Runaway Electron Beam Current Research Articles

Spatially dependent modeling and simulation of runaway electron mitigation in DIII-D

New simulations with the Kinetic Orbit Runaway electron (RE) Code KORC show RE deconfinement losses to the wall during plasma scrape off are the primary current dissipation mechanism in DIII-D experiments with high-Z impurity injection, and not collisional slowing down. The majority of simulations also exhibit an increase in the RE beam energy due to acceleration by the induced toroidal electric field, even while the RE beam current is decreasing. In this study, KORC integrates RE orbits using the relativistic guiding center equations of motion, and incorporates time-sequenced, experimental reconstructions of the magnetic and electric fields and line integrated electron density to construct spatiotemporal models of electron and partially-ionized impurity transport in the companion plasma. Comparisons of experimental current evolution and KORC results demonstrate the importance of including Coulomb collisions with partially-ionized impurity physics, initial RE energy, pitch angle, and spatial distributions, and spatiotemporal electron and partially-ionized impurity transport. This research provides an initial quantification of the efficacy of RE mitigation via injected impurities, and identification of the critical role played by loss of confinement due to plasma scrape off on the inner wall as compared to the relatively slow collisional damping.

Effect of cathode and anode materials on the high-energy electron beam in the nanosecond-pulse breakdown in gas-filled diodes

High-energy electron beam can be generated during the breakdown process in a gas diode at elevated pressures. The design of electrodes in the gas diode affects the parameters of these electron beams. In this paper, investigation on parameters of the runaway electron beam (REB) in air and nitrogen excited by nanosecond pulsed generators are presented. Experiments were carried out with gas diodes by using cathode and anode made of different materials, including stainless steel, aluminum, copper and titanium. Experimental results show that, in the case of nanosecond-pulse breakdown, the cathode material significantly affects the amplitude of the REB current pulse when either pressure or gap spacing are small. When the pressure is low, the REB current is highest for the stainless steel cathode due to its small electron work function. However, the REB current for the aluminum, copper and titanium cathodes is higher than that of stainless steel cathode at atmospheric pressure because of the Malter effect on their surfaces. The dependence of the amplitude of the REB current on the anode foils is investigated by using aluminum, titanium, copper and tantalum. The experimental results show that the atomic number of the metal, as well as mass mean free path of the electron in it contribute to the amplitude of the REB current pulse. Based on the REB currents of different anode foils, energy distribution of runaway electrons is estimated. Electron distribution with an average energy of ~55 keV is obtained when the amplitude of the applied voltage pulse across the discharge gap is 110 kV. The results may contribute to the design of the gas diode for the generation of high-energy electron beams in nanosecond-pulse breakdown.

Measurement of runaway electron beam current in nanosecond-pulse discharges by a Faraday cup

AbstractMeasurement of runaway electron beam (REB) is essential to investigate behavior of runaway electrons produced in nanosecond-pulse gas discharge. A Faraday cup is designed to measure the REB current in nanosecond-pulse discharge when the applied dV/dtis 75 kV/ns. The Faraday cup considers the impendence match with the oscilloscope and the design of the receiving part. The experimental results show that the measured REB current has a rise time of 348 ps and a full width at half maximum of 510 ps. The comparison of the measurement results by the Faraday cup and a REB collector confirm that the Faraday cup is able to measure REB current in nanosecond-pulse discharge. Furthermore, consecutive waveforms of the REB currents show stable results by using the designed Faraday cup. In addition, effects of the interelectrode gap, gas pressure, and cathode material on the REB current are investigated by the designed Faraday cup, and the measurement results provide characteristics of REB current under different conditions. The REB current decreases when the gap spacing or gas pressure increases. REB current increases with the cathode diameter. It indicates that the high-energy electrons are generated not only at the edge of the cathode but also on the side surface of the cathode.

Effect of emission on subnanosecond breakdown in a gas diode at low pressure

The paper presents experimental and numerical research results on the operation of gas diode at low pressure. A high dispersion in the runaway electron beam current (from 20 to 100 A) with respect to the average one (∼50 A) is observed for a tubular cathode with a working edge radius of 30 μm, nitrogen pressure of 30 Torr, and an interelectrode gap of 6 mm. Numerical simulation data show that the low beam current (∼20 A) is due to the early electron emission from the cathode (at the stage of low-voltage prepulse), in which the runaway electron beam is formed from the boundary of plasma layer developing early in the breakdown. The high beam current (∼100 A) is due to the delayed electron emission from the cathode, which increases the diode voltage and the runaway electron beam current. In the latter case, the runaway electron beam is formed directly at the cathode.

Effect of emission on subnanosecond breakdown in a gas diode at low pressure

The paper presents experimental and numerical research results on the operation of a gas diode at low pressure. A large scattering in runaway electron beam current (from 20 to 100 A) with regard to the average (∼50 А) is observed for a tubular cathode with a working edge radius of 30 μm, nitrogen pressure of 40 mbar, and interelectrode gap of 6 mm. Numerical simulation data show that the low beam current (∼20 A) is due to early electron emission from the cathode (at the stage of low-voltage prepulse) in which the runaway electron beam is formed from the boundary of a plasma layer developed early in the breakdown. The high beam current (∼100 A) is due to delayed electron emission from the cathode, which increases the diode voltage and the runaway electron beam current. In the latter case, the runaway electron beam is formed directly at the cathode.

Generation of dual pulses of the runaway electron beam current during the subnanosecond breakdown of atomic and molecular gases

With a diaphragm placed behind the anode foil, dual runaway electron beams have been provided in helium, hydrogen, nitrogen, and air under a pressure of several torrs to several dozen torrs and a high-voltage pulse amplitude of about 250 kV. These beams consist of two pulses with commensurable amplitudes with a time interval between them of several dozen picoseconds to several hundred picoseconds. It has been shown that the breakdown of the interelectrode gap at pressures from several torrs to several dozen torrs may occur in different regimes and dual pulses of the electron beam current are registered when the initial current through the gap is below 1 kA. It has been found that a supershort avalanche electron beam that consists of one pulse is generated when the delay of breakdown equals several hundred picoseconds. It has been shown that, when the gas pressure reaches several hundred Torr, including atmospheric pressure, the runaway electrons are detected behind the foil after the termination of the supershort avalanche electron beam pulse.

Mechanisms of plasma disruption and runaway electron losses in the TEXTOR tokamak

Based on the analysis of data from the numerous dedicated experiments on plasma disruptions in the TEXTOR tokamak the mechanisms of the formation of runaway electron (RE) beams and their losses are proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode-number magneto-hydro-dynamics (MHD) modes. It is hypothesized that the RE beam is formed in the central plasma region confined by an intact magnetic surface due to the acceleration of electrons by the inductive toroidal electric field. In the case of plasmas with the safety factor $q(0)<1$ the most stable RE beams are formed by the outermost intact magnetic surface located between the magnetic surface $q=1$ and the closest low-order rational surface $q=m/n>1~(q=5/4,q=4/3,\dots )$. The thermal quench (TQ) time caused by the fast electron transport in a stochastic magnetic field is calculated using the collisional transport model. The current quench (CQ) stage is due to the particle transport in a stochastic magnetic field. The RE beam current is modelled as a sum of a toroidally symmetric part and a small-amplitude helical current with a predominant $m/n=1/1$ component. The REs are lost due to two effects: (i) by outward drift of electrons in a toroidal electric field until they touch the wall and (ii) by the formation of a stochastic layer of REs at the beam edge. Such a stochastic layer for high-energy REs is formed in the presence of the $m/n=1/1$ MHD mode. It has a mixed topological structure with a stochastic region open to the wall. The effect of external resonant magnetic perturbations on RE loss is discussed. A possible cause of the sudden MHD signals accompanied by RE bursts is explained by the redistribution of runaway current during the resonant interaction of high-energetic electron orbits with the $m/n=1/1$ MHD mode.

Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

A new physical mechanism of formation of runaway electron beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between q = 1 and the closest low–order rational magnetic surfaces [q = 5/4 or q = 4/3,…]. It results in that runaway electron beam current has a helical nature with a predominant m/n = 1/1 component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high–energy electrons with the m/n = 1/1 MHD mode are discussed.

Anode and Cathode Spots in High-Voltage Nanosecond-Pulse Discharge Initiated by Runaway Electrons in Air

We report the experimental results with nanosecond-pulse discharges formed in the air gap between a flat electrode and a sharp electrode. The appearance of anode and cathode spots on the electrodes is studied experimentally. It is considered that bright spots on the flat cathode with positive polarity of the sharp electrode are formed due to the explosive electron emission on the cathode and the dynamic displacement current in the gap. It is also shown that with negative polarity of the sharp electrode, bright spots on the flat anode are formed after changing the polarity of the flat electrode due to the discharge oscillatory mode. Under these conditions, the explosive electron emission firstly forms on the sharp cathode. With negative polarity of the sharp electrode of the subnanosecond-pulse pulser, the runaway electron beam current is measured behind the anode foil with a time resolution of no more than 100 ps.

Runaway electron beams in nanosecond-pulse discharges

Conventional discharge (Townsend and streamer mechanisms) theories are not able to well explain the phenomenon in nanosecond-pulse discharges. Recently, much attention has been paid to the runaway breakdown due to high-energy electrons in nanosecond-pulse discharges, and some experimental data confirm that high-energy runaway electron beam is an important characteristic parameter for nanosecond-pulse discharges. In this paper, two designed collectors are used for detecting runaway electron beams in nanosecond-pulse discharges. These collectors are used to measure the runaway electron beams in discharges driven by a nanosecond-pulse generator with a pulse width of 3-5 ns and a rise time of 1.2-1.6 ns. The measuring principle of both two collectors is similar to that of Faraday cup, where high-energy electrons are collected by a metal cone, and converted into an electric signal that can be recorded by an oscilloscope. Furthermore, optimal designs of collectors are conducted in order to improve the impedance matching characteristics and to obtain better recording data. Using the above two collectors, characteristics of runaway electron beams are investigated. Experimental results show that runaway electron beams can be effectively measured by the collectors, and the optimized collector has a shorter time resolution and higher amplitude of the runaway electron beam current. When the applied voltage is 80 kV, the electron beam current can be measured with an amplitude of 160 mA and a full width at half maximum of less than 1 ns. In addition, experimental results with pulse sequences prove that the collectors have excellent reliability, and both the transient response and the time resolution are stable.

Control and dissipation of runaway electron beams created during rapid shutdown experiments in DIII-D

DIII-D experiments on rapid shutdown runaway electron (RE) beams have improved the understanding of the processes involved in RE beam control and dissipation. Improvements in RE beam feedback control have enabled stable confinement of RE beams out to the volt-second limit of the ohmic coil, as well as enabling a ramp down to zero current. Spectroscopic studies of the RE beam have shown that neutrals tend to be excluded from the RE beam centre. Measurements of the RE energy distribution function indicate a broad distribution with mean energy of order several MeV and peak energies of order 30–40 MeV. The distribution function appears more skewed towards low energies than expected from avalanche theory. The RE pitch angle appears fairly directed (θ ∼ 0.2) at high energies and more isotropic at lower energies (ε < 100 keV). Collisional dissipation of RE beam current has been studied by massive gas injection of different impurities into RE beams; the equilibrium assimilation of these injected impurities appears to be reasonably well described by radial pressure balance between neutrals and ions. RE current dissipation following massive impurity injection is shown to be more rapid than expected from avalanche theory—this anomalous dissipation may be linked to enhanced radial diffusion caused by the significant quantity of high-Z impurities (typically argon) in the plasma. The final loss of RE beams to the wall has been studied: it was found that conversion of magnetic to kinetic energy is small for RE loss times smaller than the background plasma ohmic decay time of order 1–2 ms.

Effect of cathode emissivity on runaway electron beam formation in gas-filled diode with inhomogeneous electric field

Mechanisms of the formation of runaway electron beams at the initial stage of breakdown in a gasfilled diode with sharply inhomogeneous electric field have been studied by numerical simulations using a specially developed PIC/MC code. It is established that a beam of runaway electrons can be generated either immediately at the cathode or at a discharge plasma boundary. The obtained dependence of the runaway electron beam current on the state of the cathode surface (emissivity) has the characteristic shape.

Nanosecond discharge in air at atmospheric pressure as an x-ray source with high pulse repetition rates

The properties of x-ray radiation and runaway electrons produced using a nanosecond volume discharge are examined. X-ray radiation at a pulse repetition rate of 3kHz was obtained time in a gas diode filled with air at atmospheric pressure. The current pulse width (FWHM) for runaway electrons generated in the gas diode was ∼100ps. A prepulse was observed on an oscilloscope trace of the main runaway electron beam current.