Plasma collection by an obstacle

Abstract

Plasma flow to an obstacle is examined using the two‐fluid equations. In this model the obstacle is assumed to be a two‐dimensional strip that extends to infinity in the y direction (slab geometry). An obstacle inserted into a magnetized plasma will cast a "shadow" along the magnetic field lines. The natural collection length of such an obstacle is a measure of the length of its shadow. This study shows that in a typical fusion tokamak, where c_s/Ω_(ci)d≪1 (Ω_(ci), c_s are the ion cyclotron frequency and the ion acoustic speed, respectively, d is the half‐width of the strip), the particle collection length of an obstacle can be approximated as L_∥=0.23c_sd^2/ D_⊥, if D_⊥/ c_sd ≪1; or 0.30d, if D_⊥/ c_sd ≫1. For the cases examined in this study, the inclusion of the electron–ion collisional drag parallel to B_0 changes the solution only by an insignificant amount.