Theory of self-organized dust sheaths in edge plasmas

Abstract

A concept of self-organisation is used to derive the structure of dust sheath close to the wall taking into account self-consistently dust charges and dust distribution together with electron, ion distributions and ion drift. It is shown that instead of Bohm criterion the dust sheath is determined by other criteria allowing Mach number to be in restricted value ranges. The total structure of the dust sheath in dimensionless variables is determined by the Mach number only. Dust sheaths can contain definite number of dust particles per unit area which are determined by the value of the Mach number. The dust sheaths change drastically the electrostatic potential distributions and are usually much thicker than the usual plasma sheaths. A theoretical model together with numerical computations gives all dust sheath parameter distributions and the global characteristics such as the total drop of the potential, the thickness of the sheath, the number of dust particles in the sheath and the criterion of dust levitation. The recent observations on “Textor 94” indicating the long time dust survival in edge region of the present tokamaks, the dust levitation and agglomeration in the volume of edge tokamak plasma are discussed.