The connected presheath: One-dimensional models of neighboring objects in magnetized plasmas

Abstract

One-dimensional calculations are presented of plasma flow to multiple objects larger than the ion Larmor radius, when their ion collection presheaths overlap. Situations modeled include the effects of one Langmuir probe on another probe lying on the same field line, and Langmuir probes operated close to a limiter or divertor plate in the edges of magnetic confinement experiments. The results allow quantitative analysis of the operation of Langmuir probes and especially Mach probes for velocity measurements, even when they are close to other structures. New analytic results for inviscid plasmas, and numerical results incorporating shear viscosity are presented. They show that Mach probe measurements in inviscid plasmas experience strong perturbation by connection effects especially for Mach numbers greater than about 0.5. Viscous plasmas are relatively more weakly perturbed, although some distortions occur, especially for large probes. Taking into account these distortions, the viscous model is found to be able to fit recent experimental results.