High negative charge of a dust particle in a hot cathode discharge

Dust particle levitation experiments in a plasma produced by a hot filament discharge, operating at low argon pressure, are presented. The basic characteristics of a dust grain trapped in a plate sheath edge in these experimental conditions are reported. Taking into account the sheath potential profiles measured with a differential emissive probe diagnostic, the forces applied to an isolated dust grain can be determined. Two different experimental methods yield approximately the same value for the dust charge. The observed high negative charge is mainly due to the contribution of the primary electrons emitted by the filaments as predicted by a simple model.