On the Radial Structure of the Electron Velocity Distribution and Related Macroscopic Properties in DC Column Plasmas

Abstract

The nonisothermal plasma of the axially homogeneous, cylindrical positive column of dc glow discharges represents an attractive example of a spatially bounded inhomogeneous plasma. Several attempts have been made in the past1-3 as well as are currently 4-l4 undertaken to improve the microphysical description and thus the understanding of the significant physical processes occurring in such weakly ionized nonequilibrium plasmas. These plasmas are usually enclosed by insulated walls. As a consequence a radial space charge field is established in the column to make the confinement and a stable operation of the plasma possible. An important aspect of the microphysical description of the spatially bounded plasma column is the appropriate treatment of the nonequilibrium kinetics of the active electron component taking into account the specific plasma conditions. Particularly the knowledge of the electron velocity distribution function (EVDF) and of various macroscopic properties of the electrons, which are obtained by appropriate velocity space averages over the EVDF, are of significant importance for the quantitative description of the column plasma. The occurrence of the space charge field in the column can cause a pronounced radial alteration of the velocity distribution function and of related macroscopic properties of the electrons under certain discharge conditions. Depending on the special objective of the quantitative plasma description often only a certain part of the electron kinetic quantities is of primary interest. In this respect the isotropic part 1-9 of the per one electron normalized EVDF or the so-called energy distribution function (which can be immediately obtained from the isotropic distribution function) is often of primary interest. This normalized isotropic distribution function already allows to determine several important quantities as the rate coefficients of various inelastic electron-atom collision processes in the plasma. These quantities are then used, for example, to determine the densities of different excited atom and ion species by solving their corresponding particle balance equations. However a more strict and comprehensive microphysical study of the electron