Sheath and presheath structure in the plasma–wall transition layer in an oblique magnetic field

Abstract

In this paper we present a hydrodynamic study of a current carrying transition layer that separates the plasma from electrically conducting or insulating walls. The plasma is placed in a magnetic field that could be parallel to, or intersects obliquely with the walls. The self-consistent model of the smooth presheath–sheath transition of the near-wall plasma layer developed here for a finite Debye radius to ion Larmor radius ratio (Ψ) is based on a previously developed model of the quasineutral plasma presheath [Phys. Plasmas 4, 3461 (1997)]. The potential distribution in the presheath is found to have a positive maximum with respect to the plasma–presheath interface. In the case of a wall with a floating potential, the value of the maximum decreases with the incidence angle θ of the magnetic line force, and approaches zero when θ=2°. The presheath thickness generally increases with the incidence angle, from about the electron Larmor radius up to the ion Larmor radius, and depends on the electron to ion current ratio, ion velocity at the plasma–presheath interface, and Hall parameter. Analysis of presheath-sheath transition when Ψ is in the range of 10−2 to 10−4 shows that (1) the electric field at the presheath–sheath interface is finite, (2) the critical ion velocity at the sheath edge is about (0.6–0.9) of ion sound speed, depending on the plasma parameters, and generally increases with the ion to electron current ratio. The current–voltage characteristic of the transition layer does not depend on θ, the magnetic field incidence angle, for θ⩾5°.