# Impurity transport in a mixed-collisionality stellarator plasma

## TL;DR

This paper demonstrates that impurity screening, previously thought exclusive to tokamaks, can also occur in stellarator plasmas under certain collisionality conditions, potentially improving plasma performance.

## Contribution

It reveals that impurity screening in stellarators is possible in high-collisionality impurity regimes, supported by analytical and numerical methods, challenging prior assumptions.

## Key findings

- Impurity screening can occur in stellarators with highly collisional impurities.
- Analytical and numerical evidence supports the possibility of impurity outward flux.
- Screening occurs in regimes where impurities are collisional and the bulk plasma is low-collisional.

## Abstract

A potential threat to the performance of magnetically confined fusion plasmas is the problem of impurity accumulation, which causes the concentration of highly charged impurity ions to rise uncontrollably in the center of the plasma and spoil the energy confinement by excessive radiation. It has long been thought that the collisional transport of impurities in stellarators always leads to such accumulation (if the electric field points inwards, which is usually the case), whereas tokamaks, being axisymmetric, can benefit from "temperature screening", i.e., an outward flux of impurities driven by the temperature gradient. Here it is shown, using analytical techniques supported by results from a new numerical code, that such screening can arise in stellarator plasmas too, and indeed does so in one of the most relevant operating regimes, where the impurities are highly collisional whilst the bulk plasma is in any of the low-collisionality regimes.