Ion Collection by a Cylindrical Probe in a Weak Magnetic Field

Abstract

The structure of a plasma-sheath transition region surrounding a negatively biased cylindrical probe in a weak axial magnetic field is systematically studied on the basis of the moment equations. A weakly ionized plasma is treated, and it is assumed that the mean free path is long but finite, and that the collision frequency is constant. Since the mean free path becomes short with decrease of the ion current collected, the collision effect plays an important role in the ion collection and the formation of the transition region between plasma and the so-called sheath. The position of the boundary between the transition region and plasma and its dependence on the magnetic field are determined by an effective mean free path. Our theory is a generalization of those of Bohm and others which includes the finiteness of the mean free path and the effect of a weak magnetic field. A reformed Bohm criterion is proposed.