Runaway electrons preionized diffuse discharges in SF6, argon, air and nitrogen

Summary form only given. Formation of the diffuse discharge in an inhomogeneous electric field without using of additional sources of the preionization during the breakdown in the atmospheric air and other gases is possible due to the generation of the runaway electrons (RAE) and X-ray. For RAE beam that registered behind the anode foil the term supershort avalanche electron beam (SAEB) was proposed in [1]. Objective of this work to study dynamics of intensity of discharge plasma radiation from different regions of REP DD in initial stage of breakdown and the formation of bright electrode spots on electrodes in an inhomogeneous electric field and at high pressure of air, nitrogen, argon and SF6. Experiments were performed on unique setup. This setup was designed to study discharges and SAEB generation in different gases and made it possible to measure several relevant parameters simultaneously. Thus, the setup allowed us to register the voltage across the discharge gap, current through the gap, current of SAEB and radiation of discharge plasma from different gap regions in a single pulse. Moreover, the polarity of RADAN-220 pulser in the setup could be either negative or positive. With the time resolution of ~0.1 ns the dynamics of intensity of the discharge plasma radiation from different regions of discharge gap was obtained synchronously with the voltage across the gap and discharge current. It was shown that the breakdown is occurred owing to the ionization wave, which starts from an electrode with small radius of curvature at both polarities of voltage pulses. It was determined that the ionization wave velocity decreases with increasing pressure of gases from 0.05 to 0.7 MPa. In heavy SF6 the ionization wave velocity is less than in air and nitrogen. Formation of the bright spots on the electrodes at positive and negative polarity of voltage pulses, and images of the discharge gap have been investigated. It was found, that in air, nitrogen and argon at negative polarity of voltage pulses bright spots on the flat electrode arise when conduction current changes its direction. In argon, bright spots on the flat electrode were observed at the both polarities of voltage pulses and throughout gas pressures range. It is related with less impedance of the discharge plasma and oscillatory mode of discharge in the argon, because of step ionization effect. It was shown, that at the positive polarity of the electrode with a small radius of curvature bright spots on the flat electrode appear due to the participation of the dynamic displacement current in the gap conductivity.