Modeling of dust voids in electronegative discharges under microgravity

Traces of molecular gases, like oxygen influence the size of the void observed in microgravity experiments carried out at the International Space Station. These molecular gases produce negative ions via (dissociative) attachment. A considerable amount of negative ions changes the plasma properties, especially the potential distribution and, therefore, the forces acting on a dust particle. We have investigated these phenomena by means of a two-dimensional (2-D) fluid model in which all the plasma parameters are calculated self-consistently. In this model, we have included the possibility that negative ions are formed. By changing the attachment rate, we can control the electronegativity of the gas. Heating of the dust particle material by the recombining ions and electrons and subsequent heating of the gas is taken into account, as well as the heating of the background gas by ion-neutral collisions. As in electropositive discharges the resulting thermophoretic force, however, can be neglected at low peak-to-peak voltages compared to the other forces. Results from the fluid model show that indeed the presence of negative ions influences the evolution of the void and its final size. We will show how the relevant forces change with variation of the applied voltage in electropositive and electronegative discharges. Electronegative means the presence of negative ions, not their dominance.