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Yinon Ashkenazy (The Hebrew University of Jerusalem (IL))19/09/2022, 09:30
It was proposed that breakdown nucleation can be explained as emanating from surface plastic activity due to collective dislocation dynamics within the surface layer of samples exposed to high electric fields. In the past, we have demonstrated that extensive mobile dislocation structures are routinely observed in soft large-grained OFHC copper. However, as of this day we are missing a clear...
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Inna Popov19/09/2022, 10:00
It was suggested that breakdown might result from collective dislocation effects within a surface of metal electrodes exposed to high electric fields. We applied scanning and transmission electron microscopy to check this assumption. In this talk, I present a picture of the lattice structural defects observed in soft and hard Cu – dislocation dipoles, stacking faults, twins, and grain boundaries.
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Iaroslava Profatilova19/09/2022, 10:30
The cryogenic high-voltage pulsed DC system in FREIA laboratory continues the test series of copper electrodes to establish peculiarities of BDs craters occurred at low temperature down to 4K.
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The system uses parallel plate 40 mm diameter anode and 62 mm diameter cathode with a tens micrometers inter-electrode space. 1 µs high-voltage rectangular dc pulses with a repetition rate up to 1 kHz... -
Mr Lee Millar (CERN)19/09/2022, 11:30
To synthesize the experimental results and theory pertaining to high-field phenomena, a Monte Carlo model has been developed to simulate the conditioning and operation of high-field systems. Using a grid-based approach, any arbitrary geometry and surface field distribution may be simulated in spatially resolved fashion for both RF and DC devices alike. The probabilistic behaviour of breakdown...
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Andreas Kyritsakis19/09/2022, 12:00
One of the most extensively studied characteristics of vacuum breakdown (VBD) is the conditioning process and the VBD occurrence statistics, in various systems, including Radio-Frequency (RF) accelerators and pulsed-DC large electrode systems. Despite the abundant data on the VBD statistics, drawing useful conclusions regarding the physical processes that determine various patterns within...
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Mikhail Benilov19/09/2022, 12:30
\textbf{1. Potential mechanisms of vacuum breakdown. }The field electron
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emission (FE), where electrons are released from cold negative electrodes due
to the applied electric field, is a necessary step in the development of
vacuum breakdown. It is well known that the field emission current from cold
electrodes in vacuum exceeds, by orders of magnitude, values given by the
quantum... -
Stephen PURCELL (CNRS)19/09/2022, 14:30
There has been extensive research on optically modulated and optically pulsed tip electron sources for new high field physics in the ultrafast laser community [1], electron microscopy [2] and ultrafast electron diffraction [3]. This is mostly the excitation of metallic tips or surfaces for which the quantum efficiency is extremely low eg. 10-3-10-4. This means that the high current emission...
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Serhii Lebedynskyi (Institute of Applied Physics, National Academy of Sciences of Ukraine)19/09/2022, 15:00
A mathematical model of the electrode surface morphology was proposed and
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constructed, considering submicron irregularities on its surface. It is shown that the
theoretical decrease in the field electron emission current density with an increase
in the electron yield work from the metal does not agree well with the breakdown
field experiments for different materials. It is shown that the... -
Mr Lee Millar (CERN)19/09/2022, 16:00
Alongside the numerous operational accelerators on the CERN site, several radio frequency (RF) and DC test facilities have been established to support the development of novel prototypes and investigate high-field phenomena. In these facilities, copper RF cavities are regularly operated at surface electric fields in excess of 200MV/m while DC electrodes of various materials have been tested to...
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Christoph Obermair (Graz University of Technology (AT))19/09/2022, 16:30
CERN has established several high-power RF test stands, to investigate high-field phenomena. Recently, a machine learning framework has been developed and applied to the high-gradient cavity test data from these facilities. The aim has been to search for the existence of previously unrecognized features related to the incidence of RF breakdowns. Preliminary results have shown two key features...
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Mitchell Schneider (SLAC National Accelerator Laboratory)19/09/2022, 17:00
This talk will present the status for developing high gradient X-band bulk niobium accelerating cavities. We optimized the cavity design to maximize the shunt impedance and reduce the peak surface magnetic fields. This means that a much higher gradient can be produced for the same quench field limit of niobium (230 mT), increasing the power loss to gradient2 ratio > 100 [mWm/(MV)^2]. We...
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Tauno Tiirats (University of Tartu)19/09/2022, 18:00
Open discussion on handling FEMOCS as an open source software
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Pablo Martinez-Reviriego20/09/2022, 09:00
High-Gradient accelerating cavities are one of the main research lines in the development of compact linear colliders. However, the operation of such cavities is currently limited by non-linear effects that are intensified at high electric fields, such as dark currents and radiation emission or RF breakdowns.
A new normal-conducting High-Gradient S-band Backward Travelling Wave...
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Dr Soumendu Bagchi20/09/2022, 09:30
High-gradient electric-fields are inevitably encountered in technologies ranging from accelerating structures to miniaturized electronic devices. It is now well understood that material functionality under extreme field conditions can heavily depend on the coupling between electro-thermal loading and microstructural deformation, but the fundamental mechanisms underpinning this coupling remain...
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Matthew Hopkins20/09/2022, 10:00
In this presentation we describe a new effort at Sandia National Laboratories and Texas Tech University to better understand the key physics involved in vacuum insulator flashover. In many pulsed power applications, the transmission lines from the capacitor bank to the target load move from a region of water insulation (separation) to vacuum insulation (separation). In this transition region...
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Ruth Peacock (Lancaster University (GB))20/09/2022, 11:00
There have been several pulsed DC system experiments ongoing including recent measurements of field emission current fluctuations associated with dislocation motion showing a variation in the number of events per pulse and a dependence on breakdowns. Also, results of a study comparing the optical spectra obtained during field emission experiments without breakdown from different materials,...
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Catarina Serafim (University of Helsinki (FI))20/09/2022, 11:30
During operation the LINAC4 RFQ (Radio-Frequency-Quadrupole) is exposed to high electric fields which can lead to vacuum breakdown. It is also subject to proton beam loss that can cause surface modification, including blistering, which can result in reduced electric field handling and an increased breakdown rate. An experimental study has been made to identify materials with high electric...
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Nicola Pilan20/09/2022, 12:00
Counterintuitive experimental evidences have been observed during High Voltage Direct Current (HVDC) tests of two concave, axial-symmetric, electrodes insulated by large vacuum gaps of 3 and 7 cm with voltages from 150 to 370 kVdc. The dissipation of microdischarge power during the conditioning procedure occurs mostly on the anodic side in a region close to the axis of the system where the...
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Silvia Spagnolo (Consorzio RFX)20/09/2022, 12:30
The High Voltage Padova Test Facility (HVPTF) is an experimental device for investigating High Voltage (HV) insulation in vacuum, in support of the realization of MITICA, the prototype of a neutral beam injector for ITER. Inside a high vacuum chamber, two stainless steel electrodes, separated by a few centimetres gap, can achieve an HV difference up to 800 kV During the conditioning process of...
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Salvador Barranco Cárceles (University of Edinburgh)20/09/2022, 14:30
Cold electron emission and vacuum breakdown are gaining popularity due to their significance in applications such as electron sources for mobile 3D medical imaging [1] and high-gradient particle accelerators [2]. The robust development of such devices is hindered by the lack of both effective computational models to engineer and optimise them through simulation and tools for the processing and...
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Taha Y. Posos (Michigan State University)20/09/2022, 15:00
In this work, we introduce a new technique which improves emittance of the carbon nanotube fiber field emission cathode (CNT fiber FEC) many-fold. CNT fibers remain of high interest for next generation electron source research and development as they have low turn-on voltage, high conductivity, durability, and flexibility. However, control over its emission properties is a challenge. Our...
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Mitchell Schneider (SLAC National Accelerator Laboratory)20/09/2022, 15:30
The work presented here shows evidence of the diversions from classical Fowler Nordheim in the RF regime. At high gradients in excess of 100 MV/m, it was shown that the field emitter cathode (FECs) made from ultra-nanocrystalline diamond (UNCD) follows a two-dimensional space charge dominated regime. Field emission dynamics now must be considered in a 2D regime, combining the 1D longitudinal...
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Ye Wang20/09/2022, 16:00
Metal surfaces have inevitable defects, such as dust particals, scratches, and protrusions caused by manufacture. Such defects will generate local electric field enhancement, occur vacuum breakdown, give rise to great damage to metal devices. The Compact Linear Collider (CLIC) in CERN, is one of the important examples where vacuum breakdown may affect the performance efficiency of the entire...
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Milad Ghaemikermani (University of Helsinki), Alvaro Lopez Cazalilla20/09/2022, 16:10
The application of superconducting radio frequency cavities on particle accelerators has brought the need of coating Cu with Nb thin films. Two techniques have been widely used in this field: conventional direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS). Experimentally, the application of both techniques has led to different surface morphologies...
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Andreas Kyritsakis20/09/2022, 16:20
The surface electric field has been regarded as the only parameter that determines the occurrence of a vacuum breakdown (VBD) for a given surface condition. However, recent studies have accumulated an increasing amount of evidence indicating that the ultimate limit depends strongly also on the electromagnetic power that is available to be delivered at the VBD site. Here we study this...
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Yuliia Lebedynska (Institute of Applied Physics, National Academy of Sciences of Ukraine)20/09/2022, 16:30
One of the main directions of development of acceleration technology is to increase the acceler ation rate to 100 MV / m, as, for example, in projects Compact LInear Collider (CLIC), International Linear Collider (ILC), which will reduce the linear dimensions of modern electronic accelerators, make them more compact and achieve record acceleration energies of charged particle beams (of the...
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Yuliia Lebedynska (Institute of Applied Physics, National Academy of Sciences of Ukraine)20/09/2022, 16:30
Field emission is one of the main factors, which leads to the loss of electrical insulating properties of the interelectrode vacuum gap. Solving the problem of reducing the field emission current value in accelerating structures is necessary to obtain gradient-stable materials before the occurrence of dark currents and, as a consequence, the possibility of overcoming high-vacuum high-gradient...
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Salva Barranco Cárceles (University of Edinburgh)20/09/2022, 16:40
In a world with a fast-growing and rapidly aging population, where availability and accuracy of diagnosis is key to early detection and treatment of disease and injury, the development of enhanced medical imaging techniques will improve the wellbeing of unwell members of society. The unique features of field emitters can be exploited to develop portable systems for 3D X-ray imaging, that will...
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Dr Tauno Tiirats (University of Tartu)20/09/2022, 16:50
Vacuum arcs –also known as breakdown–, i.e. electric discharges appearing in vacuum, are a major limiting factor for various applications such as particle accelerators, fusion reactors, vacuum interrupters, X-ray sources, and space applications. However, the physical mechanisms underlying the very initiation of the phenomenon still remain unclear. Recent experimental evidence indicates that...
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Matteo Hakeem Kushoro20/09/2022, 17:00
MITICA is an experiment located at Consorzio RFX which aims to create a prototype for ITER's Neutral Beam Injector (NBI). Since its design features an unprecedented potential difference (1 MV) there is an interest in researching means to prevent discharges in vacuum, which might prove fatal to the structure of the machine. In this context, High Voltage Padova Test Facility (HVPTF) is an...
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Victoria Bjelland20/09/2022, 17:10
Photoemission is a widely known physical phenomena that occurs when
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an electron with sufficient energy strikes the surface of a material and releases an electron. Nanoplasmonics is the manipulation of absorption, scattering and near-field interactions using different materials, shapes and sizes in the nanometer domain. -
Victoria Bjelland20/09/2022, 17:10
Photoemission plays an important role in several branches of physics. Usually, the photocathodes are covered in layers of photoemissive material to increase the quantum efficiency. Using plasmonics, is it possible to fabricate pure metallic photocathodes that has an increase in the number of emitted electrons?
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Mircea-George Coman (Uppsala University (SE))20/09/2022, 17:20
Conditioning of a metal surface in a high-voltage system is the progressive development of resistance to vacuum arcing over the operational life of the system.
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This is relevant for accelerator cavities where high level of performance is only achievable after long conditioning period. Beyond the accelerator research field, this is an important topic for any technology where breakdowns can... -
Sven Oras (University of Tartu)20/09/2022, 17:30
In this work Cu electrodes were characterized with AFM and SEM. The AFM topology map was used to create a field enhancement map of the surface. SEM images show different structures with multi-scale roughness. A hypothesis for CuO protrusion growth on the Cu electrodes is introduced.
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Ruth Peacock (Lancaster University (GB))20/09/2022, 17:40
Optical light spectra have been observed during field emission tests with Cu, CuCrZr, Nb and Ta electrode pairs in the CERN pulsed DC systems. Spectra for Cu and CuCrZr have been reliable and repeatable displaying an increase in light intensity proportional to the field-emitted current. The spectra obtained for Cu-based materials resemble the reflectance spectrum for Cu, which is likely the...
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Ruth Peacock (Lancaster University (GB))20/09/2022, 17:40
Beam loss occurs in H- RFQs, and has been observed for example in LINAC4 at CERN. To determine if such beam loss can induce breakdown, and to compare the robustness of different materials, tests have been done in pulsed high-voltage DC systems. Cathodes of different materials were irradiated with 1.2E19 H- p/cm2, the estimated beam loss of the LINAC4 RFQ. The irradiated electrodes were tested...
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Mr Roni Koitermaa (University of Helsinki)21/09/2022, 09:00
The complex physical mechanisms involved in the formation of vacuum arcs have been of interest for many decades. Vacuum arcs are relevant in many engineering disciplines, but the physics behind them is not yet fully understood. In recent years, there have been many experimental and computational studies focused on understanding aspects of vacuum arcs.
Our work focuses on further development...
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Chris Moore21/09/2022, 09:30
We present data from atomic-scale (nm) surface characterization using Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM), and Photoemission Electron Microscopy (PEEM) to show a connection between the surface’s local (atomic-scale) work function on the local nanostructure and spatially varying atomic step density. Atomic step-terrace structure is confirmed with scanning...
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Bing Xiao21/09/2022, 10:00
The most important physical parameter that determines the microstructure evolution in the thermal runaway and the subsequent electric pre-breakdown behaviors of nano-size metal field emitters and micro-protrusions on the metal surfaces is yet to be elucidated. In this work, we conduct a systematic multiscale-multiphysics simulations for FCC (Cu, Au and Al), BCC (Mo, W and Zr) and HCP (Ti, V...
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Anthony AYARI21/09/2022, 11:00
Metallic surfaces under an intense electric field tend to form nanoprotusions leading to very localized electron field emission and eventually a current runaway and the destruction of the emitter. The sharply pointed geometry of the protrusions strongly enhances the applied field over the topmost atoms which then permits all of the field emission to often come from only few atoms. This...
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Mykhailo Diachenko (Institute of Applied Physics, National Academy of Sciences of Ukraine)21/09/2022, 11:30
The physical nature of the high-voltage breakdown in vacuum that can occur in particle accelerators, particularly in the CLIC (Compact LInear Collider, CERN), is quite complex and, despite numerous studies, a complete theory of the process does not yet exist.
The emission of electrons from the cathode surface is usually local, due to the existence of tips on the surface. Increasing the...
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Yimeng LI21/09/2022, 12:00
With the rapid miniaturization and integration of electronic and electromechanical devices, especially for vacuum micro/nano electronic devices, it is urgent to study the influence of electrode surface condition on the field emission behaviors at nanoscale and explore its underlying principle. In this paper, the influence of surface carbon layer on field emission of tungsten nanotip was...
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Veronika Zadin (University of Tartu (EE))22/09/2022, 09:00
Many high electric field applications, for example, Compact Linear Collider in CERN are significantly limited by the presence of the phenomenon of electrical breakdowns. In case of sufficiently high applied electric field, even in ultra high vacuum conditions, electrical discharge appears, induces disturbances into the operating regime of the device, causes material damage and generally,...
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Alvaro Lopez Cazalilla22/09/2022, 09:30
Blistering is a process which usually takes place close to the surface of metals when they are irradiated, as can be seen in radio-frequency quadrupoles accelerating structures. This pronounced change of the surface morphology has been measured when the extended irradiation is done with energetic light ions.
The mechanism of continuous growth of a small bubble to a quantifiable size...
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Jyri Kimari22/09/2022, 10:00
Vacuum-facing metal surfaces are exposed to strong electric fields in many devices, such as particle accelerators, free-electron lasers and fusion reactors. Under sufficiently strong fields, current can arc through the vacuum, disrupting and damaging these devices. Despite decades of research, the precise mechanisms of the vacuum arc breakdowns are still unknown. The interplay of different...
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Dr Sneha Banerjee (Sandia National Laboratories)22/09/2022, 11:00
The tip-sample bias of a Scanning Tunneling Microscope (STM) is modulated by coupling THz pulses to the tip in order to get high transient tunneling current. This non-linear tunneling current and its parametric dependence need to be studied thoroughly to achieve efficient imaging of the sample surface. In this theoretical study, we investigate the basic scaling of rectified electrons in a...
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Dr Emanuele Spada (Consorzio RFX)22/09/2022, 11:30
The Switch-On effect of the current emission is known since long time. This effect is usually attributed to the existence of a Switch-On Voltage, after which the electrodes in the vacuum switch from a non-conductive to a conductive condition. Once reached the Switch-On Voltage the changes in the electrodes are permanent and the electric current can be measured even at lower voltages. In our...
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Richard Forbes (University of Surrey)22/09/2022, 12:00
This presentation forms part of my efforts to modernise the theories of field ion and electron emission and to encourage more consistent use of basic theory as between different groups of users. It primarily concerns the correction factor “v_F” that appears in the Murphy-Good (MG) theory of field electron emission (FE), which is based on the planar-image-rounded “Schottky-Nordheim” (SN)...
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Mr Dingjiangang Ding (Xi'an Jiaotong University)22/09/2022, 14:30
Conditioning experiment of a commercial vacuum interrupter was carried out with a gap distance of 1~3 mm. Energy of the vacuum arc following a breakdown has been controlled to be as low as several hundred mJ with a 100 kΩ current-limiting resistor. Breakdown voltage and field emission current was measured during the conditioning process. The Murphy-Good plot was employed to further study...
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Luigi Cordaro22/09/2022, 15:00
The design of the RFX-mod2 experimental fusion device requires a copper shell close to the plasma to aid in stability and magnetic confinement of the plasma [1,2]. This conductive structure, only 3mm thick, placed around the plasma, must have electrical discontinuities in both the poloidal and toroidal directions, so as to allow the penetration of electromagnetic fields into the plasma region....
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Gerard Lawler22/09/2022, 15:30
Producing higher brightness beams at the cathode is one of the main focuses for future electron beam applications. For photocathodes operating close to their emission threshold, the cathode lattice temperature begins to dominate the minimum achievable intrinsic emittance. At UCLA, we are designing a radiofrequency (RF) test bed for measuring the temperature dependence of the mean transverse...
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Jim Norem22/09/2022, 16:00
We find that a general vacuum arc model seems to require four stages: a trigger, ionization, plasma evolution, and surface damage. Within this framework, a large number of mechanisms operate, involving field emission, surface failure, ionization, plasma interactions with the surface and surface interactions with plasma. Field emission is complicated by the duty cycle, space charge, nearby...
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Abdellah Kharicha (CD Lab for mettalurgical applications of MHD)
Vacuum Arc Remelting (VAR) is typically the final melting process in the production of a wide range of metallic alloys. The process consists in applying during days a DC current of up to 40 000 A over large electrodes(~1m diameter). A strong DC arc takes place under vacuum between an electrode and a water-cooled copper crucible. For low applied current, high-speed video observation showed that...
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Michael Keidar
We will present recent results related to control of vacuum arc ignition and erosion in pulsed vacu-um arc devices. As a particular example, we will consider vacuum arc microthruster devices. To this end, we will discuss breakdown process in the case of a micro-vacuum arc thruster. In these devices ignition analysis includes effects of insulator material, cathode material deposition and...
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Gerard Lawler
High gradient RF structures capable of maintaining gradients in excess of 250 MV/m are critical in several concepts for future electron accelerators. Concepts such as the ultra-compact free electron laser (UC-XFEL) and the Cool Copper Collider (C3) plan to obtain these gradients through the cryogenic operation (<77K) of normal conducting copper cavities. Breakdown rates, the most significant...
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