LEVITATION AND DYNAMICS OF A COLLECTION OF DUST PARTICLES IN A FULLY IONIZED PLASMA SHEATH

We have examined the dynamics of a collection of charged dust particle s in the plasma sheath above a large body in a fully ionized space pla sma when the radius of the large body is much larger than the sheath t hickness. The dust particles are charged by the plasma, and the forces on the dust particles are assumed to be from the electric field in th e sheath and from gravitation only. These forces will often act in opp osite directions and may balance, making dust suspension and collectio n possible. The dust particles are supplied by injection or by electro static levitation. The ability of the sheath to collect dust particles , will be optimal for a certain combination of gravitation and plasma and dust particle parameters. In a dense dust sheath, the charges on t he dust particles contribute significantly to the total space charge, and collective effects become important. These effects will reduce the magnitude of the sheath electric field strength and the charge on the dust particles. As dust particles are collected, the dust sheath is s tretched and the largest dust particles may drop out, because the shea th is no longer able to suspend them. In a tenuous dust sheath, the in ner layer, from the surface and about one Debye length thick, will be unstable for dust particle motion, and dust will not collect there. In a dense dust sheath, collective effects will decrease the thickness o f this inner dust-free layer, making dust collection closer to the sur face possible. By linearization of the force and current equations, we find the necessary and sufficient conditions for a stable dust sheath . We have considered conditions which resemble those of planetary syst em bodies, but the results may also be of relevance to some laboratory plasmas.