Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

Abstract

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U cr , pd ) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen's curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atmospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.

Multi-scale modelling of pulsed nanosecond dielectric barrier plasma discharges in plane-to-plane geometry

“…The ionization rates in the cathode sheath, however, are negligible since it is essentially free of electrons. It has been shown that runaway ionization processes are dominant only in very short gaps and at low pressures even in extremely high electric fields [31,32]. Hence, for conditions considered in this study, the electron-impact transport and reaction coefficients provided by BOLSIG are sufficiently accurate.…”