Effect of large-angle scattering, ion flow speed and ion-neutral collisions on dust transport under microgravity conditions

Abstract

The transport of dust particles through a plasma depends mostly on the ion drag force, the neutral drag force and the electrostatic force. The standard expressions for these forces were originally derived for a single dust particle placed in a collisionless plasma, with negligible flow speeds of the ions. Recent theories show deviations from the standard expressions for the charging and the ion drag force acting on dust particles in a plasma, when there are collisions and a significant ion flow. Experiments show only a small deviation from the standard expressions for the ion drag. We have extended a self-consistent dusty plasma model for a radio-frequency discharge with recent theories regarding the calculation of the ion drag force, including the effect of ion scattering beyond the screening length, ion flow and ion-neutral collisions. A change in the dust charge due to these collisions is also considered. Inside the dust-free void, that is generated by the ion drag force, scattering beyond the screening length is very important. Inside the dust cloud however, the effect is only moderate. Ion flow speeds under typical discharge parameters are low, except near the electrodes. Therefore, the effect of the ion flow speed on the ion drag force is very small. Collisions only increase the ion drag force near the outer walls. Only there does the screening length become much larger than the ion mean-free path. The dust charge however, is strongly reduced inside the void, and near the edge of the dust cloud, which is due to the low ion flow in both regions. When we compare our model with experiments, we conclude that in the bulk of the discharge and at the void edge, large angle scattering is important and the velocity-dependent linearized Debye length is the appropriate screening length. Using small angle scattering with the electron Debye length actually overestimates the ion drag, resulting in inconsistent values of the electric field and the ion drift speed.