Axial particle density gradient in direct-current discharges

Abstract

Particle density gradients in the positive column of a dc discharge have been reported previously. An increase of the density at the cathode end of the discharge tube is due to the neutralization of ions at the cathode (cataphoresis), while a density increase at the anode end is caused by an imbalance of the momenta transferred by the electrons and ions to the neutral particles (electrophoresis). A general transport theory is presented which incorporates both effects. An approximate analytical solution shows that for a monoatomic gas, cataphoresis dominates electrophoresis when the ion mean free path for momentum transfer exceeds 2R/3 (R is tube radius). Numerical results are given for helium over a wide range of gas pressure.