Space-charge effects on drift dominated electron and plasma motion

Abstract

The electron and ion continuity equations and Poisson's equation are solved for the motion of pulses of (i) electrons and (ii) plasma in an applied electric field of approximately 6 kV cm-1. For the cases considered in nitrogen at a temperature of 293K and at pressures of 12 kPa, where E/N approximately 2*10-15 V cm-2, and drift distances are a few cm, electron drift dominates over diffusion. The relative motion of electrons and ions in a plasma pulse have characteristic features very different from ambipolar diffusion motion characteristic of low fields and pressures. An electron pulse of radius approximately 0.5 cm and density approximately 1011 cm-3 spreads rapidly due to space-charge fields, regardless of whether a uniform electric field is applied. Space-charge effects enhance the electric field at the leading edge of an electron pulse in a uniform electric field. For an initially neutral plasma of density 5*106 cm-3 in a uniform electric field, electrons move independently of the ions. When the density is increased, and space charge is significant, the electron distributions have a characteristic steep trailing edge and an almost flat leading edge. At the highest densities considered of approximately 1011 cm-3 the electric field is reduced almost to zero at the centre of the plasma by space-charge effects.