Generation Of Runaway Electron Beams Research Articles

Full conversion from ohmic to runaway electron driven current via massive gas injection in the TCV tokamak

Full conversion from ohmic to runaway electron (RE) driven current was observed in the tokamak à configuration variable (TCV) following massive injection of neon through a disruption mitigation valve into a low-density limited circular plasma. Following a partial disruption, a stable 200 kA RE beam is maintained for more than 1 s. Controlled ramp-down of the RE beam with adjustable decay rate was demonstrated. Control of the beam vertical position was achieved down to a RE current of 20 kA. RE beam formation is observed in elongated plasma configurations up to κ = 1.5. A reproducible scenario for RE beam generation without loss of circulating current is of particular interest for disruption modelling applications.

Full conversion from Ohmic to runaway electron driven current via massive gas injection in the TCV tokamak

Abstract Full conversion from Ohmic to runaway electron (RE) driven current was observed in the TCV tokamak following massive injection of neon through a disruption mitigation valve into a low-density limited circular plasma. Following a partial disruption, a stable 200 kA RE beam is maintained for more than one second. Controlled ramp-down of the RE beam with adjustable decay rate was demonstrated. Control of the beam vertical position was achieved down to a RE current of 20 kA. RE beam formation is observed in elongated plasma configurations up to κ=1.5. A reproducible scenario for RE beam generation without loss of circulating current is of particular interest for disruption modelling applications.

Different modes of runaway electron beams generated in high-pressure gases

This article presents the results of experimental studies of different modes of a runaway electron beam (RAEB) generation in high-pressure gases as well as X-rays caused by it. In particular, the mode with the greatest beam current amplitude, the one with two current pulses, that with the X-ray pulse duration of 100s ns, the mode in which a RAEB propagates in the direction opposite from an anode, and some others are described. The effect of the cathode design and material on the RAEB current amplitude and duration in atmospheric-pressure air is shown. When analyzing the most common modes, the features of the gap breakdown are used.

Study of runaway electron dynamics at the ASDEX Upgrade tokamak during impurity injection using fast hard x-ray spectrometry

To study the runaway electron (RE) dynamics during plasma discharge and develop scenarios for disruption mitigation, a hard x-ray (HXR) spectrometric system has been developed and commissioned at the ASDEX Upgrade tokamak (AUG). The diagnostic system consists of two high-performance spectrometers based on LaBr3(Ce) scintillation detectors supplied with advanced electronics and analysis algorithms. These spectrometers view the AUG tokamak chamber quasi-radially at the equatorial plane. The measurements were carried out in the RE beam generation regimes by injecting argon into a deuterium plasma. In the interaction of a developed RE beam with a heavy gas target, powerful bremsstrahlung flux is induced, reaching energy close to 20 MeV. The electron energy distributions were reconstructed from the measured HXR spectra by deconvolution methods. The experimentally obtained maximum RE energies at different discharge stages were compared with relativistic test particle simulations that include the effect of toroidal electric field, plasma collisional drag force, synchrotron deceleration force. It was observed that the electrons attain their maximum energies within 50–100 ms after the gas injection. It gradually decreases due to the drop in loop voltage, energy loss due to synchrotron radiation emission and collisions dissipation of energy with the background plasma. HXR measurements at the discharge with multiple deuterium pellet injections allowed observing the effects of plasma cooling and argon ion recombination after the pellet injections. Argon density in AUG plasma after massive gas injection was estimated using HXR measurements.

Modes of runaway electron beams during formation of diffuse discharges in air and nitrogen

Investigations of the generation of runaway electron beams (REs) and the for-mation of diffuse discharges during breakdown of gaps with a cathode, which has a small radius of curvature, have been carried out. In air and nitrogen at elevated pressure, based on the registration and analysis of the characteristics of radiation from discharge, as well as parameters of the RE beam current and dynamic dis-placement current, it is shown that, depending on the conditions (E/N, gas type and its pressure, design and material of the cathode, amplitude and front of the voltage pulse), diffe-
 rent modes of generation of runaway electron beams are realized. It was found that the ratio of the velocity of the front of the ionization wave (streamer) and the runaway electrons, as well as the design of the cathode and the delay time before the explosion of cathode microinhomogeneities, significantly affect the generation of runaway electrons. The conditions for the realization of various modes are de-termined; oscillograms of the beam current pulses and photographs of the glow of the gap are presented.

Формирование двух импульсов тока пучка убегающих электронов

The conditions for the formation of two current pulses of runaway electron beams during the breakdown of a point-to-plane and tube-to-plane gaps in high-pressure air, nitrogen, and helium are studied. It has been shown experimentally that, depending on a pressure and kind of gas, a rise time of voltage pulse with an amplitude of tens of kilovolts, three modes of generation of runaway electron beams are observed. In the first mode, a single current pulse of runaway electron beam is observed at the maximum voltage across the gap, when a streamer appears in the vicinity of the pointed electrode (cathode). Its duration is ≈100 ps. In the second mode, two current pulses of runaway electron beams are observed at a lower pressure. The first pulse is generated as in the first mode. The second pulse is generated after the gap is bridged by the streamer (the first ionization wave). The electron energy in the second pulse is significantly less than in the first one, but the duration and amplitude of second current pulse under optimal conditions are greater. The third mode is implemented at lower pressures than in the second one. The generation of runaway electrons continues after the first pulse without a pause in the quasi-stationary stage. The total current pulse duration is hundreds of picoseconds.

Generation of direct and reverse runaway electron beams in atmospheric air using anodes made of different metals

This study presents results on the generation of runaway electron beams propagating from the cathode in both directions: towards the anode and in the opposite direction. The investigations were carried out in atmospheric air excited by nanosecond voltage pulses of both polarities. In the experiments, anodes made of aluminum, brass, and tantalum were used. It was shown that the amplitude of the current pulse of an electron beam generated in the direction opposite to the anode depends on the atomic number of the metal from which the anode is made. At the same time, it is important to note that for a beam generated in the direction of the anode, such dependence was not observed.

E-beam generation in discharges initiated by voltage pulses with a rise time of 200 ns at an air pressure of 12.5–100 kPa

The effect of air pressure (12.5, 25, 50, and 100 kPa) on the generation of runaway electron beams in a non-uniform electric field when applying voltage pulses (≈35 kV) with a rise time of ≈200 ns has been studied. The results show that the discharge has various stages: streamer, diffuse, and spark. Initially, a wide streamer develops in the gap and a diffuse discharge is formed. A spark is formed ≈100 ns after the breakdown. The current pulse of a supershort avalanche electron beam (SAEB) was measured with a collector at various pressures of air. Experiments show that there are two modes of generation of runaway electrons. At an air pressure of 25–100 kPa, a single SAEB current pulse with a full width at half-maximum (FWHM) of 120–140 ps is observed. At the air pressure of 12.5 kPa, two current pulses of the electron beam are observed. FWHM of the first and second current pulses are ≈140 ps and ≈300 ps, respectively. The current pulse amplitude of the second electron beam is higher than that of the first one, but the electron energy is less.

Generation and registration of runaway electron beams during the breakdown of highly overvoltaged gaps filled with dense gases

The breakdown of the gaps providing inhomogeneous electric field strength distribution fed by voltage pulses with subnanosecond risetime has been experimentally studied. Using a four-channel ICCD camera and an ultrafast streak camera, the dynamics of plasma glow in the space between a sharp-ended tubular and a plane electrodes during the prebreakdown stage of a nanosecond discharge has been investigated. In addition, the parameters of a runaway electron beam generated during this stage of the discharge in SF6, CO2, Ar, and N2 at pressures of up to 0.3, 0.7, 1.0, and 1.2 MPa, respectively, were measured with a collector mounted behind a thin-foil anode. The generation of a runaway electron beam and fast electrons contributes to the formation of a discharge in a diffuse form that is observed in the experiments under the conditions specified above. It was shown, that the time moment, at which a runaway electron beam is detected with the collector corresponds, approximately, to the time moment when the voltage across the gap reaches its maximum value.

Study of runaway electrons in TUMAN-3M tokamak plasmas

Studies of runaway electrons in present day tokamaks are essential to improve theoretical models and to support possible avoidance or suppression mechanisms in future large-scale plasma devices. Some of the phenomena associated with the runaway electrons take place at faster time scales, and thus it is essential to probe the runaway electrons to investigate underlying physics. The present article reports a few experimental observations of runaway electron associated events, at fast time scales, using a state-of-the-art multi-detector system developed at the Ioffe Institute and recently deployed on the TUMAN-3M tokamak. The system is based on the high-performance scintillation gamma-ray spectrometers for measurements of bremsstrahlung generated during the interaction of accelerated electrons with plasma and materials of the tokamak chamber. It includes a total three detectors configured in the spectroscopic mode having different lines of sight. Along with this hardware, dedicated algorithms were developed and validated that enables the separation of piled-up pulses, maximize the dynamic range of the detector and provides a counting rate as high as 107 counts per second. The inversion code, DeGaSum, has been used for the reconstruction of a runaway electron energy distribution function from the measured gamma-ray spectra. Using this tool, experimental analysis of the runaway electron beam generation and evolution of their energy distribution in the TUMAN-3M representative plasma discharges is performed. The effect on gamma-ray count rate during the magnetohydrodynamic activities and possible changes in the runaway electron energy distribution function during sawtooth oscillations is discussed in detail. Possible maximum limit of the runaway electron energy in TUMAN-3M is investigated and compared with the numerical analysis. In addition, the probability of the runaway electron generation throughout the plasma discharge is estimated analytically and compared with the experimental observation that suggests a balance between production and loss of the runaway electrons.

Runaway electron studies with hard x-ray and microwave diagnostics in the FT-2 lower hybrid current drive discharges

Studies of the super-thermal and runaway electron behavior in ohmic and lower hybrid current drive FT-2 tokamak plasmas have been carried out using information obtained from measurements of hard x-ray spectra and non-thermal microwave radiation intensity at the frequency of 10 GHz and in the range of (53 ÷ 78) GHz. A gamma-ray spectrometer based on a scintillation detector with a LaBr3(Ce) crystal was used, which provides measurements at counting rates up to 107 s−1. Reconstruction of the energy distribution of RE interacting with the poloidal limiter of the tokamak chamber was made with application of the DeGaSum code. Super-thermal electrons accelerated up to 2 MeV by the LH waves at the high-frequency pumping of the plasma with low density ~ 2 × 1013 cm−3 and then up to 7 MeV by vortex electric field have been found. Experimental analysis of the runaway electron beam generation and evolution of their energy distribution in the FT-2 plasmas is presented in the article and compared with the numerical calculation of the maximum energy gained by runaway electrons for given plasma parameters. In addition, possible mechanisms for limiting the maximum energy gained by the runaway electrons are also calculated and described for a FT-2 plasma discharge.

Generation of runaway electron beams in high-pressure nitrogen

In this paper the results of experimental studies of the amplitude-temporal characteristics of a runaway electron beam, as well as breakdown voltage in nitrogen are presented. The voltage pulses with the amplitude in incident wave ≈120 kV and the rise time of ≈0.3 ns was used. The supershort avalanche electron beam (SAEB) was detected by a collector behind the flat anode. The amplitude-time characteristics of the voltage and SAEB current were studied with subnanosecond time resolution. The maximum pressure at which a SAEB is detectable by collector was ∼1 MPa. This pressure increases with decreasing the voltage rise time. The waveforms of the discharge and runaway electron beam currents was synchronized with the voltage pulses. The mechanism of the runaway electron generation in atmospheric-pressure gases is analyzed on the basis of the obtained experimental data.

Amplitude−temporal characteristics of a supershort avalanche electron beam generated during subnanosecond breakdown in air and nitrogen

The amplitude−temporal characteristics of a supershort avalanche electron beam (SAEB) with an amplitude of up to 100 A, as well as of the breakdown voltage and discharge current, are studied experimentally with a picosecond time resolution. The waveforms of discharge and SAEB currents are synchronized with those of the voltage pulses. It is shown that the amplitude−temporal characteristics of the SAEB depend on the gap length and the designs of the gas diode and cathode. The mechanism for the generation of runaway electron beams in atmospheric-pressure gases is analyzed on the basis of the obtained experimental data.

Generation of runaway electrons at low pressures in air, nitrogen, and argon

The influence of the air, nitrogen, and argon pressure on the generation of runaway electron beams during a breakdown in an inhomogeneous electric field is studied. Data on parameters of runaway electron beams at pressures corresponding to altitudes of more than 20 km above sea level in the Earth’s atmosphere are obtained.

Numerical study of the generation of runaway electrons in a gas diode with a hot channel

A new method for increasing the efficiency of runaway electron beam generation in atmospheric pressure gas media has been suggested and theoretically proved. The method consists of creating a hot region (e.g., a spark channel or a laser plume) with a decreased numerical density of gas molecules (N) near the cathode. In this method, the ratio E/N (E—electric field strength) is increased by decreasing N instead of increasing E, as has been done in the past. The numerical model that is used allows the simultaneous calculation of the formation of a subnanosecond gas discharge and the generation of runaway electrons in gas media. The calculations have demonstrated the possibility of obtaining current pulses of runaway electrons with amplitudes of hundred of amperes and durations of more than 100 ps. The influence of the hot channel geometry on the parameters of the generated beam has been investigated.

On the parameters of runaway electron beams and on electrons with an “anomalous” energy at a subnanosecond breakdown of gases at atmospheric pressure

The generation of runaway electron beams in gases at atmospheric pressure has been studied with a real picosecond accuracy. Their main parameters have been determined. It has been found that three groups of electrons can be separated at a subnanosecond voltage pulse in a runaway electron beam generated in air at atmospheric pressure. It has been proven that the duration of a beam pulse in air at atmospheric pressure behind an anode foil is ~100 ps.

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Disruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories. In addition, vertical stability plays a key role in long runaway beam formation. Energies up to 20 MeV are observed. Mitigation of an incoming runaway electron beam triggered by massive argon injection was found to be feasible provided that the injection takes place early enough in the disruption process. However, suppressing an already accelerated runaway electron beam in the MA range was found to be difficult even with injections of more than 2 kPa.m3 high-Z gases such as krypton or xenon. This may be due to the presence of a cold background plasma weakly coupled to the runaway electron beam which prevents neutrals from penetrating in the electron beam core. Following unsuccessful mitigation attempts, runaway electron impacts on beryllium plasma-facing components were observed, showing localized melting with toroidal asymmetries.

Development of an ITER prototype disruption mitigation valve

Disruptions in tokamaks seem to be unavoidable. Consequences of disruptions are (i) high heat loads on plasma-facing components, (ii) large forces on the vacuum vessel, and (iii) the generation of runaway electron beams. In ITER, the thermal energy of the plasma needs to be evenly distributed on the first wall in order to prevent melting, forces from vertical displacement events have to be minimized, and the generation of runaway electrons suppressed. Massive gas injection using fast valves is a concept for disruption mitigation which is presently being explored in many tokamaks. Fast disruption mitigation valves based on an electromagnetic eddy current drive have been developed in Jülich since the 1990s and models of various sizes have been built and are in operation in the TEXTOR, MAST, and JET tokamaks. A disruption mitigation valve for ITER is of necessity larger with an estimated injected gas volume of ∼20kPam3[7] for runaway electron suppression and all materials used have to be resistant to much higher levels of neutron and gamma radiation than in existing tokamaks. During the last 5 years, the concept for an ITER prototype disruption mitigation valve has been developed up to the stage that a fully functional valve could be built and tested. Special emphasis was given to the development and functional testing of some critical items: (i) the injection chamber seal, (ii) the piston seal, (iii) the eddy current drive, and (iv) a braking mechanism to avoid too fast closure of the valve, which could damage the injection chamber seal. The concept of the valve will be introduced and detailed solutions and testing results for the critical items presented.

Generation of runaway electrons and X-ray emission during breakdown of atmospheric-pressure air by voltage pulses with an ∼0.5-μs front duration

Results are presented from experiments on the generation of runaway electron beams and X-ray emission in atmospheric-pressure air by using voltage pulses with an ∼0.5-μs front duration. It is shown that the use of small-curvature-radius spherical cathodes (or other cathodes with small curvature radii) decreases the intensity of the runaway electron beam and X-ray emission. It is found that, at sufficiently high voltages at the electrode gap (Um ∼ 100 kV), the gap breakdown, the formation of a spark channel, and the generation of a runaway electron beam occur over less than 10 ns. At high values of Um behind the anode that were reached by increasing the cathode size and the electrode gap length, a supershort avalanche electron beam with a full width at half-maximum (FWHM) of up to ∼100 ps was detected. At voltages of ∼50 kV, the second breakdown regime was revealed in which a runaway electron beam with an FWHM of ∼2 ns was generated, whereas the FWHM of the X-ray pulse increased to ∼100 ns. It is established that the energy of the bulk of runaway electrons decreases with increasing voltage front duration and is ⩽30 keV in the first regime and ⩽10 keV in the second regime.

New Features of the Generation of Runaway Electrons in Nanosecond Discharges in Different Gases

This paper presents new experimental data on the generation of runaway electron beams and X-rays in repetitively pulsed discharges in helium and air with an inhomogeneous electric field. Both negative and positive voltage pulses of duration 1-5 ns and amplitude in the incident wave up to 18 kV were applied to a “tube-to-plane” gap at a pulse repetition frequency of up to 1 kHz. It is shown that at a helium pressure of several and tens of torrs, whatever the polarity of the main voltage pulse, an electron beam is generated on arrival of reflected negative voltage pulses in the gap. X-rays were detected over a wide pressure range of helium and air, including atmospheric pressure. The breakdown of atmospheric-pressure air gaps with a special cathode design at a rate of voltage rise of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> V/s was studied with subnanosecond and picosecond time resolution. At a voltage amplitude higher than 100 kV, a fast electron beam in atmospheric-pressure air was detected in the space downstream of the cathode, which was made of thin wires arranged parallel to a thin flat foil. The current of the fast electron beam downstream of the cathode depended strongly on the anode material.