Theory of the collisional presheath in a magnetic field parallel to the wall

Abstract

In the limit of a small Debye length (λD→0) the plasma boundary layer in front of a negative absorbing wall is split up into a collision-free planar space charge sheath and a quasineutral presheath, where the ions are accelerated to fulfill the Bohm criterion. Apart from ion inertia, the mechanism of the presheath depends on an additional effect controlling the ion field acceleration. The present paper considers a stationary presheath dominated by the deflection of the ion orbits in a magnetic field parallel to the wall. The ion transport is provided by charge exchange collisions with cold neutrals. The potential profiles and ion distributions resulting from the self-consistent kinetic analysis are compared with expectations based on a previous hydrodynamic model. The transition from ‘‘closed’’ to ‘‘open’’ ion orbits results in typical deviations near the sheath edge. The kinetic Bohm criterion is found to be fulfilled marginally.