ION DRAG AND PLASMA-INDUCED THERMOPHORESIS ON PARTICLES IN RADIOFREQUENCY GLOW-DISCHARGES

A comparison is made of the forces governing the transport of negative ly charged particles electrostatically suspended in the plasma of a sy mmetric parallel-plate radiofrequency glow discharge with isothermal w alls. The two forces driving the particles symmetrically from the plas ma centreline to the sheath edges are the ion drag force and the plasm a-induced thermophoresis due to the thermal gradient appearing in the gas heated by plasma power dissipation. A general expression of the io n 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 ener gy distribution using the analytical expression for the ion orbital mo mentum 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-charact erized experimental discharge conditions in Ar. It appears that ion dr ag 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 i nteractions result in a drastic reduction of the ion drift velocity th rough the plasma bulk.