Modeling of nonlocal slow-electron kinetics in a low-pressure negative-glow plasma

Abstract

The kinetics of slow electrons is studied in a low-pressure negative-glow plasma (NGP). A method based on the nonlocal approach is proposed, which allows the nonlocal (nonequilibrium) nature of slow electrons to be accounted for in a physically transparent and numerically efficient manner. The slow electrons are divided into trapped (cold, Maxwellian) and free (superthermal, non-Maxwellian). It is shown that the superthermal (free) electrons are particularly important because they carry current and supply energy to the system of cold (trapped) electrons. A nonlocal energy-balance equation for the trapped electrons is derived, in which heating by superthermal electrons and diffusion cooling are found to be among the most important mechanisms. Simple expressions for the diffusion-cooling rate and the wall potential are determined. The proposed method is validated by the numerical solution of the full kinetic equation for a NGP in Ar. The energy and space distributions of electron fluxes are analyzed, and flux reversal (in energy space) is observed and explained. A comparison to experiment is carried out and close agreement is obtained. The proposed method can be useful in building fully kinetic, self-consistent models of various NGP-based discharge devices.