Moving Striations in Direct Current Glow Discharges

Abstract

A theoretical examination of the ion balance equations for a plasma, such as the positive column of a direct current glow discharge might be, shows that a uniform plasma is not always possible. When ionization proceeds by a two-stage process involving accumulated metastable excited atoms, as may be the case in the noble gases, small perturbations of the ion concentrations from their equilibrium values may not remain small. Under such conditions, a spatially uniform steady plasma cannot exist. A general stability criterion is developed which may be used whenever accurate expressions are known for the ion and metastable production and loss rate terms. Application of the stability criterion is made using several approximations.The instability of a spatially uniform plasma suggests that the ion concentrations may vary along the direction of current flow. The ion balance equations are re-examined for wave-like solutions. Two approximate solutions showing some agreement with experimental observations on moving striations are obtained. The first solution, ignoring variations in the metastable concentration, yields ion and electron concentration waves traveling in the direction of the current flow with velocity probably considerably greater than the positive ion drift velocity. The amplitude of the positive ion wave is much greater than that associated with the electrons.The second approximate solution ignores diffusion and represents a concentration wave dependent solely upon production and loss processes. It can travel in either direction, depending upon conditions in the plasma. General expressions for velocity, frequency, and wave number are given, but these cannot be evaluated numerically without better expressions for some of the quantities involved.