SARAF tour (3 groups alternating between: 1) accelerator 2) target 3) control and RF)
Laser Plasma Accelerators (LPA) rely on the control of the electrons motion with intense laser pulses [1]. Improvements of electron beam parameters such as divergence, energy or energy spread are possible thanks to this fine optical control. This manipulation of electrons with intense laser pulses allows a fine mapping of the longitudinal and radial components of giant electric fields that can...
We will present overview of plasma modeling and experiments related to vacuum arc devices. The character of the plasma flow depending on the anode geometries relatively to the expanding plasma jet will be analyzed. Specifics of the high-current vacuum arc will be examined including several effects associated with high-current arc behavior in a magnetic field and transition from diffusion to...
The Plasma Window is a novel apparatus that utilizes a stabilized plasma arc as interface between vacuum and atmosphere or pressurized targets without solid material. Additionally, the plasma has a lensing effect on charged particles. Furthermore, plasma from a plasma window was extended over 2 cm into atmosphere to form a plasma shield (Plasma Shield is basically a Plasma Window extended to...
In this talk we will introduce a new effort at Sandia to better understand the mechanisms involved in vacuum field emission from real, contaminated surfaces. We concentrate on the vacuum field emission of electrons from a surface based strongly on the role of initial emission as a necessary precursor to vacuum discharge. Although other ongoing work at Sandia is concerned with the evolution of...
High-field and breakdown experiments are being carried out at CERN using a pulsed dc set-up. The objective of the pulsed dc experiments are to complement the ongoing high-gradient radio frequency experimental program and for optimizing material treatments, parameters of conditioning process and methods of minimization of breakdown events in accelerating structures. The latest hardware...
A spectrometer for detection of the dark and breakdown currents during conditioning of new accelerating structures for CLIC, is operated at one of the 12 GHz stand-alone test-stands at CERN. The spectrometer consists of a dipole magnet, a variable collimator, and a fluorescent screen read out by a fast camera.
Built for high repetition rate operation it can measure the spatial and energy...
This presentation aims to provide brief updates on our current understanding of the theory of field electron emission (FE), on progress in putting FE theory onto a more scientific basis, and on progress in interpreting measured FE current-voltage [Im(Vm)] characteristics. Possibly the main use of FE theory in vacuum breakdown is to provide good formulae for use in simulators, with some...
Electron emission from nanometric emitters becomes increasingly interesting due to its involvement to vacuum breakdown phenomena and various other vacuum nanoelectronics applications. The most commonly used theoretical tool for the calculation of electron emission is still nowadays the Fowler-Nordheim (F-N) equation, although it has been shown that it is inadequate for nanometrically sharp...
When an individual post-like field emitter (modelled as standing on one of a pair of parallel planes) is sufficiently resistive, and current through it sufficiently large, then voltage loss occurs along it. This Poster shows this voltage loss is directly and inextricably linked to a reduction in the field enhancement factor (FEF) at the post apex. A formula relating apex-FEF reduction to this...
In a Vacuum Arc with a Black Body Assembly (VABBA), cathode material is
emitted into a closed chamber formed by the end surface of a water-cooled cylindrical
cathode and a cup shaped refractory anode that is heated by the arc. Material is eroded from
the cathode spots as plasma and MPs. They impinge on the hot anode and are re-evaporated
from it, forming a dense high-pressure...
Many optical elements such as mirror, window, amplifier, and wavelength conversion elements are made from oxide materials. The elucidation of damage threshold and mechanism is indispensable to the development of high-power system. Especially, in high-repetition-rate system, the accumulation of defects such as color center causes the damage.
In this work, we propose a kinetic model including...
Certain adaptations of metal vapor vacuum arcs can be a promising approach as high charge state ion sources for various applications including relativistic heavy ion colliders, heavy ion inertial confinement fusion and with modifications to boron phosphorous as well as for As, and Sb ions for ion implantation for semiconductor manufacturing as well. Two approaches have shown good promise:...
This note suggests improvements in presenting field electron emission (FE) theory. It relates to the so-called principal Schottky-Nordheim (SN) barrier function "v" used in Murphy-Good-type FE theory. I argue that: (1) we should separate the mathematics of v from its applications in tunnelling theory; (2) we need to change the independent variable used; and (3) we can improve the notation.
It...
For the development of high gradient accelerating structures of CLIC and other similar machines it is vital to understand the mechanism of vacuum breakdown. Previous investigations into the phenomena were performed through lengthy and costly RF conditioning of accelerating structures and cells. In an attempt to more efficiently investigate the effects of different materials and conditioning...
Surface defects are thought to play an important role in vacuum breakdowns. Possible nanoprotrusions can significantly enhance the electric field, leading to increased field emission, atom evaporation, and eventually arcing. Atomic scale simulations are very useful in studying dislocation behaviour and surface defects. However, the main material of interest, copper, is in practice covered by...
In order to investigate the evolution of species type during a vacuum breakdown, a spectroscope with a ICCD was adopted to detect the spectrum during the breakdown, and the state of species can be deduced from the spectra. A tip-to-tip gap was installed in a vacuum chamber; the gap distance varied from 0.5 mm to 3 mm; and copper and tungsten were chosen as the cathode and anode material,...
Fowler-Nordheim (FN) theory uses "smooth-surface" conceptual emitter models. It is not accurate theory because it disregards the existence of atoms, and takes the emitter surface charge as confined to an infinitesimally thin classical layer. Developing fully self-consistent field emission theory will be intensively difficult, and has received limited attention. It could take 50-100 years....
Y. Jiang, (1) S.V. Shchelkunov, (1,2) X.Chang, (1) and J.L. Hirshfield, (1,2)
1 Beam Physics Laboratory, Yale University, New Haven, CT 06511 (USA)
2 Omega-P R&D, Inc. New Haven, CT 06511 (USA)
ABSTRACT
Theory and simulations predict that metallic microwave accelerating structures, including RF guns, should operate at high acceleration gradients with reduced breakdown rates and/or with...
X-band high gradient accelerating structures has been studied in Tsinghua University, especially the choke-mode damped structure. Several single-cell standing-wave structures have been built for high power test. One choke-mode structure achieved a max gradient of 75 MV/m and the choke breakdown limited further increasing of the gradient. Postmortem inner surface inspection of the choke-mode...
The objective of this paper is to determine influence of depth of cathode melted layer on vacuum insulation. A 12kV vacuum interrupter with a pair of rod-plane electrode was designed. A DC current source is applied on the test vacuum interrupter, in which the rod electrode is chosen as anode. A drawn vacuum arc is used to generate different depth of melted layer on the rod electrode in an...
The initial stage of vacuum breakdown is still lack of direct experimental evidence because it is difficult to observe and measure. The objective of this study is to observe the light emission in the initial stage by optical diagnosis.We adopted a ICCD camera to catch a series of images in a breakdown and two optical filters to distinguish the light from copper atom (Cu I ) or singly charged...
Increasing satellite bus voltages from 28-100 V to 300-600 V is presently under investigation [1]. High voltages will be required to power new generations of ion and Hall effect thrusters [2], with higher power efficiencies, lower power system costs and substantial mass savings. At the same time, increasing bus voltage could lead to a higher risk of electrical breakdown for several satellite...
In the frame of the CLIC (Compact Lineal Collider) study for the development of a two-beam accelerating technology for a future TeV–scale e+e- linear collider, electrical breakdown (BD) phenomena are deeply studied. BDs are caused by the interaction of the oxygen free electronic copper (Cu-OFE) with vacuum arcs at high gradient operation. BD phenomena lead to isolated or clusters of craters on...
Numerous devices featuring diverse applications contain metal that is exposed to high electric fields and operate under a vacuum. At high electric fields, an occurrence of plasma formation inside the vacuum can lead to breakdown [BD] and failure of the device. Despite the fact that the detailed mechanisms controlling BD have been the subject of extensive study for the past several decades,...
A new type of micropropulsion device is presented – the inline-screw-feeding vacuum-arc thruster (ISF-VAT). This thruster couples a conventional ”triggerless” vacuum arc ignition system with a feeding mechanism that maintains a steady state discharge performance. The feeding mechanism implements a screw action on a central cathode rod. At a predetermined rate, a complete and uniform erosion of...
RF breakdown has a negative impact on a RF cavity’s performance, especially with the presence of strong magnetic fields. This issue can arise in designs of muon ionization cooling channel, RF guns, klystrons and in many other applications. The MuCool Test Area at Fermilab is the facility that allows us to study the effects of static magnetic field on RF cavity operation. As a part of this...
The study of surface flashover behavior in nanoscale dielectrics has broad application in nano- and microfabricated on-chip electronics such as energy converters, logic/memory, mass spectrometers, field emission devices, and compact accelerators. Most microscale devices incorporate planar dielectric architectures (e.g. gate oxides in transistors) without exposed surfaces. Therefore, relatively...
A novel coaxial resonator to investigate two-surface multipactor discharges on metal and dielectric surfaces in the gap region under vacuum conditions (~10-8 mbar) has been developed and tested. The resonator is ~ 100 mm in length with an outer diameter of ~ 60 mm (internal dimensions). A pulsed RF source delivers up to 30 W average power over a wide frequency range 650-900 MHz to the RF...
Review of BD related studies in HIP
A common hypothesis for explaining the occurrence of vacuum arcs is that high electric fields enable growth of nanotips on the metallic electrode surfaces. These tips are locally enhancing the field enough to cause field emission and start processes that eventually cause an arc. We have developed a Kinetic Monte Carlo (KMC) model that describes the atom diffusion on metal surfaces as thermally...
In 2011 the hybrid electrodynamics – molecular dynamics code HELMOD was developed to study the effect of electric field to the stability of metal surface. That code has been successfully used in several projects. However, to cope with the forthcoming challenges, the efficiency of the software must be enhanced. This will be done by transferring the electric field solver from the finite...
Breakdown events are a very important issue in high-voltage vacuum holding studies. These events are usually and efficiently studied using heuristic predictive models based on some experimentally-given parameters. However, the complete microscopic view is still lacking. A different, but related topic is the dark-current-emission which have been first theoretically studied by Fowler and...
It has been suggested that BD nucleation is a possible outcome of a stochastic process where
the dislocation population is undergoing a critical transition leading to critical protrusion formation.
In the past few years we have used a combination of post mortem microscopy work as well as acoustic emission studies and field emission studies in order to try and track the dislocation...
Triggered vacuum spark gaps are useful as fast, high current switches in a variety of pulsed power applications. We will present a 3D PIC-DSMC model of a triggered vacuum switch in which the arc is triggered by running a current through a semi-conductive film adjacent to the cathode surface. Joule heating of the film results in both electron and material emission and the gap ultimately breaks...
Increasing interest in using DFT methods to model charged surfaces is illustrated by the MeVARC 2017 programme. However, the need to understand charged surfaces at the atomic level has long existed, most notably since Bahadur and Müller first "saw atoms" in 1956, using their new field ion microscope technique [1], and needed to explain how it worked. This didn't happen for another 20 years,...
High field enhancement factor, in range 50-100 is commonly measured quantity in CLIC accelerator design. Such field enhancement values are usually associated with high aspect ratio surface irregularities appearing under applied electric fields. However, if dynamic surface change is present during ramping of the electric field or due to the lower than estimated work function value, change in...
Around 1900, it was shown that voltage breakdown could occur at a predetermined surface field, rather than through an electron avalanche. The first simple explanation of this data was given about three years later. In spite of this early progress, the field of vacuum breakdown is still not settled science. We describe our self-consistent model of rf breakdown that seems to provide useful...
