The influence of magnetization strength on the sheath: Implications for flush-mounted probes

Abstract

When the projected ion Larmor radius is larger than the Debye length, ion trajectories near a surface are unmagnetized and a magnetic presheath develops, whose electric field gradient serves to deflect the ion flow toward the surface normal so that it enters the sheath at the sound speed. When ions are very strongly magnetized, they remain tied to the field lines, even inside the sheath: there is no mechanism available to allow the existence of a magnetic presheath, and the Bohm criterion only applies to the parallel velocity, not the normal velocity. There are important consequences for flush-mounted probes because the shape of the ion branch of the I-V characteristic depends on the degree of sheath magnetization, a wide range of which has been investigated by means of one-dimensional particle-in-cell simulations. The results explain in a qualitative way the behavior of the ion current characteristics obtained using the Tokamak de Varennes (TdeV) [J. P. Gunn et al., Rev. Sci. Instrum. 66, 154 (1995)] flush-mounted probe system.