On the generation mechanism of supershort avalanche electron beam during a nanosecond discharge in high pressure gases

Abstract

Electron acceleration up to high energies in high pressure gases (X-ray and runaway electron generation) has been known since the beginning of the 1960s. This phenomenon occurs both in the Earth's atmospheric discharges and in laboratory conditions. The present work is aimed at the study of the different factors influencing the generation mechanism of runaway electron beams with maximal amplitudes in laboratory discharges. The foregoing analysis of available data and experimental results obtained at the Laboratory of Optical Radiation of HCEI, SB RAS, confirms the generation mechanism for runaway electron beams proposed in [1] and developed in [2]. It is shown that at atmospheric pressures of different gases, maximal amplitudes of supershort avalanche electron beams (SAEB) are attainable in the case of diffuse discharge and electron acceleration between the anode and the ionization wave front, which propagates at a rate of about 10 cm/ns. The dense plasma is formed at the cathode at the expense of the electric field concentration, the initial preionization by the fast electrons, and the formation of electron avalanches. A supershort avalanche electron beam of highest amplitudes is generated as the critical field is reached between the ionization wave front and the anode. The SAEB was recorded both at high (in helium at 15 atm) and low (single to tens of torr) pressures in the gas diode. To obtain SAEB with maximal amplitudes and a pulse length of ∼100 ps at the half height, the gas diode should be filled with helium at a pressure about 60 torr. To achieve SAEBs with maximal amplitudes, it is necessary to use gas diodes and cathodes of an optimal construction. The motion of the ionization wave front is also responsible for the production of electrons with an energy > eU max . It is the effect of the ionization wave on the generation of runaway electrons that is the only explanation for SAEB amplitudes of tens of amperes in an atmospheric pressure air at a discharge current greater than 1 kA. Attaining the highest currents in the gap requires voltage pulses with a rise time of ∼0.3 ns and amplitude of hundreds of kilovolts applied to the cathode, a long sharp edge of the cathode, and an insulator of the gas diode shielding the side walls of the transmission line.