THE ANALYSIS OF THE MUTUAL DEPENDENCE OF IMPACT IONIZATION AND ELECTRON ENERGY DISTRIBUTION IN ELECTRIC PROPULSION DEVICES

Abstract

The object of this study is the set of physical processes in the non-equilibrium plasma of electric propulsion devices. The subject of the study is impact ionization, specifically the feedback effect on the electron energy spectrum. The aim of the study is to derive expressions for the impact ionization characteristics in the right-hand sides of the mathematical model equations describing electric propulsion devices based on the compromise kinetic-fluid model. The objectives of the study are: to analyze existing continuum mechanics models that incorporate the impact ionization process building upon the results presented in the author's previous works; to write an expression for the average values over the electron energy spectrum of the ionization volume coefficient and cross-section and to analyze the behavior of the calculated values taking into account the interdependence of ionization processes and the formation of the electron energy distribution. The methods used in the study include an analytical investigation of existing process models to determine their limits of applicability in rarefied plasma, as well as a theoretical analysis of the impact of ionization on the electron energy spectrum, taking into account the characteristics of plasma confined by potential barriers in a boundary bipolar layer. Calculated energy-dependent distributions above the potential barrier yielded lower values than those obtained for a Maxwellian distribution; however, higher values were observed below the ionization potential at elevated electron temperatures. Based on an analysis of the influence of the electron energy spectrum on the ionization intensity and the influence of the electromagnetic field on electron transport in energy space, a positive feedback was demonstrated: at relatively high electron temperatures, the need to transport them "upward" from the region below the ionization potential leads to an excess of electrons in this region. In turn, this surplus increases the energy-averaged ionization cross-section, further reinforcing the electron excess in the lower energy spectrum.