Collisional transport of impurities with flux-surface varying density in stellarators

TL;DR

This paper extends neoclassical impurity transport calculations in stellarators to include flux-surface density variations, revealing the significant influence of potential perturbations and electric fields on impurity flux, especially in Wendelstein 7-X.

Contribution

It generalizes previous models to account for flux-surface density variations and analyzes their impact on impurity transport in stellarators, particularly W7-X.

Findings

Flux-surface density variations affect impurity transport.

Potential perturbations can dominate impurity flux.

Classical transport can surpass neoclassical in W7-X.

Abstract

High-Z impurities in magnetic confinement devices are prone to develop density variations on the flux-surface, which can significantly affect their transport. In this paper, we generalize earlier analytic stellarator calculations of the neoclassical radial impurity flux in the mixed-collisionality regime (collisional impurity and low-collisionality bulk ions) to include the effect of such flux-surface variations. We find that only in the homogeneous density case is the transport of highly collisional impurities (in the Pfirsch-Schl\"{u}ter regime) independent of the radial electric field. We study these effects for a Wendelstein 7-X (W7-X) vacuum field, with simple analytic models for the potential perturbation, under the assumption that the impurity density is given by a Boltzmann response to a perturbed potential. In the W7-X case studied, we find that larger amplitude potential perturbations cause the radial electric field to dominate the transport of the impurities. In addition, we find that classical impurity transport can be larger than the neoclassical transport in W7-X.