Generation of Runaway Electrons near Micro-Inhomogeneities on the Cathode Surface in Subnanosecond Self-Sustained Discharges in a Wide Range of High Pressures

Abstract

The results of numerical 3D modeling of the development of an electron avalanche initiated by a field emission electron in a small-sized region of an amplified electric field near the microinhomogeneities at the cathode have been presented. The simulation has been carried out for the discharge gaps with an initially homogeneous distribution of the electric field with a reduced intensity significantly lower than that required by the electron runaway criterion. The possibility of the transition of the field emission electrons initiating avalanches and the electrons in these avalanches into runaway regime has been investigated. The microinhomogeneities in the form of a cone, metal droplets, and boundaries between pores or microcraters have been considered. The calculations were carried out for nitrogen in the pressure range from atmospheric to 40 atm. It has been shown that the initial energy obtained near the microinhomogeneity can significantly facilitate the transition of the electron into the runaway mode. And the electron will continue to run away in a discharge gap electric field weak according to the runaway criterion. It has been shown that this effect is especially noticeable at gas pressures above 10 atm. A comparative analysis of the simulation results with the experimental data obtained by us on the switching characteristics of a discharge gap filled with nitrogen when exposed to voltage pulses with subnanosecond fronts of different steepness has been carried out. This made it possible to divide the ranges of experimental conditions into those when only the amplification of the electric field near the microinhomogeneities is sufficient for the runaway of electrons and when the electric field of an avalanche of critical or close to critical size is additionally necessary for the runaway.