https://iopscience.iop.org/article/10.1088/1361-6595/ab5c57/pdf

Runaway electrons in diffuse gas discharges

Conditions under which the number of runaway electrons in atmospheric-pressure air reaches ∼5 × 1010 are determined. Recommendations for creating runaway electron accelerators are given. Methods for measuring the parameters of a supershort avalanche electron beam and X-ray pulses from gas-filled diodes, as well as the discharge current and gap voltage, are described. A technique for determining the instant of runaway electron generation with respect to the voltage pulse is proposed. It is shown that the reduction in the gap voltage and the decrease in the beam current coincide in time. The mechanism of intense electron beam generation in gas-filled diodes is analyzed. It is confirmed experimentally that, in optimal regimes, the number of electrons generated in atmospheric-pressure air with energies T > eUm, where Um is the maximum gap voltage, is relatively small.

Parameters of a supershort avalanche electron beam generated in atmospheric-pressure air

This paper presents the results of the experimental studies of a pulsed discharge in atmospheric pressure air in an inhomogeneous electric field for various parameters of voltage pulses. It is shown that in a wide range of experimental conditions, including those with a positive electrode of small curvature radius, a diffuse discharge is ignited in the gap. In particular, a diffuse discharge is ignited at a pulse repetition frequency of 1 kHz and a voltage pulse amplitude of ∼25 and ∼40 kV across a high-resistance load. With voltage pulses of ∼ 220 kV in amplitude and low repetition frequencies, an extended (∼70 cm) diffuse discharge is observed in gaps of 13–40 mm. It is confirmed that the diffuse form of discharges in an inhomogeneous electric field at increased pressures is attributed to the generation of runaway electrons and x-rays.

Runaway electron preionized diffuse discharges in atmospheric pressure air with a point-to-plane gap in repetitive pulsed mode

In this work, the spectra of electron beams produced in air-filled diodes at atmospheric pressure were studied for different cathode designs. The feasibility of correct reconstruction of the electron beam spectrum from an experimental dependence of its attenuation factor in foils of different thicknesses was demonstrated. The electron energy distributions were calculated on minimum a priori assumptions by regularization of an ill-posed problem—a Fredholm integral equation. The spectra of a subnanosecond electron beam generated in the gas gap during the voltage pulse rise time were reconstructed and analysed. A time-of-flight spectrometer study and reconstruction of the spectrum from the data on e-beam attenuation confirmed the fact that groups of electrons with two-three characteristic energies can be generated in gas-filled diodes. In experiments, electrons of energy greater than that corresponding to the nominal voltage amplitude across the gap were detected.

https://iopscience.iop.org/article/10.1088/0022-3727/43/30/305201/pdf

Spectrum of fast electrons in a subnanosecond breakdown of air-filled diodes at atmospheric pressure

The conditions for the generation of runaway electron beams with maximum amplitudes and soft X-rays with maximum exposure doses in a nanosecond discharge in atmospheric-pressure air were determined. A supershort avalanche electron beam (SAEB) with a current of amplitude ~50 A, a current pulse of full-width at half-maximum (FWHM) ~ 100 ps, and a current density up to 20 A/cm2 was recorded downstream of the gas diode foil. It is shown that the maximum of the SAEB current amplitude shifts in time relative to the voltage pulse rise as a collector is displaced over the foil surface. A source of soft X-rays with an FWHM of less than 200 ps and an exposure doze of ~3 mR per pulse was designed based on a SLEP-150 pulser (maximum voltage amplitude ~140 kV, FWHM ~1 ns, and pulse rise time ~0.3 ns). It is demonstrated that X-ray quanta with an effective energy of ~9 keV make a major contribution to the exposure dose.

Supershort Avalanche Electron Beams and X-rays in Atmospheric-Pressure Air

With a RADAN 303-A pulser (a rise time of 150 ps and a maximum voltage of 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter-size gap operated at subatmospheric pressure. Diagnostics include capacitive voltage dividers which allow the determination of voltage across and current through the gap with a temporal resolution defined by the digitizer (20 Gs/s, 6 GHz) used. A scintillator-photomultiplier combination with different metal absorber foils and a temporal resolution of 2 ns is used as X-ray detector to obtain a rough energy spectrum of the X-rays and electrons in the range of 10-150 keV. Discharges are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and ldquofree-fallrdquo conditions with negligible gaseous ionization for the rest of the gap. High-energy electrons (> 60 keV) are observed up to atmospheric pressure. Time-to-breakdown curves versus pressure have been measured for different applied voltage rise times. They resemble Paschen curves with a steep increase toward low pressure and a slow increase toward high pressure. The major experimental findings and particularly the time-to-breakdown curves are confirmed using simple force-equation modeling. Monte Carlo calculations simulating collisional ionizations and developing electron avalanches in three dimensions have been used to verify and explain the experimental results.

Breakdown Delay Times for Subnanosecond Gas Discharges at Pressures Below One Atmosphere

Gas breakdown in quasi homogeneous electric fields with amplitudes of up to 3 MV/cm is investigated The setup consists of a RADAN 303 A pulser and pulse slicer SN 4, an impedance-matched oil-filled coaxial line with a lens-transition to a biconical line in vacuum or gas, and an axial or radial gap with a width on the order of mm, with a symmetrical arrangement on the other side of the gap Capacitive voltage dividers allow to determine voltage across as well as conduction current through the gap, with a temporal resolution determined by the oscilloscope sampling rate of 20 GS/s and an analog bandwidth of 6 GHz The gap capacitance charging time and voltage risetime across the gap is less than 250 ps Previous experiments at TTU with a slightly larger risetime have shown that breakdown is governed by runaway electrons, with multi-channel formation and high ionization and light emission in a thin cathode layer only In argon and air, time constants for the discharge development have been observed to have a minimum of around 100 ps at several 10 torr A qualitative understanding of the observed phenomena and their dependence on gas pressure is based on explosive field emission and gaseous ionization for electron runaway conditions

Ultrafast gas breakdown at pressures below one atmosphere

Summary form only given. With a RADAN 303 A pulser (risetime 150 ps, maximum voltage 150 kV into matched load), fast breakdown in argon and air is investigated. An oil-filled coaxial transmission line is coupled with a lens to a biconical section and a radial millimeter size gap operated at sub-atmospheric pressure. On the other side of the gap, the arrangement is symmetrically continued to represent a matched load. Pulse risetime at the gap is increased to about 180 ps. With capacitive dividers the voltage across the transmission line separating incident and reflected pulses is measured, which allows to determine voltage across and current through the gap. Temporal resolution is defined by the digitizer (20 Gs/s, 6 GHz). Breakdown usually happens during the rising part of the applied voltage pulse. Breakdown curves, i.e. breakdown voltage or time-to-breakdown vs. pressure, have been measured for different applied dV/dt's (from 2times1014 V/s to 8times1014 V/s) and they resemble Paschen curves with a steep increase toward low pressure, and a slow increase toward high pressure. The major findings, such as shifts of the minimum formative time toward increasing pressure with increasing dV/dt, are discussed in terms of similarity laws. Discharges for this case are characterized by runaway electrons over much of the pressure range, with a strong excitation and ionization layer at the cathode surface, and "free-fall" conditions with negligible gaseous ionization for the rest of the gap. Monte-Carlo simulations for the initial stage of the discharge are expected to confirm and quantify the experimental findings.

Scaling Laws for Sub-Nanosecond Breakdown in Gases with Pressures Below One Atmosphere

Summary form only given. The X-ray emission of highly overvolted spark gaps under electron runaway conditions is investigated. The pulse source, a RADAN 303 A, is connected to a test chamber through an oil-filled coaxial line, a coupling lens, and a biconical transmission line section, with a symmetrical arrangement attached on the opposite side of the chamber with a matching load. The test chamber allows pressure variation from 10-6-670 torr with argon or dry air used as a background gas. Voltage pulses with amplitudes of 40-150 kV, risetimes less than 200 ps, and FWHM less then 300 ps are applied across hemispherical electrodes with 1 mm spacing. A scintillator-photomultiplier combination with a temporal resolution of 2 ns is used as X-ray detector. Metallic absorber foils of different thicknesses are used to obtain a rough energy spectrum of the x-rays and electrons in the range of about 10 to 150 keV. Results show a high electron-energy component (>60 keV) existing up to atmospheric pressure, and an intense soft component (5 to 20 keV) at pressures around 100 torr. The observations are compatible with gaseous ionization and runaway conditions for extremely high E/p.

W. Justis

Papers

Breakdown Delay Times for Subnanosecond Gas Discharges at Pressures Below One Atmosphere

Ultrafast gas breakdown at pressures below one atmosphere

Scaling Laws for Sub-Nanosecond Breakdown in Gases with Pressures Below One Atmosphere

X-ray Emission from Subnanosecond Gas Breakdown