Parallel impurity dynamics in the TJ-II stellarator

TL;DR

This paper reviews impurity density variations and transport in the TJ-II stellarator, deriving a parallel inertia force expression and applying a fluid model to explain observed emissivity asymmetries.

Contribution

It introduces a compact form of the parallel inertia force for arbitrary toroidal geometry and applies a fluid model to interpret impurity asymmetries in TJ-II plasmas.

Findings

Parallel inertia force is significant for TJ-II conditions.

The fluid model explains impurity radiation asymmetries.

Model matches observed stationary and transient features.

Abstract

We review in a tutorial fashion some of the causes of impurity density variations along field lines and radial impurity transport in the moment approach framework. An explicit and compact form of the parallel inertia force valid for arbitrary toroidal geometry and magnetic coordinates is derived and shown to be non-negligible for typical TJ-II plasma conditions. In the second part of the article, we apply the fluid model including main ion-impurity friction and inertia to observations of asymmetric emissivity patterns in neutral beam heated plasmas of the TJ-II stellarator. The model is able to explain qualitatively several features of the radiation asymmetry, both in stationary and transient conditions, based on the calculated in-surface variations of the impurity density.