Charge of dust on surfaces in plasma

Experimental investigations are reported on the charging of dust particles resting on conducting and insulating surfaces beneath plasma. The experiment is configured to give insight into the behavior of dust on planetary surfaces exposed to solar wind plasma. In the experiments, the surfaces are agitated so that the particles drop through a small hole into a Faraday cup that measures the charge on each dust particle. Both conducting (Ni) and insulating dusts (SiO2, Al2O3 and JSC-1 lunar regolith simulant) are investigated. The conducting surface is given a bias voltage above the floating potential to create conditions in which the electron flux is dominant or a bias voltage below the floating potential for which the ion flux is dominant. The dust charges more positively in ion-flux-dominant conditions and more negatively in electron-flux-dominant conditions. The insulating dusts retain the charge for a long period of time (> 1 h) after the plasma is turned off. The charge decays more quickly if the surface is agitated, causing the dust to roll around, or if there is ultraviolet illumination. The conducting dust often has a lower level of charge than the insulating dust because the charge is conducted from the dust particle into the conducting surface. Conducting and insulating dusts on an insulating surface have approximately the same level of charge.