Experimental levitation of dust grains in a plasma sheath

Abstract

Dust grains have been observed to levitate above the surface of the Moon and as spokes in Saturn's rings. In order to gain a better understanding of these observations, we have performed levitation experiments on dust grains in a low‐density plasma. Plasma sheath potential profiles, measured by an emissive probe, are used to determine the spatial dependence of the electric force on a grain in the sheath. The observed levitation height agrees with the values calculated using orbital‐motion‐limited charging theory and force balance equations. Levitating grains were also exposed to an ultraviolet light source to induce photoemission. Three types of dust were investigated: polystyrene divinylbenzene microspheres 10.0 ± 0.5 μm in diameter, glass microballoons <38 μm in diameter, and JSC‐1 (lunar regolith simulant) <25 μm in diameter. Our experimental results show that (1) various types and sizes of grains can levitate in a plasma sheath above a conducting surface; (2) levitating grains of a standard size float at a height corresponding to that predicted by theory; (3) exposure to UV light causes the grain levitation height to decrease slightly as a result of less negative charge; and (4) a mechanism to inject grains into the sheath is not necessary if the electric field is sufficiently strong.