Pfirsch–Schlüter impurity transport in stellarator edge plasmas with large radial gradients

Abstract

Large radial gradients of the bulk ion temperature and density in edge plasmas cause heavy impurity ions to redistribute themselves within flux surfaces, thereby making the impurity density vary poloidally (and, in a stellarator, toroidally). This redistribution reduces the radial impurity flux. In this paper, the effects of stellarator geometry on this mechanism are investigated, which lead to qualitatively different results compared with axisymmetric systems, in which the plasma can rotate freely. We derive analytic expressions for the radial impurity flux and the impurity density. The resulting differential equations are solved analytically in some limiting cases and numerically for W7-X and W7-AS.