Runaway Electron Beam Research Articles

Control-oriented model of disruption generated RE beam at FTU

In this paper control-oriented dynamical models of current, horizontal and vertical displacement for Runaway Electrons (RE) beam in the plateau phase are presented. The dynamical models were obtained through gray-box identification techniques on experimental data collected on post-disruptive RE beam shots in the Frascati Tokamak Upgrade (FTU). Based on the identified models, the design of simple robust controllers to improve RE beam mitigation strategies is proposed and simulations results are provided.

The effects of kinetic instabilities on the electron cyclotron emission from runaway electrons

In this paper we show that the kinetic instabilities associated with runaway electron beams play an essential role for the production of high-level non-thermal electron–cyclotron-emission (ECE) radiation. Most of the non-thermal ECE comes from runaway electrons in the low-energy regime with large pitch angle, which are strongly scattered by the excited whistler waves. The power of ECE from runaway electrons is obtained using a synthetic diagnostic model based on the reciprocity method. The electron distribution function is calculated using a kinetic simulation model including the whistler wave instabilities and the quasilinear diffusion effects. Simulations based on DIII-D low-density discharge reproduces the rapid growth of the ECE signals observed in DIII-D experiments. Unlike the thermal ECE where radiation for a certain frequency is strongly localized inside the resonance region, the non-thermal ECE radiation from runaway electrons is nonlocal, and the emission-absorption ratio is higher than that of thermal electrons. The runaway electron tail is more significant for ECE with higher frequencies, and the ECE spectrum becomes flatter as RE population grows. The nonlinear behavior of the kinetic instabilities is illustrated in the oscillations of the ECE waves. The good agreement with the DIII-D experimental observations after including the kinetic instabilities clearly illustrate the significance of the scattering effects from wave-particle interactions, which can also be important for runaway electrons produced in disruptions.

X-ray radiation and runaway electron beams generated during discharges in atmospheric-pressure air at rise times of voltage pulse of 500 and 50 ns

AbstractThe parameters of X-ray radiation and runaway electron beams (RAEBs) generated at long-pulse discharges in atmospheric-pressure air were investigated. In the experiments, high-voltage pulses with the rise times of 500 and 50 ns were applied to an interelectrode gap. The gap geometry provided non-uniform distribution of the electric field strength. It was founded that at the voltage pulse rise time of 500 ns and the maximum breakdown voltage Um for 1 cm-length gap, a duration [full width at half maximum (FWHM)] of a RAEB current pulse shrinks to 0.1 ns. A decrease in the breakdown voltage under conditions of a diffuse discharge leads to an increase in the FWHM duration of the electron beam current pulse up to several nanoseconds. It was shown that when the rise time of the voltage pulse is of 500 ns and the diffuse discharge occurs in the gap, the FWHM duration of the X-ray radiation pulse can reach ≈100 ns. It was established that at a pulse-periodic diffuse discharge fed by high-voltage pulses with the rise time of 50 ns, an energy of X-ray quanta and their number increase with increasing breakdown voltage. Wherein the parameter Um/pd is saved.

Наносекундный объёмный разряд в воздухе, инициируемый пикосекундным пучком убегающих электронов

Наносекундный объёмный разряд в воздухе, инициируемый пикосекундным пучком убегающих электронов

Streamers at the Subnanosecond Breakdown of Argon and Nitrogen in Nonuniform Electric Field at Both Polarities

An ICCD camera was used to study plasma glow at the stage of the streamer (ionization wave) formation in the tip–plane gap with a length of 3 mm filled with argon or nitrogen at a pressure of 12.5–400 kPa. Positive and negative nanosecond voltage pulses were applied across the gap. Images of streamer were obtained at different time at its propagation along the gap. A streak-camera equipped with a spectrometer was used to measure time evolution of the radiation intensity of nitrogen molecules at a wavelength of 337.1 nm in several regions along the gap at the negative polarity. Average streamer velocity (1.8 cm/ns) was estimated from experimental data at atmospheric pressure of nitrogen. Amplitude–time characteristics of voltage, discharge current and the current of runaway electron beam behind the aluminum-foil anode with a thickness of 10 μm were measured. Reasons for a diffuse discharge under the given experimental conditions were discussed.

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.

Resolving runaway electron distributions in space, time, and energy

Areas of agreement and disagreement with present-day models of runaway electron (RE) evolution are revealed by measuring MeV-level bremsstrahlung radiation from runaway electrons (REs) with a pinhole camera. Spatially resolved measurements localize the RE beam, reveal energy-dependent RE transport, and can be used to perform full two-dimensional (energy and pitch-angle) inversions of the RE phase-space distribution. Energy-resolved measurements find qualitative agreement with modeling on the role of collisional and synchrotron damping in modifying the RE distribution shape. Measurements are consistent with predictions of phase-space attractors that accumulate REs, with non-monotonic features observed in the distribution. Temporally resolved measurements find qualitative agreement with modeling on the impact of collisional and synchrotron damping in varying the RE growth and decay rate. Anomalous RE loss is observed and found to be largest at low energy. Possible roles for kinetic instability or spatial transport to resolve these anomalies are discussed.

Dissipation of post-disruption runaway electron plateaus by shattered pellet injection in DIII-D

We report on the first demonstration of dissipation of fully avalanched post-disruption runaway electron (RE) beams by shattered pellet injection in the DIII-D tokamak. Variation of the injected species shows that dissipation depends strongly on the species mixture, while comparisons with massive gas injection do not show a significant difference between dissipation by pellets or by gas, suggesting that the shattered pellet is rapidly ablated by the relativistic electrons before significant radial penetration into the runaway beam can occur. Pure or dominantly neon injection increases the RE current dissipation through pitch-angle scattering due to collisions with impurity ions. Deuterium injection is observed to have the opposite effect from neon, reducing the high-Z impurity content and thus decreasing the dissipation, and causing the background thermal plasma to completely recombine. When injecting mixtures of the two species, deuterium levels as low as ∼10% of the total injected atoms are observed to adversely affect the resulting dissipation, suggesting that complete elimination of deuterium from the injection may be important for optimizing RE mitigation schemes.

A Compact Setup Based on a Gas Diode for Studying of Cathodoluminescence

A compact setup for investigating the cathodoluminescence was created on the basis of a gas diode, a compact GIN-55-01 pulse–periodic generator, and an industrial spectrometer. Under the excitation by a beam of runaway electrons with a pulse duration (full width at half maximum) of ~100 ps, the cathodoluminescence spectra of natural and synthetic diamonds, calcite, cesium iodide, zinc selenide, zirconium dioxide, sapphire, gallium oxide (III), cadmium sulfide, zinc sulfide, calcium fluoride, and other crystals were recorded. The prospects of using gas diodes, which were developed at the Institute of High Current Electronics (Siberian Branch, Russian Academy of Sciences), and pulsers, which were created by the FID-Tekhnologiya Company (St. Petersburg), were shown when studying the properties of pulsed cathodoluminescence.

Generation of Diffuse Jets and Runaway Electron Beams in Air, SF6, and Helium at Low Pressures

Nanosecond discharges in air, SF6, and helium at pressures of units–tens of Torr are studied. Spatial inhomogeneities of diffuse jets and autographs of runaway electron (RAE) beams are recorded in all three gases during a discharge in a nonuniform electric field. It is shown that diffuse jets change their shape and their lengths increase and change from pulse to pulse in the discharge gap as the pressure decreases; further, it is confirmed that the RAE beam amplitude increases as the gas pressure decreases. It is assumed that inhomogeneities observed in the diffuse jets and RAE beams can be associated with transient luminous events in the Earth’s atmosphere that have sizes of tens of kilometers and occur at high altitudes at low pressures under high thunderstorm activity.

Emission from Crystals Irradiated with a Beam of Runaway Electrons

An investigation of the spectral and amplitude-temporal characteristics of emission from different crystals, promising in terms of their application as detectors of runaway electrons, is performed. This emission is excited by subnanosecond electron beams generated in a gas diode. It is found out that at the electron energies of tens–hundreds of kiloelectronvolts, the main contribution into the emission from CsI, ZnS, type IIa artificial and natural diamonds, sapphire, CaF2, ZrO2, Ga2O3, CaCO3, CdS, and ZnSe crystals comes from the cathodoluminescence; the radiation pulse duration depends on the crystal used and sufficiently exceeds the Cherenkov radiation pulse duration. It is demonstrated that the latter radiation exhibits low intensity and can be detected in the short-wave region of the spectrum in the cases where a monochromator and a high-sensitivity photomultiplier tube (PMT) are used.

Studies of runaway electrons via Cherenkov effect in tokamaks

The paper concerns measurements of runaway electrons (REs) which are generated during discharges in tokamaks. The control of REs is an important task in experimental studies within the ITER-physics program. The NCBJ team proposed to study REs by means of Cherenkov-type detectors several years ago. The Cherenkov radiation, induced by REs in appropriate radiators, makes it possible to identify fast electron beams and to determine their spatial- and temporal-characteristics. The results of recent experimental studies of REs, performed in two tokamaks - COMPASS in Prague and FTU in Frascati, are summarized and discussed in this paper. Examples of the electron-induced signals, as recorded at different experimental conditions and scenarios, are presented. Measurements performed with a three-channel Cherenkov-probe in COMPASS showed that the first fast electron peaks can be observed already during the current ramp-up phase. A strong dependence of RE-signals on the radial position of the Cherenkov probe was observed. The most distinct electron peaks were recorded during the plasma disruption. The Cherenkov signals confirmed the appearance of post-disruptive RE beams in circular-plasma discharges with massive Ar–puffing. During experiments at FTU a clear correlation between the Cherenkov detector signals and the rotation of magnetic islands was identified.

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

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.

Ionization Waves During the Subnanosecond Breakdown Initiated by Runaway Electrons in High-Pressure Nitrogen and Air

The experimental investigations of a subnanosecond breakdown initiated by runaway electrons in air and nitrogen at the pressures within 0.013–0.4 MPa are performed. The temporal development patterns of the discharge plasma glow in different regions of the discharge gap are registered with a fast photodiode, a four-channel ICCD-camera, and an ultrafast streak camera. It is shown that the breakdown occurs in the form of ionization waves propagating from the electrode with a small radius of curvature. It is found that a runaway electron beam behind the anode foil at the nitrogen and air pressure ~0.1 MPa is detected with the collector at the point of time corresponding to the maximum gap voltage.

Luminescence of Ga2O3 Crystals Excited with a Runaway Electron Beam

The spectra and amplitude–time characteristics of the radiation of studied Sn and Fe-doped Ga2O3 crystals excited with a runaway electron beam and an excilamp with a wavelength of 222 nm were investigated. The main contribution to the luminescence of samples in the region of 280–900 nm under excitation with a beam was shown to be made by cathodoluminescence. In the Fe-doped crystal, a new cathodeand photoluminescence band was detected within a wavelength range of 650–850 nm. In the Sn-doped crystal, Vavilov–Cherenkov radiation was detected in the region of 280–300 nm using a monochromator and a photomultiplier.

Fulfillment of Similarity Principles for Pulsed Discharges in a Highly Inhomogeneous Field at High Pressures Under Conditions of Runaway Electron Generation

An analysis of the fulfillment of the similarity law pτ = f(E/p) under conditions of a pulsed discharge triggered in a gas diode with a highly inhomogeneous field at the voltage in the incident wave >100 kV is performed. It is shown that in this case within the pressure range 1–12 atm the deviations from the similarity principles E/p(pτ) and Ubr(pd) are due to the nonconservation of proportions in the gas-filled diode geometry. Using a collector, the beam of runaway electrons is for the first time registered behind the anode foil in nitrogen at the pressure from 5 to 12 atm.

On the Mechanism of Maintenance and Instability of the Overvoltage Low-Pressure Discharge Forming a High-Current Runaway Electron Beam

Results of experiments on the study of dynamics of an overvoltage discharge at the low pressure р = 0.5–2.5 Torr up to its transition to the high-current low-voltage regime are presented, and the instability mechanism leading to a sharp voltage drop across the discharge is suggested.

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.

Runaway electron mitigation by 3D fields in the ASDEX-Upgrade experiment

Disruption-generated runaway electron (RE) beams represent a severe threat for tokamak plasma-facing components in high current devices like ITER, thus motivating the search of mitigation techniques. The application of 3D fields might aid this purpose and recently was investigated also in the ASDEX Upgrade experiment by using the internal active saddle coils (termed B-coils). Resonant magnetic perturbations (RMPs) with dominant toroidal mode number n = 1 have been applied by the B-coils, in a RE specific scenario, before and during disruptions, which are deliberately created via massive gas injection. The application of RMPs affects the electron temperature profile and seemingly changes the dynamics of the disruption; this results in a significantly reduced current and lifetime of the generated RE beam. A similar effect is observed also in the hard-x-ray (HXR) spectrum, associated to RE emission, characterized by a partial decrease of the energy content below 1 MeV when RMPs are applied. The strength of the observed effects strongly depends on the upper-to-lower B-coil phasing, i.e. on the poloidal spectrum of the applied RMPs, which has been reconstructed including the plasma response by the code MARS-F. A crude vacuum approximation fails in the interpretation of the experimental findings: despite the relatively low β () of these discharges, a modest amplification (factor of ) of the edge kink response occurs, which has to be considered to explain the observed suppression effects.

On efficiency during the generation of runaway electrons beams in stationary open discharge

Measurements of energetic efficiency were accomplished in generation of runaway electrons beams for helium, air and water vapor with various cathode materials. The efficiency near 80% was demonstrated for runaway electrons beam generated in helium and water vapour.