Ion drag and plasma-induced thermophoresis on particles in radiofrequency glow discharges

Abstract

A comparison is made of the forces governing the transport of negatively charged particles electrostatically suspended in the plasma of a symmetric parallel-plate radiofrequency glow discharge with isothermal walls. The two forces driving the particles symmetrically from the plasma centreline to the sheath edges are the ion drag force and the plasma-induced thermophoresis due to the thermal gradient appearing in the gas heated by plasma power dissipation. A general expression of the ion drag force as a function of the ratio of the ion drift velocity and thermal velocity is obtained by a proper integration over the ion energy distribution using the analytical expression for the ion orbital momentum transfer cross section derived by Kilgore et al. (J. Appl. Phys. 73 7195 (1993)). The ion drag force is then compared with the plasma-induced thermophoresis on submicrometre-size particles in well-characterized experimental discharge conditions in Ar. It appears that ion drag is usually larger than thermophoresis on an isolated particle in a pristine discharge. However, thermophoresis always dominates over ion drag in dusty discharges where plasma-particle and particle-particle interactions result in a drastic reduction of the ion drift velocity through the plasma bulk.