vertical magnetic field is present. Analytically, one can illustrate that for a given uniform $B_{z}$, and $B_{\phi}(r,z)$, a nontrivial $J_{r}$ and $J_{z}$ will be retrieved via Ampere’s law. As a result, the MHD equations in 2D cylindrical geometry yield a rotational velocity dependent on both $B_{z}$ and $B_{\phi}(r,z)$. Starting from the analytical solution, we assess how much rotation is expected in a MagLIF-like target. Then we complete our analysis by using the FLASH, a fully explicit adaptive mesh refinement code, that studies the viability of this new approach to magnetized target fusion. FLASH was developed in part by the DOE NNSA ASC and DOE Office of Science ASCR-supported Flash Center at the University of Chicago.

        Speaker: Marissa Adams (University of Rochester)
    • 16:00 17:30
      8.5 Power Supplies and Modulators II Seminole C ()

      Seminole C

      Convener: Mark Sinclair (AWE)
      • 16:00
        Roadmap on the Development of Klystron Modulators for ESS 30m

        The European Spallation Source will require, by its completion, a pulsed Linac capable of delivering a proton beam with a peak power of 125MW and a pulse width of 2.86ms and a pulse repetition rate of 14Hz. The required klystrons are fed by 33 high power long pulse modulators, each rated for 115kV/100A;3.5ms/14Hz. Due to their high power levels, not only the quality of the pulses delivered is of great concern, but also the power quality on the AC grid is a challenge. Additional constraints like cost, footprint, efficiency, reliability/maintainability made it impossible procuring a solution already available in the market by the time of decision.
        This contribution will focus on the modulator development roadmap for ESS. It will describe the obtained results and lessons learned from the first phase of the project, where two commercial solutions were procured and tested leading to unsatisfactory results.
        Concurrent to the commercial approach, ESS decided to launch an internal R&D project aiming at developing a new class of modulators, particularly suited for long pulse and high power applications. The topology (Stacked Multi-Level) is modular and based on several HV modules connected in series using High Frequency Transformers, fed from a primary low voltage inverters. In order to cope with power quality requirements, Active Front Ends are used at the first stage of the capacitor chargers. A reduced scale prototype, capable of powering one klystron, was built and successfully validated on a dummy load and on the real load. The series production of the first batch of 12 full scale units was outsourced on a built-to-print basis. The first series unit, capable of feeding 4 klystrons in parallel, was recently tested at full power on a HV dummy load. Experimental results of both the prototype and first series unit will be presented and discussed.

        Speaker: Dr Carlos Martins (European Spallation Source)
      • 16:30
        Optimal Design of a High Voltage High Frequency Transformer and Power Drive System for Long Pulse Modulators 15m

        The stacked multi-level (SML) klystron modulator topology has been suggested as an alternative to conventional pulse transformer based topologies in an attempt to improve output pulse performance and reduce system size for long pulse applications. In this topology, a power converter chain including a high frequency transformer generates the output pulse in a pulse modulation/demodulation scheme, effectively eliminating the direct size-pulse length dependency while allowing higher degree of freedom in design.

        However, increased complexity necessitates careful consideration from a system perspective to ensure appropriate component selection and design. First, from the perspective of the semiconductor switches, the pulsed nature of the load must be taken into account. High modulator average and peak powers are combined into a power cycling problem where lifetime issues must be managed when selecting semiconductor technology and converter operating frequency. Simultaneously, these considerations are directly coupled to the design of the high voltage high frequency transformer, the largest component in the SML chain, key in reducing modulator footprint and volume. In addition, appropriate passive components must be chosen with respect to transformer leakage inductance, switching frequency and switch ratings to constrain voltage overshoot without deteriorating system efficiency.

        In this paper, these integrated design considerations are combined with a catalog of IGBT switches available on the market to form an optimization algorithm set to minimize transformer volume, indicative of system oil tank volume, while ensuring high system efficiency and long semiconductor lifetime. The impact of required system lifetime as well as tradeoffs between system efficiency and volume are studied. Finally, the algorithm is used to outline the design procedure for a system rated for pulse amplitude 115 kV / 20 A, pulse length 3.5 ms, pulse repetition rate 14 Hz, efficiency>90%, lifetime>25 years. The derived design is validated in both circuit simulation and through finite element analysis.

        Speaker: Max Collins (Lund University)
      • 16:45
        Saturating Pulse Transformer Circuits Using Advanced Magnetic Materials 15m

        The APERIODIC research group at the University of New Mexico has been investigating novel triggering technologies for compact pulsed power. These technologies include the use of advanced magnetics and novel topologies to develop high performance compact solid state electrical trigger systems*. One of the technologies under investigation is the use of advanced magnetic materials as the basis of saturating pulse transformer (SPT) circuits.

        High performance triggering of high voltage three-electrode gas switches places difficult demands on the trigger generator system. For optimum performance, the trigger system must be capable of driving the trigger electrode(s) to a potential of up to 100 kV with 10’s of nanosecond risetimes. Achieving these performance specifications with a compact all-solid-state design using conventional topologies is impossible.

        The use of an SPT in an L-C circuit topology, as described by Fan and Liu [1], provides a means to overcome the limitations of conventional topologies to achieve the desired performance in an all-solid-state compact trigger pulser. However, there is still a significant amount of research and development that needs to be done to fully exploit the potential of this technology. The work at UNM has focused on two areas. The first is the use of advanced magnetic materials for the core of the SPT. The second is the development of circuit simulations that incorporate realistic models of the magnetic behavior of the core. This is important because the SPT L-C circuit has several free parameters that are mutually interactive – making it a system that is well suited to optimization through simulation.

        • This work supported by The Office of Naval Research under STTR N00014-15-P-1214

        • X. Fan and J. Liu, A compact, all solid-state LC high voltage generator, Review of Scientific Instruments 84, 064703 (2013); doi: 10.1063/1.4808314

        Speaker: Jon Pouncey (University on New Mexico)
      • 17:00
        Integrated Klystron Test Stand 15m

        Diversified Technologies, Inc. (DTI) recently delivered an Integrated Klystron Test Stand for klystrons under development at the Naval Research Laboratory (NRL) and Communication and Power Industries, Inc. (CPI). The test stand provides an HV beam and depressed collector power supplies, mod-anode modulator, controls, and circuit/klystron protection. The Integrated Klystron Test Stand simplifies and speeds the ability of the user to safely and efficiently test and exercise the klystron over the full range of its capabilities.
        This test stand design draws directly on previous DTI solid-state systems and shares common design elements based on DTI’s patented solid-state switching technology—which has a history of reliable operational performance across more than 600 high voltage systems around the world. A single capacitor and solid-state cathode switch provide the peak beam power while providing protection for the klystron in the event of an arc. The switch opens and removes cathode voltage within ~ 1 μs after an arc is detected.
        A control cabinet houses the main system controls and interface, including most of the power distribution and a Programmable Logic Controller (PLC) for system sequencing, parameter and fault monitoring. The PLC sequences are based on DTI’s klystron transmitter systems, with the addition of the flexibility and programmable sequencing in voltage, pulsewidth, and frequency required for klystron conditioning and testing across a range of parameters, rather than just the full power operation. A standard DTI switching power supply delivers a 10 to 32 kV DC high voltage input to the modulator. This high stability/low noise supply uses an advanced PWM inverter which gives excellent voltage and current regulation over the full output range. Nominal output behavior is 0.1% ripple and +/- 0.2% voltage regulation, with fast response to transients.

        Speaker: Ms Rebecca Simpson (Diversified Technologies, Inc.)
      • 17:15
        Design of a Wide Band Test system with Interchangeable Antenna Modules 15m

        Applied Physical Electronics, L.C. (APELC) has built a suite of wide band antennas, using a fat dipole geometry with an integrated resonator. Each antenna uniquely radiates a damped sinusoid, resulting in several cycles of energy at the predetermined center frequency, and with a wide band of frequency content. The unique aspect to this technology is the capability of using a single pulsed power source to drive different antennas. This paper describes a system consisting of a single Marx generator sourcing five unique wide band antennas, with center frequencies of 60, 100, 250, 400 and 500 MHz. The system design and results are discussed.

        Speaker: Dr Jon Mayes (Applied Physical Electronics L.C.)
    • 16:00 17:45
      9.1 Optical, X-ray, FIR and Microwave Diagnostics Gold Coast I/II (Double Tree at the Entrance to Universal Orlando)

      Gold Coast I/II

      Double Tree at the Entrance to Universal Orlando

      Convener: Peter Bruggeman (University of Minnesota)
      • 16:00

        Due to potential applications in plasma medicine and surface modifications, atmospheric pressure plasma jets (APPJs) receives great attentions by scholars in the last decade. Recently, the plasma induced flow instability and ionization propagation along thin mixing layer have been proposed and studied experimentally and numerically [1,2]. In respect to plasma-surface interaction, the mixing near surface is even important while understanding discharge physics and chemistry. In this report, the two-dimension filtered Rayleigh scattering is adopted to study the mole concentration distribution of helium and air near dielectric surface.
        The plasma jet impingement is obtained by flowing helium to dielectric surface. The laser sheet generated by high spectral purity system at 532 nm is aligned approaching to the dielectric surface. By applying the iodine cell in the observation pathway and fitting the laser wavelength to spectral absorption curve of iodine, an effective suppression of narrow-band stray light from surface is achieved, while allowing the doppler broadened Rayleigh signal through and captured by camera, which shows great advantage in plasma-surface interaction.
        By applying the method described above, the two-dimension mole fraction of helium and air near the surface is obtained. The error coming from laser energy variation and wavelength jitter is evaluated as well as the effect of stray light is discussed. The experimental outcomes are further compared with numerical results showing great consistency. The discharge effect on instability of mole fraction of gas is also discussed.

        1. A. Lietz, E. Johnsen and M. Kushner, “Plasma-induced flow instabilities in atmospheric pressure plasma jets” Appl. Phys. Lett., 111,114101 (2017).
        2. E. Doremaele, V. Kondeti and P. Bruggeman, “Effect of plasma on gas flow and air concentration in the effluent of a pulsed cold atmospheric pressure helium plasma jet” Plasma Sources Sci. Technol. 27 095007 (2018).
        Speaker: Mr Yuanfu Yue (University of Minnesota)
      • 16:15

        The increasing use of atmospheric-pressure nanosecond pulsed plasma jets (APNPJs) in biomedical and environmental applications has motivated fundamental studies of the APNPJs including measurements of plasma properties such as the electron density and temperature. Quantifying these properties helps us to understand the roles of electrons during the initiation and development of the discharge, as well as the electron impact-related chemical kinetics resulting in generations of reactive plasma species. This study applies the laser Thomson scattering technique to resolve the spatial distribution and temporal development of the electron density and temperature in an APNPJ. A 1-mm in-width helium plasma jet was generated using a tubular dielectric barrier discharge (DBD) electrode driven by 150 ns, 7 kV pulses at a repetition rate of 4 kHz. Ring-shaped profiles, with higher values on the outer edge and lower values in the center, were observed for the electron density as the plasma jet exits the nozzle and converges as it travels away. A peak electron density of $4\times10^{19} cm^{-3}$ was observed at an axial distance of 7-8 mm from the nozzle surface, after convergence of the ring. In addition, comparisons of these measurements with previous studies, including the one for a low repetition rate APNPJ [1], are discussed.

        Acknowledgements: This work is supported by the Air Force Office of Scientific Research under Award FA9550-17-1-0257, and in part by the 2018 AFRL summer faculty fellowship.

        Speaker: Chunqi Jiang (Old Dominion University)
      • 16:30

        We report on the spatial and temporal distribution of electric field strength in a nanosecond atmospheric pressure helium plasma jet during the evolution of the discharge when impinging on an ITO glass substrate. We used a non-invasive optical spectroscopy technique based on polarization-dependent stark splitting and shifting of the He I at 492.19 nm ($2p$ $\space ^1P^0 $ − $4d$ $\space ^1D$) line and its forbidden ($2p$ $\space^{1}P^{0}$ − $4f$ $\space^1F$) counterpart. The wavelength separation between allowed and forbidden lines is dependent on the electric field strength due to the Stark effect. For the He I at 492.19 nm, the separation between allowed and forbidden components can be written as a third order polynomial function of the electric field$^1$. The electric field is deduced from the experimentally measured separation. For our experimental conditions, the peak electric field value was measured to be ⁓ 15 kV/cm at the streamer head and it reduces to ⁓ 9 kV/cm at the streamer tail.
        The results show strong interference of $N_2$ second positive system emission (ν = 1-7) in the low E-field regions and also the presence of a field free component in the He I line in spite of the time resolved measurements on a time scale of 4 ns. The impact of these factors on the accuracy of the technique and the possibility to measure surface electric fields is also discussed.

        Acknowledgement: This work is partly supported by a Department of Energy Early Career Research Award (DE-SC0016053).

        1.$\space$M. M. Kuraica and N. Konjević, “Electric field measurement in the cathode fall region of a glow discharge in helium”, Applied Physics Letters, June 4, 1997, pp. 1521-1523.

        Speaker: Ms Mahsa Mirzaee (Department of Mechanical Engineering, University of Minnesota)
      • 16:45
        Incoherent laser Thomson scattering diagnostics for streamer discharge in He gas 15m

        Streamer discharge plasma, a type of non-thermal plasma, has received global attention as a source of reactive radicals, and is used for many applications such as ozone generation, decomposition of NOx and other gas pollutants, cleaning water, disinfection, deodorization, and medical applications. The tip of streamer discharge, known as the streamer head, in particular contributes to radical production. The peak electric field is located in the streamer head on the axis of symmetry of the discharge, likely resulting in many radical types. Very remarkable results in NO removal efficiency and superior ozone generation yield performed by streamer discharge have reported. Improving gas treatment methods requires understanding of physical characteristics of streamer discharge and streamer head, for example, electron temperature and electron density.
        This study investigates characteristics of streamer discharge by observing the propagation process of streamer head in a needle to conic electrode with positive voltage using a high speed gated emICCD camera. Then, incoherent laser Thomson scattering (LTS) diagnostic for streamer discharge and streamer head with positive voltage was performed. LTS diagnostic is considered to be the most reliable technique measuring electron temperature and density in plasma simultaneously. In addition, LTS diagnostic has high resolution temporally and spatially, therefore, LTS diagnostic can measure location dependence of electron temperature and density in streamer discharge including streamer head. The measurement point was 1 mm and 2 mm from tip of the high voltage needle electrode, and Thomson scattering signals were measured at the point of initial phase of streamer head propagation. In the results, electron temperature of streamer discharge was 3 ~8 eV, electron density of streamer discharge was 1020 m-3 ~ 1021 m-3 order. This study has proven that LTS diagnostic can measure electron temperature and density in streamer discharge plasma.

        Speaker: Kyohei Eguchi (Graduate school of Science and Technology, Kumamoto University - Japan)
      • 17:00
        Advanced streamer imaging techniques 15m

        Streamers are the precursors to sparks, which have undesired effects in many high voltage applications. An example of this is a circuit breaker where high voltages are switched by physically opening the electrical circuit, thereby creating an arc. This arc, which short circuits the separated electrodes is extinguished by flushing with an electronegative gas. After extinguishing, the circuit breaker needs to remain open for a successful switching operation. Therefore, new streamers which initiate at the high voltage electrodes and create electrically conducting paths, need to be prevented.

        The electronegative gas of choice for the best performance in extinguishing arcs is SF6. However, due to its extreme potency as a greenhouse gas, alternative gasses are under investigation. This research focuses mainly on streamer discharge properties in CO2 in addition to air and N2.

        Due to the complex nature of streamers and the simplified conditions used in numerical modeling of streamers, we attempt to simplify the streamer morphology in our experiments by reducing the applied voltage. These simplified streamers can then be imaged using stroboscopic and stereoscopic techniques. Using path tracking, grouping and triangulation algorithms, the 3-D morphology can be reconstructed. The reconstructed streamers can then be compared in detail to simulations.

        When the streamer morphology becomes too complex and cluttered for these techniques, the cylindrical symmetry of the point to plane geometry can be used to perform an Abel inversion. Single shot streamer images are not cylindrically symmetric, but when numerous discharges are stacked, this symmetry appears.

        Both methods operate in their respective streamer morphology complexity regime, where the full 3D reconstruction certainly extracts more detailed information compared to the stacked Abel inversions. The main challenge now lies in detailed diagnostics for the full regime.

        Speaker: Siebe Dijcks (TU/e)
    • 19:00 22:00
      Night Out 3h

      Universal City Walk Block Party

    • 07:30 08:15
      Continental Breakfast 45m Universal Center ()

      Universal Center

    • 07:45 08:15
      Memorial Session for Roger White (2011 Peter Haas Award) 30m Seminole Ballroom ()

      Seminole Ballroom

    • 08:15 08:30
      Announcements 15m
    • 08:30 09:30
      Plenary Wed - Alexander Kim (2019 Erwin Marx Award) Seminole Ballroom ()

      Seminole Ballroom

      Convener: Bryan Oliver (Sandia National Laboratories)
      • 08:30
        The Story of the LTD Development 1h

        In this presentation the LTD development story will be presented, which begins in the 1980's when the microsecond plasma opening switches (POS) were the focus of many pulse power scientists' research all over the world. The accompanying problems of this technology resulted in the development of the microsecond LTD to replace the long pulse Marx generator coupled to plasma opening switches. Boris Kovalchuk's design of the microsecond LTD cavity, in which the primary energy storage capacitors are directly integrated into the LTD structure, like the SPHINX accelerator of France, was the first crucial step towards the development of this new technology. This led to the invention of the fast (100 ns) LTDs, a natural step to follow which eliminated completely the need for pulse compression and power amplification. Most recently the LTD technology was further developed to generate output pulses of trapezoidal shape with very fast rise time (< 10ns) and flat top, named "Square Pulse LTDs." This newest version of the technology, which replaces the sinusoidal pulse output of the standard LTD, is ideally suited for applications such as flash radiography, Z-pinch, high power microwaves, etc. In this presentation a number of different types of LTDs will be described and their operation analyzed. In addition, since an LTD cavity encloses many spark gap switches, the statistical regularity of these switches, important for the viability of LTD technology, will be discussed.

        Speaker: Alexander A. Kim (Institute of High Current Electronics, Siberian Branch, Russian Academy of Sciences)
    • 09:30 10:00
      AM Break 30m
    • 10:00 12:00
      1.1 Basic Phenomena II Seminole A/B (Double Tree at the Entrance to Universal Orlando)

      Seminole A/B

      Double Tree at the Entrance to Universal Orlando

      Convener: Patrick Wong (Michigan State University)
      • 10:00
        Direct observation of the current evolution in a small-scale self-compressing plasma column 30m

        We report on the direct measurement of the evolution of the azimuthal magnetic field ($B_{\theta}$) in a small-scale, self-compressing plasma column using spectroscopic methods. This measurement allows for calculating the radial distribution of the axial current density which is crucial for understanding the dynamics of the plasma compression, and for improving predictions of detailed MHD simulations. The direct measurement of the magnetic field in self-compressing plasma systems is challenging, since the plasma density rises to few $10^{19} \ cm^{-3}$, resulting in strong spectral line broadening that fully obscures the Zeeman-split pattern.
        In our experiment, a 27-kA current pulse with a rise time of 170 ns drives the implosion of an oxygen column with an initial radius of ~3 mm. The main diagnostics is a high-resolution, polarization-based spectroscopic setup. Simultaneous measurement of the $\sigma$+ and $\sigma$- components of the Zeeman pattern enables the determination of the magnetic field from the separation of these two components. Additionally, we obtain the electron temperature and the electron density simultaneously with the magnetic field.

        The detailed time and space resolved characterization of the magnetic field and plasma parameters results in a comprehensive picture of the plasma compression dynamics. During the compression stage, we observe that the entire discharge current is located in the imploding plasma. At stagnation, however, the current flowing in the stagnating plasma decreases significantly and the ‘missing’ current resides at much higher radii.

        The data presented here can be used to build a detailed model of the current evolution that includes the plasma resistivity and inductance. This may explain the mechanism of the current re-distribution, which is relevant for numerous similar experiments in the pulsed power community.

        This work was supported in part by the Cornell Multi-University Center for High Energy-Density Science (USA), the US-Israel Binational Science Foundation, and the AFOSR (USA).

        Speaker: Christine Stollberg (Weizmann Institute of Science)
      • 10:30
        Identification of the Corona Point in Point-to-Plane Geometries in Atmospheric Air 15m

        In highly non-uniform electric fields, corona appears when a geometry-dependent threshold voltage is reached. A further increase in voltage results in circuit-limited breakdown. As the electric field becomes more uniform, the difference between the corona-onset voltage and the breakdown voltage becomes smaller until finally breakdown commences directly. The gap distance and voltage where the corona onset curve meets the breakdown curve is known as the Corona Point.
        The Corona Point is associated with the sustaining field – the minimum electric field which supports streamer propagation. This is of interest as it forms the basis for an additional requirement on streamer propagation to supplement the Meek-Raether avalanche-to-streamer criterion for electrical breakdown. The Corona Point is determined using a point-to-plane experimental setup that allows precise changes in electric field uniformity by moving the point closer to the plane. The potential on the electrode is ramped slowly to ensure that the threshold electric field for corona-onset is measured by triggering the digitizer with a photomultiplier sensitive in the 300-600nm range.

        Speaker: Leonardo Rossetti (University of New Mexico)
      • 10:45
        On three different ways to quantify the degree of ionization in sputtering magnetrons 15m

        High power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition (IPVD) technique that has received significant interest lately [1]. However, proper definitions of the various concepts of ionization are lacking while quantification and control of the fraction of ionization of the sputtered species are crucial in magnetron sputtering, and in particular in HiPIMS deposition. Here we distinguish between three approaches to describe the degree (or fraction) of ionization: the ionized flux fraction $F_{\rm flux}$, the ionized density fraction $F_{\rm density}$, and the fraction $\alpha$ of the sputtered metal atoms that become ionized in the discharge (sometimes referred to as probability of ionization). By studying a reference HiPIMS discharge with a Ti target, we show how to extract absolute values of these three parameters and how they vary with peak discharge current. Using a simple model, we also identify the physical mechanisms that determine $F_{\rm flux}$, $F_{\rm density}$, and $\alpha$ as well as how these three concepts of ionization are related. This analysis finally explains why a high ionization probability does not necessarily lead to an equally high ionized flux fraction or ionized density fraction.

        [1] J. T. Gudmundsson, N. Brenning, D. Lundin and U. Helmersson, High power impulse magnetron sputtering discharge, Journal of Vacuum Science and Technology A, 30(3) (2012) 030801

        [2] A. Butler, N. Brenning, M. A. Raadu, J. T. Gudmundsson, T. Minea and D. Lundin, On three different ways to quantify the degree of ionization in sputtering magnetrons, Plasma Sources Science and Technology, 27(10) (2018) 105005

        Speaker: Prof. Jon Tomas Gudmundsson (University of Iceland)
      • 11:00
        Nonlinear Electron Power Absorption in Capacitively Coupled Radio Frequency Discharges 15m

        In low pressure capacitively coupled radio frequency (CCRF) discharges, the expansion of the plasma sheaths generate highly energetic electron beams traversing the discharge gap and supporting the plasma via ionization. The penetration of these electrons into the plasma bulk can lead to significant plasma oscillations and propagation of electrostatic waves. Consequently, the electron power absorption as well as the RF current indicate significant nonlinear dynamics in the low pressure regime. In this work, we investigate the nonlinear electron power absorption by means of 1d3v Particle-In-Cell / Monte Carlo Collisions (PIC/MCC) simulations in geometrically symmetric and asymmetric CCRF discharges. Pronounced electron power gain and loss dynamics are observed in the region between the plasma bulk and the plasma sheaths during the phase of sheath expansion. Oscillations observed in the RF current — which is a global parameter that can be easily measured by a current probe in an experiment — can be clearly attributed to these discharge dynamics.

        Speaker: Sebastian Wilczek (Ruhr University Bochum)
      • 11:15
        Observation of positive and negative nanosecond pulsed streamers in a coaxial electrode using a quadruple emICCD camera system 15m

        The environmental improvements by non-thermal plasma have been actively studied all over the world. Generally, a pulsed discharge having a time duration of 100s ns consists of two discharge phases including the primary streamer and secondary streamer discharges. It is also well known that the streamer heads always have the largest electric field during entire discharge process and meanwhile, the propagation behavior between electrodes is strongly influenced by the polarity of the applied voltage. In recent studies, the nanosecond (ns) pulse power generator with a short pulse duration of 5 ns achieved the higher energy efficiency for exhaust gas treatment and ozone generation than other discharge methods. However, the fundamental characteristics of ns pulsed discharge are still unclear. In our previous study, although several propagation processes of the discharge were revealed using a single high-speed gated emICCD camera, it could not clarify on the full details because the continuous propagation images were unable to obtain, therefore, a combination of different discharge shots were photographed by changing exposure onset time of the emICCD camera. In the present study, the newly developed high-speed imaging system combined with four emICCD cameras, which can observe the single ns pulsed discharge phenomenon at the same time or over time using delay generator built in each camera. The propagation of the positive and negative streamer heads was observed using this imaging system. In the experiment, ns pulsed discharge was generated in a coaxial electrode. The propagation behavior such as propagation velocity, thickness of streamer heads, were compared in more detail between streamer heads with different voltage polarities.

        Speaker: Hitoshi Yamaguchi
      • 11:30
        Atomistic Study of Polarization response in Functionalized Barium Titanate Nanoparticles 15m

        To further the goal of optimized material design for pulsed power components, we aim to achieve a fundamental understanding of the model nonlinear dielectric material BaTiO3, through concurrent experimental and theoretical study. This effort is intended to lead to improved synthesis and design control, and a validated model to enable material predictions. The ultimate goal is to improve energy storage and discharge characteristics through design models for dielectric materials that translate results from the molecular level to macroscopic device level.
        In order to achieve this goal, it is first necessary to understand the atomistic level polarization response of ferroelectric BaTiO3 to the presence of surface ligands used in the material synthesis process. This response is of sufficient magnitude to obscure the energetics of surface and defect chemistry, and is highly sensitive to simulation boundary conditions. In this work, we apply density functional theory to explore the effects of different choices of surface termination and initial starting conditions on the polarization response of the material to the presence of adsorbed molecules and point defects. The examined choices include constrained optimization techniques, preconditioned ferroelectric phases, surface mirroring, and inclusion of compensating electrode layers. The advantages and shortcomings of each option are discussed both in the current context and in relation to previous literature results. Where possible, we place results in the context of our ongoing concurrent experiments.

        Speaker: Renee Van Ginhoven (AFRL)
    • 10:00 12:00
      10.3 System Modeling, Thermal, EMI and Circuits Gold Coast III/IV (Double Tree at the Entrance to Universal Orlando)

      Gold Coast III/IV

      Double Tree at the Entrance to Universal Orlando

      Convener: Heather O'Brien (Army Research Laboratory)
      • 10:00
        Investigation of Low Amplitude Lighting Strikes On Low Voltage Electrical Systems 30m

        This study serves to investigate the voltage and current development in commercial electrical systems and the impact of various protection schemes. Various tests are conducted on miniature mock electrical systems to determine breakdown thresholds and mechanisms. The area of interest includes low current amplitude lightning impulses in the range of 2 kA to 5 kA peak. The test setup is constructed from a 4 stage, 44 kJ Marx generator capable of 400 kV impulses. To further control the testing, a low inductance ground path is used along with a variable resistor, inductor circuit to control the impulse characteristics. The risetime is adjusted from 500 ns to 5 µs with peak current from 2 kA to 5 kA. Under test are small sections of conduit systems of 10 ft to 50 ft with various junctions and connections to mimic commercial electrical systems. These flaws or lack thereof are evaluated for breakdown threshold and current flow direction. Common wires under test are 10 and 12 AWG both stranded and solid core THHN along with different types of MOVs placed at various points in the system to mimic typical lightning protection schemes. Voltage and current measurements are taken at the entry and exit points in the conduit system under test. A photomultiplier tube (PMT) is used for diagnostic measurement of arc characteristics inside the conduit during lightning impulses. Resulting voltage and current waveforms are presented for different various risetime impulses and setups ups along with MOV effects.

        Speaker: David Barnett (Texas Tech University)
      • 10:45
        Characterization of Sustained Series dc Arc Duration for Advanced Detection Schemes 15m

        Series arcs in dc power systems can occur if energized wires split, or load connections are physically interrupted. Compared to their parallel counterparts, series dc arcs decrease load current, making detection more challenging. Series dc arc models, along with accompanying detection methods have been studied in the past . However, few studies link load and source impedance to the timing of series dc arcs. In this paper, using a broad set of data taken at different RC loads with a fixed loop impedance, the minimum required time for a sustained series dc arc to occur was determined. Analytic models describing the transient behavior of series dc arcs are used to link the load and line impedance to this necessary timing condition. The findings in this paper can guide the design of future dc power systems, ensuring that detection and protection schemes operate within the minimum time window.

        Speaker: Bailey Hall (The Ohio State Univeristy)
      • 11:00
        Three-Dimensional Model of the Saturn Accelerator Water Tri-plate Transmission Line Connection to the Vacuum Insulator Stack 15m

        Calculation of the power flow from the 36 pulse forming lines to the vacuum region of Saturn has always been complicated by the three-dimensional structure of the rod and bottle connections to the vacuum insulator stack. Recently we have completed a 3-D calculation of the bottle configuration and found a large error in previous impedance estimates. We have used this calculation to determine impedance and to construct a 2-D model of each of the 36 bottles of each level of the insulator using the Transmission Line Matrix (TLM) technique. These TLM models are then used in a 2-D model for each of the three levels of the insulator. Each model starts at a measured forward-going pulse in the water tri-plate, and ends at the Brehmstrahlung load at the center of the machine. Because of transmission line lengths, and because of the short pulse lengths, each can be considered independent of the others. Thus the combination of the three models represents a quasi-3-D model of the load region of the machine. The results of these calculations agree well with measurement and thereby provide confidence in simulation predictions for those areas where measurements are not possible. Details of the 3-D bottle calculation, the TLM model, and results of the load region simulations using this model will be given.

        • Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
        Speaker: Dr Ken Struve (Sandia National Laboratories)
      • 11:15
        Lumped Circuit Model of Multi-Pulse Laser Triggered Gas Switch with Braginskii Resistivity 15m

        L3 Applied Technologies is developing series pulsed forming water transmission lines for Los Alamos National Laboratory. The Series Pulse-Line Integrated Test Stand (SPLITS) consists of a set of four, 5.5 ohm coaxial water pulse forming lines in series. Each water line is capable of producing a -300 kV pulse when driving a matched resistive load.

        The University of New Mexico (UNM) is performing SPICE circuit modeling to simulate the performance of SPLITS. Microcap 10 is used as the SPICE program to model the first two, 120ns pulse forming lines with laser triggered, gas filled switches at varying currents and separation distance. One of the challenges of this effort is to model a time-varying resistance of the gas-filled switch. A solution to this is to implement Braginskii’s gas arc resistance equation, which is time-varying, within the model. To further improve on the Braginskii model, a time-varying conductivity element will added in. Future work to understand the conductivity of a gas arc as it changes temporally may include time-resolved interferometry can be used to measure radial growth rates and densities. From there a relation can be made to the spitzer conductivity. Ongoing work is seeking to gain further understanding of this model and derive time-varying conductivity from time-resolve radial position measurements.

        • Work supported by LANL contract #522006
        Speaker: Joe Chen (University of New Mexico)
    • 10:00 12:00
      2.1 Intense Beam Microwave Generation Gold Coast I/II (Double Tree at the Entrance to Universal Orlando)

      Gold Coast I/II

      Double Tree at the Entrance to Universal Orlando

      Convener: Prof. Rebecca Seviour (Supervisor)
      • 10:00
        Limits to High Power Amplification 30m

        JW Luginsland1*, JJ Watrous1, DH Simon2, and BW Hoff2
        1 Confluent Sciences, LLC
        2 Air Force Research Laboratory

        There has been tremendous progress in the power levels of high-power electromagnetic sources, such as klystrons, magnetrons, and TWTs that produce power into the gigawatt (GW) range. The vast majority of these sources are oscillators, where there is not a controlled phase signal associated with the high-power signal. In general, this is not surprising – amplification typically denotes linearity with respect to some input signal, while high power inevitably implies non-linear processes that generate intense relativistic AC electron beams. The vacuum electronics community has performed significant analysis on the sources for oscillation, typically involving undesired electromagnetic feedback from reflections in the device. The limits of high-power amplification in the GW range could be caused by these kinds of reflections, but also from non-linear processes associated with the intense space-charge in high-power microwave devices. One device that has both amplifier and oscillator configuration is the relativistic klystron driven by intense relativistic annular electron beams. In fact, the oscillator configuration explicitly introduces engineered feedback to produce oscillation. Here we report on a new model that captures both amplification and oscillation in the relativistic klystron, building on previous oscillator models(1). We interrogate this model with analytic and numerical methods to develop insight into the limits of high-power amplification. This model is also compared with particle-in-cell simulation and experimental performance. Additional efforts to develop a similar model for a TWT(2) will be detailed.

        1. Luginsland, Lau, Hendricks, Coleman “A model of injection-locked relativistic klystron oscillator.” IEEE Trans. Plasma. Sci, 24, 935, 1996
        2. Hoff, Simon, French, Lau, Wong “Study of a high power sine waveguide traveling wave tube amplifier centered at 8 GHz.” Phys. Plasma, 23, 103102, 2016.

        Work was supported by FA9550-19RDCOR025.

        Speaker: Dr John Luginsland (Confluent Sciences, LLC)
      • 10:30
        Rep-rated Testing of a Compact Magnetron with Diffraction Output (MDO) and Plans for Testing the Full MDO* 15m

        The University of New Mexico (UNM) proposed a compact magnetron with diffraction output (MDO) that is consistent with a permanent magnet guide magnetic field [1,2]. This compact high power microwave (HPM) source was tested at UNM and the particle-in-cell simulations accurately predicted its experimental operation [3]. This compact MDO was then tested at NSWCDD using a rep-rated power modulator. The power modulator was developed in a manner to drive a variety of potential relativistic RF sources with varying impedance and voltage requirements, providing flexibility for potential changes in drive voltage, impedance, and pulse width/repetition rate. UNM’s compact MDO with a permanent magnet was the first source to be tested. The initial pulse width and repetition rate settings ranged from 50 – 250 ns and up to 10 Hz, respectively, allowing the investigation of pulse shortening, efficiencies at high repetition rate conditions, and possible interactions between the two conditions. The experimental results of this testing will be presented. Finally, plans for testing UNM’s full S-band MDO [4] with a superconducting magnet will also be described.

        1. C. Leach, S. Prasad, M. Fuks, and E. Schamiloglu, “Compact Relativistic Magnetron with Gaussian Radiation Pattern,” IEEE Trans. Plasma Sci., vol. 40, 3116-3120 (2012).
        2. C. Leach, S. Prasad, M. Fuks, and E. Schamiloglu, “Compact A6 Magnetron with Permanent Magnet,” Proc. 2012 IEEE International Vacuum Electronics Conference (Monterey, CA, 24-26 April 2012), p. 491-492.
        3. A. Sandoval, “Experimental Verification of A6 Magnetron with Permanent Magnet” (M.S. Thesis, University of New Mexico, Albuquerque, NM, 2018).
        4. M. Fuks and E. Schamiloglu, “70% Efficient Relativistic Magnetron with Axial Extraction of Radiation Through a Horn Antenna,” IEEE Trans. Plasma Sci., vol. 38, 1302-1312 (2010).

        *Research at UNM was supported by ONR Grants N00014-16-1-2352 and N00014-16-1-3101. Research at NSWCDD was supported by ONR 30, ONR 35, and JNLWD.

        Speaker: Prof. Edl Schamiloglu (University of New Mexico)
      • 10:45
        Experimental Results of a Metamaterial-Enhanced Resistive Wall Amplifier Prototype* 15m

        Theory and simulation predict the Metamaterial-Enhanced Resistive Wall Amplifier (MERWA) could be developed into a high power amplifier with wide instantaneous bandwidth$^1$. Similar to a Resistive Wall Amplifier and Easitron$^2$, the MERWA produces exponential slow space charge wave growth for a velocity modulated beam. In contrast to the Resistive Wall Amplifier and Easitron, our research suggests the MERWA’s slow space charge wave gain occurs in the presence of a lossy metamaterial-circuit’s backwards (anomalously dispersive) electromagnetic wave. Due to the backward wave interaction and associated risk of backward wave oscillation, an important tradeoff exists involving the amount of circuit loss and its effect on oscillation, bandwidth, and gain. Prototype metamaterial circuits made of meandered wires$^3$ have been constructed out of copper (low loss) and stainless steel (high loss) for the purpose of proof-of-concept experiments to verify existence of MERWA gain. This talk will summarize results of simulated models and experimental hot test measurements using the prototypes including effects of varying circuit loss or introducing severs to prevent oscillation.

        1. T. Rowe, J. H. Booske, and N. Behdad, “Metamaterial-Enhanced Resistive Wall Amplifiers: Theory and Particle-InCell Simulations,” IEEE Trans. Plasma Sci., vol. 43, no. 7, 2015, pp. 2123-2131.
        2. C. K. Birdsall and J. R. Whinnery, "Waves in an electron stream with general admittance walls," Journal of Applied Physics, vol. 24, no. 3, pp. 314-323, 1953.
        3. T. Rowe, N. Behdad, and J. H. Booske, “Metamaterial-Enhanced Resistive Wall Amplifiers Design Using Periodically Spaced Inductive Meandered Lines,” IEEE Trans. Plasma Sci., vol. 44, no. 10, 2016, pp. 2476-2484.

        *Work supported by the Air Force Office of Scientific Research under Award No. FA9550-16-1-0509 and by a graduate fellowship from the Directed Energy Professional Society.

        Speaker: Patrick Forbes (Electrical and Computer Engineering Department University of Wisconsin-Madison)
      • 11:00
        High Power Amplification Experiments on a Recirculating Planar Crossed-Field Amplifier 15m

        The Recirculating Planar Crossed-Field Amplifier (RPCFA) is an S-band high-power microwave amplifier adapted from the Recirculating Planar Magnetron developed at the University of Michigan.1 The RPCFA has demonstrated amplification in excess of 16 dB for input signals up to 40 kW at frequencies ranging from 2.40 to 3.05 GHz. Pulsed power for the RPCFA is provided by the Michigan Electron Long Beam Accelerator with Ceramic insulator stack (MELBA-C), which generates pulses of -300 kV, 1 – 10 kA, for approximately 400 ns. The injected RF power is produced by various magnetrons ranging in both power (10-40 kW) and frequency (2.40-3.05 GHz), as well as a moderate power (~1 kW) generator with continuously variable frequency.

        The RPCFA has been fabricated and tested experimentally, verifying the results of MAGIC particle-in-cell simulations.2 The RPCFA has demonstrated amplification over a continuous frequency band from 2.6 to 3.05 GHz. For injected powers up to 40 kW, the amplifier is unsaturated, producing output powers approximately proportional to the input. These amplified, output microwave power levels have high variance and measures have been taken to understand and improve reproducibility. Cathodes with carbon brazed emitters have been tested to improve electron emission and create a more reproducible beam. These cathodes have decreased the variation of emission current, which has decreased the variation of amplification. Current research is focused on delivering input powers on order of 1 MW. A pulse forming network has been built to drive a 2.5 MW-rated MG5193 magnetron. Two existing magnetrons have consistently generated up to 2 MW on a test-stand.3 Amplification at MW input drive would confirm the high-power capabilities of the design and may also reduce variations in gain due to the stronger fringing RF fields.

        Speaker: Mr Steven Exelby (University of Michigan)
      • 11:15
        Inter-Digital Magnetron and Rippled-Field Magnetron: Two Remarkable Reincarnations of a Voltage-Tunable Magnetron* 15m

        Originally all magnetrons were nonrelativistic [1]. The inter-digital magnetron (IDM) [2] is, perhaps, the most peculiar magnetron variant, operating at anode voltages $\leq$ 5 kV. The rippled-field magnetron (RFM) [3], another variant, is relativistic, operating at voltages $\geq$ 1 MV. It came to our attention that the RFM [3] is the relativistic analog of the IDM [2].

        The IDM consists of a cathode and two sets of interleaving anode fingers (teeth, vanes) joined alternately to opposite faces of a cylindrical cavity. When voltage is applied to the cathode, the electrons initially circle in crossed radial electric $E_r$ and axial magnetic $B_z$ fields, and are intercepted by the inter-digital anode. The resulting interleaving anode current flowing along each anode finger in opposite axial directions produces a $B_r$ varying in the $\theta$-direction, superimposed onto $B_z$.

        One may replace both cathode and anode interleaving fingers of the IDM [2] with a corresponding set of interleaving anode and cathode permanent magnets, also oriented alternately in the $z$-direction in such a manner that results in $B_r$ varying in the $\theta$-direction, and superimposed onto $B_z$. Assuming that the set of interleaving cathode permanent magnets may be covered by an electron emitting smooth surface and, correspondently, the set of interleaving anode permanent magnets may be covered by the electron absorbing smooth surface with both covers facing the magnetron interaction space, one may effectively have a RFM [3].

        Theoretical analyses and particle-in-cell simulations of both the IDM and RFM will be presented, which should provide for wide frequency tuning by varying the voltage.

        1. G.B. Collins (Ed.), Microwave Magnetrons (Vol. 6) (McGraw-Hill, New York, 1948).
        2. J.F. Hull and A.W. Randals, "High-Power Interdigital Magnetrons," Proc. IRE, vol. 36, 1357 (1948).
        3. G. Bekefi, "Rippled-Field Magnetron," Appl. Phys. Lett., vol. 40, 578 (1982).

        *Supported by DARPA Grant #N66001-16-1-4042

        Speaker: Dr Andrey Andreev (University of New Mexico)
      • 11:30
        High-Power Microwave Generation by a Double-Anode Virtual Cathode Oscillator 15m

        The virtual cathode oscillator (vircator) is one of the promising devices oscillating high-power microwaves. Simplicity and high-power capability are advantages. However, the low efficiency and frequency stability are serious problems. To improve oscillation efficiency, strengthen the feedback of the electromagnetic wave to the electron beam is important. To strengthen feedback, double-anode is effective because it strengthens the modulation of beams. In this paper, we dealt double-anode to improve output power. Experiments were carried out on a repetitive pulsed power generator “ETIGO-IV” (maximum output: 400 kV, 13 kA, 120 ns, 1 Hz). The output microwaves are diagnosed for peak power and energy by using horn antennas. The microwave frequency is obtained by fast-Fourier analysis of the signal recorded by a high-speed digital oscilloscope. From the experimental result, the microwaves are obtained peak power of ~100MW. These results are shown that the output of the virtual cathode oscillator can be progress by using the double-anode. In addition, particle-in-cell simulations were carried out by using a simulation code “MAGIC.” Simulation results are compared with experimental results to examine the effect of the double-anode and possible ways of further improvement of microwave efficiency.

        Speaker: Mr Kazuki Nagao (Nagaoka University of Technology)
      • 11:45
        Problems of development a cold-cathode magnetron in pulse mode for application in an accelerator 15m

        High efficiency and low construction costs of magnetron have a significant benefit for an accelerator RF feeding. Main disadvantage of magnetron is poor frequency stability. The disadvantage may overcome due to injection phase lock. This solution is ready for the pulsed mode operation of accelerator. Next step of a magnetron advantage development is application a secondary emission cold cathode. It allows next decrease construction costs and much increase lifetime. The last one allows decrease operation costs. Important problem is ignition of self-supported secondary emission from a cold cathode of a magnetron. Ignition of magnetron occurs on slop of the high voltage pulse applied to the cathode. After initiation the electron emission continues at the top of the following part of the pulse. Such an igniting method can contribute to the deterioration of frequency stability due to subsequent voltage fluctuations on the subsequent flat part of the pulse. Another way for good frequency stability is application magnetron type amplifier. That is an amplitron. Generally the amplitron have secondary emission cold cathode. The ampliton are ignited by input RF power. Main disadvantage of the amplitron is low gain. The gain is 13-20 db at high power. In compare injection phase lock scheme need input power to magnetron on 30 db and more low than magnetron power. That is means “amplification” more 30 db. Another amlitron disadvantage is more complicated design in compared with magnetron. Magnetron may be ignited by input RF power too. However, the optimal start frequency does not coincide with the generation frequency. So fast magnetron frequency tuning is need for development. Another approach is application stabilitron scheme of amlitron with ignition by slop of the high voltage pulse. It may allow achieving good frequency stability without seed RF power. Application of a coaxial magnetron is possible for similar goal.

        Speaker: Prof. Sergiy Cherenshchykov
    • 10:00 12:00
      4.6 Fast Z Pinches I Seminole D/E ()

      Seminole D/E

      Convener: Steve Richardson (Naval Research Laboratory)
      • 10:00
        New insights in pulsed power driven explosion of underwater wires and wire arrays 30m

        Pulsed power driven underwater wire explosions are accompanied by the efficient generation of strong shockwaves. In the case of cylindrical or quasi-spherical wire arrays, convergence of these shockwaves results in high energy density conditions with multi Mbar pressures being obtained on axis, even in compact ‘table-top’ experiments. However much of the physics underlying wire explosion and shockwave interactions remains undiagnosed – making modelling efforts difficult and prone to misinterpretation.
        Recently, we have performed the worlds first high current pulsed power experiments coupled to a synchrotron. The resultant multiframe, phase contrast radiography images provide absolute density measurements at high resolution. On the ID19 beamline at ESRF we explored the explosion of aluminium, copper and tungsten wires, using a ~30kA, 500ns current source. As the wires expanded and ionised unexpected striation instability growth was observed inside the dense wire material. In two wire experiments, interacting a shockwave launched into the water with the surface of the exploding wires produced a new test bed for Richmeyer Meshkov instability growth. With a cylindrical array of wires, multiple shock reflections were observed and the increase in density of the water at convergence of the shockwaves seen.
        As part of the talk we will compare the quantitative data produced with leading hydrodynamic codes. We will show that underwater wire explosions can be used for the determination of phase transitions which are accompanied by a weak shock generation; as well as show direct evidence of Al wire combustion obtained through spectroscopy. Finally we will discuss how the techniques can be extended, exploring the use of underwater arrays for flyer acceleration, and the production of different hydrodynamic instabilities such as the Kelvin Helmholtz.
        This work was sponsored by Sandia National Laboratories, First Light Fusion, EPSRC, NNSA under DOE Agreement Nos. DE-NA0003764 and DE-SC0018088 and the Israeli Science Foundation.

        Speaker: Simon Bland (Imperial College London)
      • 10:30
        Thomson Scattering on Laboratory Plasma Jets to Study Current Polarity Effects 15m