4th International Workshop on Mechanisms of Vacuum Arcs (MeVArc 2013)

4 Nov 2013, 08:00 → 7 Nov 2013, 22:00 Europe/Zurich

Hotel Les Aiglons

Alexia Augier (CERN), Flyura Djurabekova (University of Helsinki), Matthew Hopkins (Sandia National Laboratories), Sergio Calatroni (CERN), Walter Wuensch (CERN)

Introduction

Vacuum arcs are a concern in nearly every vacuum electronic device, consequently are present in a very wide range of applications. Sometimes they form the basis for device operation, but all too often they are the primary failure mode.
Understanding the physical processes of an arc requires the expertise from many disciplines – material science, surface physics, plasma physics, high-voltage systems and radio frequency to name a few.
The purpose of this workshop series is to bring together scientists and engineers from many different applications and disciplines to discuss the latest efforts to understanding of vacuum arcs. We cover theory, simulation and experiments.
The multidisciplinary nature of vacuum arcs and vacuum devices provides a rich environment for finding physics of shared interest from multiple sources.

Some specific topics include:

• High electric field gradient devices (e.g., high-gradient accelerators)
• Modelling and simulation
• Specialized experiments
• Diagnostic methods

We welcome new areas of investigation in addition to the above. The link "Previous Workshops" can help you better understand the character of our workshop.

The workshop will run from Tuesday morning, 5 November to mid-day Thursday 7 November at the Hotel Les Aiglons in Chamonix. On Thursday afternoon there will be free time for exploring Chamonix or informal discussions.

Scientific program

The scientific (and social) program can be seen in the link above "Timetable."

Registered participants are now invited to make proposals for presentations and posters. Please send a title and a brief, two to four line, abstract to Walter Wuensch specifying oral or poster. We are targeting 30 minute (although this may change) oral presentations including discussion, maximum one per participant. The poster session will be held on Wednesday afternoon.

Please note that the program will evolve so please check periodically for the latest information.

Mini-School: Introduction to Radio Frequency Acceleration

The workshop will be preceded by a "Mini-School: Introduction to Radio Frequency Acceleration" on Monday 4 November 2013, also at the Hotel Les Aiglons. The objective of the mini-school is to provide non-accelerator CLIC collaborators more background on high-gradient acceleration. Everyone is welcome to participate.

The program for the mini-school is available via the link "Timetable."

Please contact Walter Wuensch for further information about this event.

Visit to CERN

A visit to the CERN will held on Friday 8 November 2013, after the workshop. Tours of various facilities, especially high-gradient test areas, will occur in the morning and early afternoon. Conference rooms for collaboration meetings or further discussion can be arranged.

Transportation from Chamonix to CERN late Thursday afternoon will be provided. Participants in this event can stay at the CERN hostel on Thursday night and are encouraged to make their reservation as soon as possible.

Please contact Sergio Calatroni for further information about this event.

Registration

Registration is opened. There is NO registration fee (Coffee breaks and lunches are included in the workshop).

Participants are requested to pay their room directly with the hotel (see Accommodation and Registration menus for more details)

Monday, 4 November

09:30 → 12:30 Pre-workshop - Introduction to rf acceleration

09:30 Introduction to the concepts of rf acceleration

dc, standing wave, traveling wave, phase velocity, synchronism, energy/power transfer

Speaker: Walter Wuensch (CERN)

Slides MeVArc_rf_concepts.pdf MeVArc_rf_concepts.pptx

10:30 Coffee and discussion

11:30 Fields and features of accelerating structures

resonant cavities, periodic structures, field patterns, couplers

Speaker: Walter Wuensch (CERN)

Slides MeVArc_accelerating_structures.pdf MeVArc_accelerating_structures.pptx

12:30 → 14:30 Lunch and discussion

14:30 → 16:30 Pre-workshop - Introduction to rf acceleration 2

14:30 Operation of rf structures at high-powers and gradients

rf systems, test stands, breakdown, signals, processing, scaling laws

Speaker: Walter Wuensch (CERN)

Slides MeVArc_high-gradient.pdf MeVArc_high-gradient.pptx

15:30 Coffee and discussion

19:30 → 21:30 Dinner

Tuesday, 5 November

09:00 → 12:30 Experimental 1 - rf systems

Convener: Matthew Hopkins (Sandia National Laboratories)

09:00 High-gradient test results from CERN XBox-1

Speaker: Ben Woolley (Lancaster University (GB))

Slides XBOX-1-B-WOOLLEY-5th-NOV-13.pdf XBOX-1-B-WOOLLEY-5th-NOV-13.pptx

09:30 Studies of breakdowns at high-gradients in TBTS/CTF3

Breakdown studies and results in Two-Beam Test Stand in CTF3, where accelerating structures are being tested at gradients up to 150MV/m, will be summarized. The planned modifications will be outlined.

Speaker: Alexey Dubrovskiy (Joint Inst. for Nuclear Research (RU))

Slides Studies_of_breakdowns_at_high-gradients_in_TBTS-CTF3.pdf

10:00 Laser triggered RF breakdown experiment

Speaker: Jiaru Shi (Tsinghua University)

Slides 201311_Jiaru_MeVArc.pdf 201311_Jiaru_MeVArc.pptx

10:30 Coffee

11:00 Recent High Gradient Experiments at SLAC

I will review results of our dual-mode cavity experiment, update on cryo experiments with normal conducting single-cell structure and on reproducibility test of hard CuAg.

Speaker: Dr Valery Dolgashev (SLAC)

Slides

• Film-FIB-Rigato.mp4
• Recent_high_gradient_tests_at_SLAC_Dolgashev_4nov2013.pdf
• Recent_high_gradient_tests_at_SLAC_Dolgashev_4nov2013.pptx

11:30 Research and Achievements on X-band accelerator structures at LNF

The next generation of linear accelerators is highly demanding in terms of accelerating gradients. To upgrade performances of X band Linacs at 11.424 GHz many resources are devoted to achieve higher accelerating gradients and at the same time obtain the highest possible reliability. In the framework of a large collaboration among SLAC (USA), KEK (Japan) and INFN-LNF, our laboratories have been involved in the design, manufacture and test of short high power standing wave (SW) sections operating at 11.424 GHz. In particular, because electroforming is a very attractive technique to manufacture compact structures avoiding the soft brazing while maintaining mechanical properties and high vacuum requirements, recently an electroformed SW structure has been also realized and tested a room temperature. Among other R&D activities, characterization and tests of molybdenum coatings are in progress. Copper coated by molybdenum via sputtering under vacuum is another promising approach to increase the accelerating gradient of RF cavities working at higher frequencies. In this contribution we present the status of the different activities on high gradient SW electroformed structures, the first measurement on high frequency hybrid accelerators and the characterization of thick molybdenum coatings.

Speaker: Bruno Spataro (INFN - LNF)

Slides Spataro_MeVArc_2013.pdf Spataro_MeVArc_2013.pptx

12:00 Multi-Harmonic Cavities with Suppression of Surface Pulsed Heating and Field Emission

Multi-harmonic cavities that are designed for RF breakdown experiments are described. The fundamental TM010 and its higher harmonic, either TM020, or TM011, or TM012 modes of the cavities are to be excited coherently by an external RF source in the expectation of lowering surface field emission and/or surface pulsed heating, so as to reduce breakdown probability and possibly achieve an increase in acceleration gradient.

Speaker: Jiang Yong (Yale University)

Slides YJiang_MeVArc4_Presentation.pdf YJiang_MeVArc4_Presentation.pptx

12:30 → 14:30 Lunch

14:30 → 18:00 Theory and Simulation 1 - Materials and Surfaces

Convener: Sergio Calatroni (CERN)

14:30 Linking intermittent plasticity and RF breakdown

Stochastic models have been proven successful in describing response of systems characterized by intermittent plasticity. We use similar type of models to identify possible mechanisms for RF breakdown. Specifically we explore the option that breakdown is initiated due to a correlated dislocation avalanche, caused by stresses in the sample.

Speaker: Dr Yinon Ashkenazy (The Hebrew University of Jerusalem (IL))

Slides mevarc2013_ashkenazy_v01.1.pdf mevarc2013_ashkenazy_v01.1.pptx

15:00 Computer simulation of Cu surface behavior under high external electic field before and after plasma impact

Speaker: Flyura Djurabekova (University of Helsinki)

Slides Djurabekova_MeVARC2013.pdf Djurabekova_MeVARC2013.pptx

15:30 Electro-mechanical and thermal simulations of surface under electric field

To investigate and understand the interaction between the surface, bulk material and the electric field, we simulated and studied field emitters by analyzing electro-mechanical and thermal effects, possible interactions between the emitters and dynamic field enhancement factor changes due to the mechanical response of the tips using finite element method. To simulate accurately temperature changes in the emitters, we added nanoscale size effects to thermal and electrical conductivities.

Speaker: Dr Vahur Zadin (University of Tartu)

Slides Electro-mechanical_and_thermal_simulations_of_surface_under_electric_field.pdf Electro-mechanical_and_thermal_simulations_of_surface_under_electric_field.ppt

16:00 Coffee

16:30 Manufacturing of void-filled cathodes for breakdown experiments

Speaker: Anders Jaakko Korsback (University of Helsinki (FI))

Slides AndersKorsback_presentation_MeVArc2013.pdf AndersKorsback_presentation_MeVArc2013.pptx

16:50 MD simulations of Fe precipitates in Cu

Spherical Fe precipitates were simulated with MD in Cu under high electric fields to investigate their effect on dislocations in the Cu lattice. Different depths of the precipitates were studied. Fe precipitates were also placed on Cu grain boundaries.

Speaker: Simon Vigonski

Slides MD_simulations_of_Fe_precipitates_in_Cu_-_Vigonski_presentation.pptx

20:00 → 22:00 Dinner

Wednesday, 6 November

09:00 → 10:30 Experimental 2 - dc systems and high-voltage technology

Convener: Dr Yinon Ashkenazy (The Hebrew University of Jerusalem (IL))

09:00 Latest results from CERN DC Spark System I

Obtaining a low breakdown rate in CLIC accelerating structures is a critical requirement for the successful operation of the proposed collider. To complement the RF study of breakdown phenomena the CERN DC spark systems were constructed. Recently system I has been vastly improved and can now operate at a repetition rate of 1kHz and is also able to monitor the electrode gap size. In this talk the latest improvements and results from system I will be presented. These will include the turn on time of the breakdown, the breakdown position within the pulse, the breakdown rate, the burning voltage and breakdown distribution statistics as well as how these parameters are affected by the electric field and gap size. Comparisons will also be made to results obtained from C-band and X-band RF tests and simulations.

Speaker: Nicholas Shipman (University of Manchester (GB))

Slides Shipman_MeVArc.pdf Shipman_MeVArc.pptx

09:30 Impact of dry ice cleaning on the enhanced field emission from flat Cu samples

Enhanced field emission (EFE), resulting in dark currents and electric breakdowns, is one of the main gradient limitations for the CLIC accelerating structures (actual design Eacc = 100 MV/m, Epeak = 240 MV/m) [1]. Measurements on diamond-turned, flat (Ra = 158 nm) Cu samples showed first EFE at surface fields Es = 130 MV/m [2]. In order to reduce EFE, we have installed a commercial dry ice cleaning (DIC) system in our clean room (class iso 5). As expected, the number density of emitters (N) was significantly decreased from N = 52 cm-2 to N = 12 cm-2 at Es = 190 MV/m after DIC. Furthermore we have tested two diamond-turned and chemically etched (SLAC treatment, Ra = 150 nm) Cu samples after DIC resulting in first EFE at 230 MV/m. Locally measured I(V) characteristics of the strongest emitters revealed field enhancement factors β = 10 – 90 on the diamond-turned sample and β = 10 – 85 on the chemically etched samples. SEM and EDX investigations of the located emission sites will be presented at the workshop.

Speaker: Stefan Lagotzky (University of Wuppertal)

Slides Lagotzky_MeVArc2013.pptx

10:00 Impact of electro-chemical cleaning on the dielectric strength in sealed vacuum interrupters

Vacuum interrupters (VI's) are used in medium voltage circuit breakers to protect electrical distribution systems. Today the world wide annuel production exceeds 2 million units. Size reduction of VI's and application in solid insulated systems tend to increase the dielectric stresses inside these VI's. Cleaning is a key parameter to obtain reproducible dielectric results. Here the impact of electro-chemical cleaning will be discussed. Although considerable progress is obtained in the dielectric strength over the last 5 years, a great difference persists to exist with respect to Cern published results.

Speaker: Hans Schellekens (Schneider Electric)

Slides Cern_Chamonix_2013.pdf

10:30 → 11:00 Coffee

11:00 → 12:30 Theory and Simulation 2 - Plasma: part 1

Convener: Dr Yinon Ashkenazy (The Hebrew University of Jerusalem (IL))

11:00 Paschen Concepts in Non-Uniform Gas

Christopher H. Moore, Matthew M. Hopkins, Jeremiah J. Boerner, Paul S. Crozier, Stan G. Moore The study of arc discharge often begins with Paschen’s analysis of breakdown processes under a number of assumptions. These assumptions are usually not valid in practice. We investigate changes to breakdown voltage when violating the assumption of uniform background gas.

Speaker: Matthew Hopkins (Sandia National Laboratories)

Slides Paschen_Concepts_in_a_Non-Uniform_Gas.pdf

11:30 Molecular Dynamics Simulations > of Ion Irradiation of a Surface under an Electric Field

I will talk about how the sputtering yield of Cu is increased in the presence of an electric field. The sputtering yield is important from the point of view of the plasma formation, and we need to be able to explain where all the neutral atoms are coming from that are needed for the plasma. The sputtering yield is used as an input parameter for e.g. the PIC simulations, so it's good to understand what it happening there.

Speaker: Stefan Anton Parviainen (Helsinki Institute of Physics (HIP))

Slides parviainen-expanded.pdf parviainen.odp

12:00 Ignition of explosive electron emission pulses - ectons as initiation of vacuum discharge stages - the breakdown, the spark, and the arc

M. M Tsventoukh, G.A.Mesyats and S.A.Barengolts The vacuum discharge implies a formation of plasma from the electrode material for a large current transfer. It consists of three stages - vacuum breakdown, vacuum spark, and final - vacuum arc. The basic feature of all these stages - explosive electron emission (EEE) pulses - ectons that arise from microcenters at the cathode and are responsible for an electron emission current of a large density and large magnitude. Ignition of the EEE pulses under the external action of plasma and power fluxes at a surface of a fine structure is considered. The work was supported in part by RFBR grants 12-08-33031 and 13-08-01397 G. A. Mesyats 2013 IEEE Trans. Plasma Sci. 41 676 S. A. Barengolts, G. A. Mesyats, and M. M. Tsventoukh 2010 Nucl. Fusion 50 125004 S. A. Barengolts, G. A. Mesyats, and M. M. Tsventoukh 2008 JETP 107 1039

Speaker: Mikhail Tsventoukh

Slides Vacuum_Discharge___Initiation_EEE.pdf Vacuum_Discharge___Initiation_EEE.ppt

12:30 → 14:30 Lunch

14:30 → 18:00 Poster session

14:30 Catastrophic Breakdown in Nonlinear Optical Crystals under Continuous-wave Laser Irradiation

High-power continuous-wave (CW) lasers are used in many industries, including laser processing, fiber-optic communication, and laser display. Laser-induced breakdown damage in optical materials limits the laser power. We propose a nonlinear absorption mechanism in optical crystals under CW laser irradiation. The absorption mechanism works for catastrophic CW laser breakdown in an optical crystal such as LiNbO3 or LiTaO3, in which the bandgap energy is less than two times the photon energy and which has a long-lived excited state like a polaron.

Speaker: Susumu Kato (National Institute of Advanced Industrial Science and Technology)

14:30 Determination of dislocations density in Cu-OFE for CLIC project by using EBSD

In the frame of the CLIC (Compact Lineal Collider) Study, we are especially interested in a better understanding of the superficial modifications on oxygen free electronic copper (Cu-OFE) caused by the interaction with vacuum arcs at high gradient operation. Radio-frequency breakdown (BKD) phenomena are responsible of some of the surface damages observed in CLIC accelerate structures which are made out the cited material. Recent finite element simulations carried out in Helsinki University about deformation on pure copper surface applying a force similar to the applied by an electric field, shows a relation between the BKD affected zone and the concentration of dislocations on the material. More generally, dislocations could have certain relation with the field emission sites even in areas away from BKD affected zones. A crucial aspect of determining the role of dislocations in BKD is to find a diagnostic technique which determines their presence and/or density. The study of the bibliography presents the Electron Backscatter Diffraction (EBSD) like a valid technique for the assessment of the dislocation density in specimens subjected to a plastic deformation. In EBSD, diffraction patterns are generated when electrons are diffracted by the lattice planes in the specimen and they are formed on a suitably placed phosphor screen. Patterns consist of a set of Kikuchi bands which are characteristic of the sample crystal structure and orientation. In deformed samples the dimensions of crystal lattice are distorted (due to higher dislocation density), which leads to a greater angular distribution (variation) of the mentioned diffracted crystallographic planes that results in decreased Kikuchi band contrast and blurred Kikuchi band edges. This distortion of the EBSD pattern can be directly related with the residual strain of the specimen. The aim of this study, therefore, is to legitimatise the EBSD technique for the determination of dislocation density in Cu-OFE and to establish, if it is possible, a relation between them and the BKD phenomena on this material.

Speaker: Ana Teresa Perez Fontenla (Universidad Nacional de Educacion a Distancia UNED)

Poster Poster_MeVarc_2013.pdf

14:30 Dielectric testing of vacuum circuit breakers for high-voltage applications above distribution voltage level

Vacuum circuit breakers (VCB) are well established in the distribution voltage level (up to 52 kV) of the electrical power supply system. In order to establish this technology at higher voltages, which may be desirable in order to reduce the use of sulfur-hexafluoride, the usual insulating and arc quenching medium in high-voltage circuit breakers, the dielectric withstand reliability – especially when switching capacitive loads – must be increased. In this contribution, we show our approach to investigate the field emission current as well as the micro discharges of commercial VCB for test voltages up to 200 kV.

Speaker: Benjamin Baum (TU Darmstadt)

Poster MevArc_2013_Poster_Baum.pdf

14:30 Instrumentation developments in DC spark setups at CLIC

DC spark systems are used at the CLIC group at CERN to study electrical breakdown in vacuum. Recent instrumentation developments are presented. Capacitive gap distance measurement has been implemented to allow non-contact setting and feedback control of gap distance. Temperature control has been implemented to allow for the study of the effect of temperature on breakdown rate. Optical spectroscopy has been added to allow for plasma characterization.

Speakers: Anders Jaakko Korsback (University of Helsinki (FI)), Iaroslava Profatilova (Nat. Acad. Sci., Inst. Appl. Phys. (UA)), Nicholas Shipman (University of Manchester (GB)), Tomoko Muranaka (CERN)

Slides Korsback_poster_1.pdf Korsback_poster_1.pptx

14:30 MD simulations of Fe precipitates in Cu

Spherical Fe precipitates were simulated with MD in Cu under high electric fields to investigate their effect on dislocations in the Cu lattice. Different depths of the precipitates were studied. Fe precipitates were also placed on Cu grain boundaries.

Speaker: Simon Vigonski (University of Tartu)

Poster MD_Simulation_of_Fe_Precipitates_in_Cu_-_Vigonski.pdf

14:30 Simulation of Breakdown in Cu-Cr Metal Vapor

Slides Simulation_of_Breakdown_in_Cu-Cr_Metal_Vapor.pdf

14:30 Spectroscopic analysis of DC breakdowns

Optical spectroscopy is a powerful diagnostic method for investigating plasma and surfase processes in a breakdown. Optical spectrometer used in combination with DC system at the CLIC group at CERN for studying breakdown. Description of experimental setup are presented. Spectra of DC breakdown between copper electrodes are produced.

Speaker: Iaroslava Profatilova (Nat. Acad. Sci., Inst. Appl. Phys. (UA))

Slides Profatilova_Spectroscopic_analysis_of_DC_breakdowns_MeVarc_2013.pdf

14:30 Surface flashover of high purity alumina during a pulsed electric field

The LHC injection kicker magnets include beam screens to shield the ferrite yoke against the effects of the high intensity beam: the screening is provided by conductors lodged in the inner wall of a high purity alumina support tube. The alumina must have a low rate of flashover. This screening will be further improved by additional conductors; however these must not compromise the good high-voltage behaviour. Extensive studies have been carried out to better satisfy the often conflicting requirements for low beam coupling impedance, fast magnetic field rise-time, ultra-high vacuum and good high voltage behaviour. The new design will be presented together results of high voltage tests. Significant pressure rises, due to electron-cloud, can occur in and nearby the alumina tube: the predominant gas desorbed from surfaces is hydrogen. Similarly temperature rise of the ferrite yoke can result in an increase in pressure. A series of high voltage tests are planned for the laboratory in which various gases are injected into a test tank: this will allow a careful and systematic study of the effect of pressure upon surface flashover of the ceramic tube. In addition various coatings are under investigation for further reducing surface flashover. The plans for these tests and a summary of the coatings under investigation will be presented.

Speakers: Hugo Alistair Day (University of Manchester (GB)), Dr Mike Barnes (CERN)

Slides MeVARc_Nov2013.pdf

14:30 The Object Kinetic Monte Carlo method

Object Kinetic Monte Carlo (OKMC) is a stochastic simulation method, suitable for modelling of systems were the processes are on a time scale of seconds to years. The general method of OKMC will be presented based on an example of evolution of nanostructure in model alloys of steels employed in reactor pressure vessels of nuclear reactors. We will discuss the possibilities and restrictions of the OKMC method for future applications for simulations of materials operated under high electric fields.

Speakers: Flyura Djurabekova (University of Helsinki), Ville Jansson (University of Helsiniki)

Poster 2013_MeVArc_poster.pdf

14:30 Voltage holding prediction in multi electrode – multi voltage systems insulated in vacuum

The work is aimed at proposing a criterion for the DC voltage breakdown prediction of a complex system insulated by large vacuum gaps , the model is based on the implementation of the micro particle (clump) induced breakdown Cranberg-Slivkov theory into a statistical approach

Speaker: Nicola Pilan (Consorzio RFX)

Poster Pilan_poster__MeVArc_2013.ppt

Slides Pilan_poster__MeVArc_2013.pdf

16:00 → 16:30 Coffee

19:30 → 21:00 Banquet

Thursday, 7 November

09:00 → 12:30 Theory and Simulation 2 - Plasma: part 2

Convener: Flyura Djurabekova (University of Helsinki)

09:00 Modeling approaches to vacuum arc plasmas

We will present the state of the art modeling approach to describe the plasma (primarily interelectrode) of the vacuum arc. The model is based on the free boundary plasma expansion. The character of the plasma expansion depends on the anode geometries (disk anode, ring anode, and small anode relatively to the expanding plasma). Peculiarities of the high-current vacuum arc will be discussed. Several effects associated with high-current arc such as behavior in a magnetic field and transition from diffusion to constricted arc will be discussed. New applications of the vacuum arc will be discussed as well.

Speaker: Michael Keidar (George Washington University)

Slides

• ndne-field.mp4
• ndne-nofield.mp4
• Vacuum_arc_modeling.pdf
• Vacuum_arc_modeling.ppt

09:30 Particle-in-cell simulation of vacuum arc ignition and development

A full arc cycle (ignition-growth-steady-state-extinction) is simulated using the particle-in-cell (PIC) code ArcPic2D. The model features electrons, singly charged ions, and neutrals which are tracked in a rotational symmetric geometry with 2 position components and 3 velocity components between two parallel plates. Modelled interactions are long range electrostatic fields (PIC), coulomb collisions, elastic collisions e-Cu, charge exchange, and impact ionization. The surface model includes Fowler-Nordheim (FN) electron emission, Yamamura-Tawara sputtering and heatspike sputtering, single electron yield, and neutral evaporation. The circuit is a small capacitor which depletes from the current in the arc. Using this model, we are able to show a qualitatively correct ignition and development of a vaccuum arc copper plasma, and are predicting the conditions on and above the surface during a breakdown.

Speaker: Kyrre Ness Sjobaek (University of Oslo (NO))

Slides

• 2013-11-07-MevArc_Kyrre.pdf
• 5800V-0ohm_dens_log.mp4
• 5800V-5000ohm_dens_log.mp4
• 5800V_bf2-dens_log.mp4
• R0-5800V-potential1D.mp4
• R0-5800V-quiver_zoom.mp4
• R0-5800V-vhist_e.mp4
• R5000-5800V-potential1D.mp4
• R5000-5800V-quiver_zoom.mp4
• R5000-5800V-vhist_Cu.mp4

10:00 1D PIC/DSMC computer modeling of near-cathode plasma layers and expansion of cathode plasma flare of vacuum arc cathode spot

A 1D PIC/DSMC model was developed to simulate the near-cathode layer of plasma with the parameters typical for plasma of vacuum arc cathode spot. The model takes into account the main types of inelastic and elastic collisions (including the coulomb one) of particles in the plasma as well as evaporation and thermo-field electron emission from the cathode. A wide range of characteristics of the quasistationary plasma layer was calculated with the help of the model. It was obtained that the cathode spot can operate in two modes: quasistationary mode with radius ~ 100 microns and current density ~ 10^6 A/cm^2, and nonstationary mode with radius ~ 1 microns and current density >10^7 A/cm^2. The simulation of the expansion of cathode spot plasma flare, which appears at the vacuum breakdown, was performed with the help of the same model. It is obtained that the plasma flare is a dense high ionized plasma cloud expanding to vacuum with velocity of about 210^6 cm/s, that is in good agreement with the known experimental results.

Speaker: Dmitry Shmelev (Institute of Electrophysics RAS)

Slides MeVArc_9_Shmelev.pdf MeVArc_9_Shmelev.ppt

10:30 Coffee

11:00 PIC simulation of vacuum arc

A better understanding of vacuum arc is essential in current breaking vacuum switches. There are multiple process involved during the arcing process not limited to 1) Explosive emission from cathode surfaces due to thermal runaway and effect of space charges 2) influence of particles 3) influence of anode in case of high current arc 4) modification of contact surface. In order to incorporate these processes a multi-scale model is needed to simulate vacuum arc. With that aim I am proposing atomistic approach based on partial in cell method to simulate the vacuum arc, commercial PIC code MAGIC 2D/3D will be used for the same. Ionization caused by the impact of primary and secondary emission electrons can be modelled both in 2D and 3D using both particle-only and hybrid particle-fluid models. Energetic primary electrons and low energy secondary ionization products which are also capable of ionizing neutrals can also be treated with the code. Processes of electron attachment to neutral species, recombination with ions, and neutralization of ions can also be simulated. Assuming the presence of a field emitter at the cathode initially, we can identify the conditions for plasma formation and can study the transitions from field emission stage to a fully developed arc.

Speaker: Mrunal Parekh

Slides

• 10mm_1kv.mp4
• 10mm_1kv_only_secel.mp4
• 20mm_1kv.mp4
• arc.avi
• MeVArc_2013.pdf
• MeVArc_2013.pptx

11:30 Computational Efficiency for Kinetic Simulation of Vacuum Arcs

Matthew M. Hopkins, Jeremiah J. Boerner, Christopher H. Moore, Paul S. Crozier, Stan G. Moore, Matthew T. Bettencourt, Russell Hooper The spatial, temporal, and model capability demands for simulating vacuum arc discharges are enormous. The simulation must evolve from an initial collisionless vacuum (or near vacuum) state through a sputtering phase with surface interaction and low collisionality and ionization, into a growing quasi-neutral plasma with increasing collisionality and ionization, to an explosive growth electron avalanche process, and finally to a steady current-carrying arc plasma. The modeling demands change drastically as each of these phases is encountered. We describe a number of model advances to address these challenges.

Speaker: Matthew Hopkins (Sandia National Laboratories)

Slides Computational_Efficiency_for_Kinetic_Simulation_of_Vacuum_Arcs.pdf

12:30 → 13:30 Lunch

13:30 → 17:00 Free time for exploring Chamonix or discussion

18:00 → 19:30 Bus to CERN