Theory of positive columns in electronegative gases

Abstract

A steady state theory of the positive column of a glow discharge in an electronegative gas is presented. Excitation and ionization are assumed to occur by single-electron collisions with neutral molecules. Both electron attachment and detachment are included in the continuity equations, the latter being due to long-lived excited neutral molecules taken to be uniformly distributed in the gas. Fluid-type momentum equations are used to describe the motion of positive and negative ions and of the electrons. By retaining Poisson’s equation throughout the treatment it is possible to impose physically realistic boundary conditions on all three charged species. It is found that the radial distribution of negative ions in the column is substantially different from that of the positive ions and the electrons. This is caused by the inwardly directed drift velocity of the negative ions, which confines them almost completely to the central region of the discharge column. Since the axial concentration of the negative ions relative to that of the electrons depends on the ratio of the coefficients of attachment to detachment, the concentration can reach very high values indeed when the rate of detachment is low.