Numerical investigation of a high-pressure gas medium preionization by runaway electrons

Abstract A comparative simulation of the generation and acceleration of runaway electrons in the discharge gap during the initiation of the discharge by nanosecond and subnanosecond pulses is carried out. We used a numerical model based on the PIC-MCC method. Calculations were carried out for N2 6 atm pressure. Numerical simulation of a formation process of the electron avalanche initiated by an electron field-emitted from the top of the cathode microspike was carried out taking into account the motion of each electron in the avalanche. Characteristic runaway electron trajectories, runaway electron energy gained during the motion through the discharge gap, times required for runaway electrons to reach the anode were calculated. We compared our results with calculations using well-known differential equation of electron acceleration using braking force in Bethe approximation. We solved this equation also for braking force based on real (experimental) ionization cross section. The reasons for the discrepancy in the calculation results are discussed.

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Why do Electrons with “Anomalous Energies” appear in High-Pressure Gas Discharges?

EPJ Web of Conferences ◽

10.1051/epjconf/201816701005 ◽

2018 ◽

Vol 167 ◽

pp. 01005 ◽

Cited By ~ 3

Author(s):

Andrey Kozyrev ◽

Vasily Kozhevnikov ◽

Natalia Semeniuk

Keyword(s):

Runaway Electron ◽

Experimental Studies ◽

Boltzmann Kinetic Equation ◽

Travelling Wave ◽

Runaway Electrons ◽

Gas Discharges ◽

High Energies ◽

Self Consistent ◽

Discharge Gap ◽

Maximum Voltage

Experimental studies connected with runaway electron beams generation convincingly shows the existence of electrons with energies above the maximum voltage applied to the discharge gap. Such electrons are also known as electrons with “anomalous energies”. We explain the presence of runaway electrons having so-called “anomalous energies” according to physical kinetics principles, namely, we describe the total ensemble of electrons with the distribution function. Its evolution obeys Boltzmann kinetic equation. The dynamics of self-consistent electromagnetic field is taken into the account by adding complete Maxwell’s equation set to the resulting system of equations. The electrodynamic mechanism of the interaction of electrons with a travelling-wave electric field is analyzed in details. It is responsible for the appearance of electrons with high energies in real discharges.

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Suppression of runaway electron generation by massive helium injection after induced disruptions on TEXTOR

Journal of Plasma Physics ◽

10.1017/s0022377815001051 ◽

2015 ◽

Vol 81 (5) ◽

Cited By ~ 5

Author(s):

A. Lvovskiy ◽

H. R. Koslowski ◽

L. Zeng ◽

Keyword(s):

Time Delay ◽

Runaway Electron ◽

Critical Density ◽

Runaway Electrons ◽

Electron Generation ◽

Disruption Mitigation ◽

Order Of Magnitude ◽

Short Time ◽

Short Time Delay

Disruptions with runaway electron generation have been deliberately induced by injection of argon using a disruption mitigation valve. A second disruption mitigation valve has been utilised to inject varying amounts of helium after a short time delay. No generation of runaway electrons has been observed when more than a critical amount of helium has been injected no later than 5 ms after the triggering of the first valve. The required amount of helium for suppression of runaway electron generation is up to one order of magnitude lower than the critical density according to Connor & Hastie (1975) and Rosenbluth & Putvinski (1997).

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Progress in HXR diagnostics at GOLEM and COMPASS tokamaks

Journal of Instrumentation ◽

10.1088/1748-0221/17/01/c01033 ◽

2022 ◽

Vol 17 (01) ◽

pp. C01033

Author(s):

J. Cerovsky ◽

O. Ficker ◽

V. Svoboda ◽

E. Macusova ◽

J. Mlynar ◽

...

Keyword(s):

Diagnostic Tool ◽

Runaway Electron ◽

Sampling Rate ◽

Data Acquisition System ◽

Scintillation Detectors ◽

Low Density ◽

Runaway Electrons ◽

X Ray ◽

Radiation Induced ◽

Pulse Analysis

Abstract Scintillation detectors are widely used for hard X-ray spectroscopy and allow us to investigate the dynamics of runaway electrons in tokamaks. This diagnostic tool proved to be able to provide information about the energy or the number of runaway electrons. Presently it has been used for runaway studies at the GOLEM and the COMPASS tokamaks. The set of scintillation detectors used at both tokamaks was significantly extended and improved. Besides NaI(Tl) (2 × 2 inch) scintillation detectors, YAP(Ce) and CeBr3 were employed. The data acquisition system was accordingly improved and the data from scintillation detectors is collected with appropriate sampling rate (≈300 MHz) and sufficient bandwidth (≈100 MHz) to allow a pulse analysis. Up to five detectors can currently simultaneously monitor hard X-ray radiation at the GOLEM. The same scintillation detectors were also installed during the runaway electron campaign at the COMPASS tokamak. The aim of this contribution is to report progress in diagnostics of HXR radiation induced by runaway electrons at the GOLEM and the COMPASS tokamaks. The data collected during the 12th runaway electron campaign (2020) at COMPASS shows that count rates during typical low-density runaway electron discharges are in a range of hundreds of kHz and detected photon energies go up to 10 MeV (measured outside the tokamak hall). Acquired data from experimental campaigns from both machines will be discussed.

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Kinetic modelling of runaway electron generation in argon-induced disruptions in ASDEX Upgrade

Journal of Plasma Physics ◽

10.1017/s0022377820000793 ◽

2020 ◽

Vol 86 (4) ◽

Cited By ~ 1

Author(s):

K. Insulander Björk ◽

G. Papp ◽

O. Embreus ◽

L. Hesslow ◽

T. Fülöp ◽

...

Keyword(s):

Runaway Electron ◽

Kinetic Modelling ◽

Electron Distribution Function ◽

Real Space ◽

Tokamak Plasmas ◽

Runaway Electrons ◽

Plateau Phase ◽

Electron Generation ◽

Current Dissipation ◽

Threshold Amount

Massive material injection has been proposed as a way to mitigate the formation of a beam of relativistic runaway electrons that may result from a disruption in tokamak plasmas. In this paper we analyse runaway generation observed in eleven ASDEX Upgrade discharges where disruption was triggered using massive gas injection. We present numerical simulations in scenarios characteristic of on-axis plasma conditions, constrained by experimental observations, using a description of the runaway dynamics with a self-consistent electric field and temperature evolution in two-dimensional momentum space and zero-dimensional real space. We describe the evolution of the electron distribution function during the disruption, and show that the runaway seed generation is dominated by hot-tail in all of the simulated discharges. We reproduce the observed dependence of the current dissipation rate on the amount of injected argon during the runaway plateau phase. Our simulations also indicate that above a threshold amount of injected argon, the current density after the current quench depends strongly on the argon densities. This trend is not observed in the experiments, which suggests that effects not captured by zero-dimensional kinetic modelling – such as runaway seed transport – are also important.

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Runaway electron diagnostics for the COMPASS tokamak using EC emission

EPJ Web of Conferences ◽

10.1051/epjconf/201920303006 ◽

2019 ◽

Vol 203 ◽

pp. 03006

Author(s):

Michal Farnik ◽

Jakub Urban ◽

Jaromir Zajac ◽

Ondrej Bogar ◽

Ondrej Ficker ◽

...

Keyword(s):

Runaway Electron ◽

Horn Antenna ◽

Runaway Electrons ◽

Electron Cyclotron Emission ◽

Frequency Range ◽

Front End ◽

Suprathermal Electrons ◽

Cyclotron Emission ◽

Heterodyne Radiometer ◽

Generation Mechanisms

An electron cyclotron emission (ECE) diagnostic of suprathermal electrons was utilised for runaway electron (RE) experiments purposes in the COMPASS tokamak. Our vertical ECE (V-ECE) system consists of a 16-channel heterodyne radiometer and an E-band horn antenna with a 76.5-88 GHz frequency range front-end. Simulations used for the design of the diagnostic showed a possibility of detecting the emission of low-energy (50-140 keV) runaway electrons. We realized measurements with both extraordinary (X-) and ordinary (O-) mode linear polarizations. The amplitudes of the X-mode and O-mode signals are similar, which can be explained by depolarised reflected radiation. V-ECE measurements in low-density flattop discharges and in discharges with massive gas injections of high-Z elements show correlations with other RE diagnostics. Our results are in the agreement with the principles of the primary runaway generation mechanisms.

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On the observed energy of runaway electron beams in air

Laser and Particle Beams ◽

10.1017/s0263034611000541 ◽

2011 ◽

Vol 29 (4) ◽

pp. 425-435 ◽

Cited By ~ 43

Author(s):

G.A. Mesyats ◽

A.G. Reutova ◽

K.A. Sharypov ◽

V.G. Shpak ◽

S.A. Shunailov ◽

...

Keyword(s):

Electric Field ◽

Runaway Electron ◽

Electron Beams ◽

Beam Current ◽

Acceleration Of Particles ◽

Runaway Electrons ◽

Electrode Gap ◽

Cathode Voltage ◽

Wide Range ◽

Widespread Belief

AbstractExperiments with an air electrode gap have been performed where the current/charge of a picosecond beam of runaway electrons was measured over a wide range (up to four orders of magnitude) downstream of the absorbing foil filters. Measurements and calculations have made it possible to refer the beam current to the rise time of the accelerating voltage pulse to within picoseconds. It has been shown that, in contrast to a widespread belief, the runaway electron energies achieved are no greater than those corresponding to the mode of free acceleration of electrons in a nonstationary, highly nonuniform electric field induced by the cathode voltage. The experimental data agree with predictions of a numerical model that describes free acceleration of particles. It has been confirmed that the magnitude of the critical electric field that is necessary for electrons to go into the mode of continuous acceleration of electrons in atmospheric air corresponds to classical notions.

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Численное исследование возможности генерации убегающих электронов в формирующемся катодном слое самостоятельного объемного разряда высокого давления

Журнал технической физики ◽

10.21883/jtf.2020.05.49173.70-19 ◽

2020 ◽

Vol 90 (5) ◽

pp. 740

Author(s):

В.В. Лисенков

Keyword(s):

Electric Field ◽

Gaseous Medium ◽

Cathode Layer ◽

Volume Discharge ◽

Runaway Electrons ◽

Field Amplification ◽

Streamer Channel ◽

Formation Conditions ◽

Gas Medium ◽

Plasma Streamer

Calculations of the formation of the cathode layer of an self-sustained high-pressure volume discharge with pre-ionization of the gas medium excited by nano-and subnanosecond voltage pulses are carried out. It is shown that at pressures of ~ 1 atm at the final stage of the cathode layer formation, conditions for the generation of escaping electrons arise. We also considered the transition of electrons into runaway mode from the area of electric field amplification in front of the plasma (streamer) channel, which originates from the top of the micro-spike on the cathode. It is shown that at pressures of ~ 10 atm, electrons can transfer into runaway mode immediately after emission from the top of micro-spike in amplified electric field. Thus obtained runaway electrons can create pre-ionization of the gaseous medium and provide the formation of the initial phase of the discharge in volume form in the system without external pre-ionization.

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Generation of runaway electrons and X rays in an inhomogeneous electric field at high gas pressures

Laser and Particle Beams ◽

10.1017/s0263034616000719 ◽

2016 ◽

Vol 34 (4) ◽

pp. 748-763 ◽

Cited By ~ 8

Author(s):

V.F. Tarasenko ◽

E.Kh. Baksht ◽

D.V. Beloplotov ◽

A.G. Burachenko ◽

M.I. Lomaev ◽

...

Keyword(s):

Atmospheric Pressure ◽

Runaway Electron ◽

Experimental Studies ◽

Maximum Pressure ◽

X Rays ◽

Runaway Electrons ◽

Inhomogeneous Electric Field ◽

Temporal Characteristics ◽

Beam Currents ◽

Gas Pressures

AbstractResults of experimental studies of the amplitude–temporal characteristics of a runaway electron (RE) beam, as well as breakdown voltage and discharge current with a picosecond time resolution are presented. The maximum pressure, at which a RE beam is detectable, decreases with increasing the voltage rise time. The waveforms of the discharge and RE beam currents are synchronized with those of the voltage pulses. It is shown that the amplitude–temporal characteristics of the RE beam depend on the designs of the gas-filled diode and cathode, as well as the gap length. The mechanism for the generation of REs in atmospheric-pressure gases is analyzed on the basis of the obtained experimental data.

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Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

Open Physics ◽

10.2478/bf02475565 ◽

2004 ◽

Vol 2 (4) ◽

Cited By ~ 8

Author(s):

Alexey Tkachev ◽

Sergei Yakovlenko

Keyword(s):

Electric Fields ◽

Critical Voltage ◽

Runaway Electrons ◽

Local Criterion ◽

Fast Electrons ◽

Electron Multiplication ◽

Dense Gases ◽

Non Local ◽

Discharge Gap

AbstractNew understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U cr, pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam.The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation.The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.

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Radiation reaction induced non-monotonic features in runaway electron distributions

Journal of Plasma Physics ◽

10.1017/s0022377815000513 ◽

2015 ◽

Vol 81 (5) ◽

Cited By ~ 20

Author(s):

E. Hirvijoki ◽

I. Pusztai ◽

J. Decker ◽

O. Embréus ◽

A. Stahl ◽

...

Keyword(s):

Runaway Electron ◽

Minimum Energy ◽

Approximate Expression ◽

Radiation Reaction ◽

Threshold Condition ◽

Runaway Electrons ◽

High Energies ◽

Electron Distributions ◽

Space Dynamics ◽

Very High

Runaway electrons, which are generated in a plasma where the induced electric field exceeds a certain critical value, can reach very high energies in the MeV range. For such energetic electrons, radiative losses will contribute significantly to the momentum space dynamics. Under certain conditions, due to radiative momentum losses, a non-monotonic feature – a ‘bump’ – can form in the runaway electron tail, creating a potential for bump-on-tail-type instabilities to arise. Here, we study the conditions for the existence of the bump. We derive an analytical threshold condition for bump appearance and give an approximate expression for the minimum energy at which the bump can appear. Numerical calculations are performed to support the analytical derivations.

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