# The importance of the classical channel in the impurity transport of optimized stellarators

## TL;DR

This paper demonstrates that in certain stellarator configurations, classical impurity transport can be comparable to neoclassical transport, especially in high-collisionality regimes, challenging the usual dominance of neoclassical effects.

## Contribution

It provides a comparative analysis of classical and neoclassical impurity transport in Wendelstein 7-X and LHD stellarators, highlighting conditions where classical transport is significant.

## Key findings

- Classical transport is comparable to neoclassical transport in W7-X for all radii.
- In LHD, classical transport is negligible except near sign changes in neoclassical transport.
- Electrostatic potential variations significantly affect neoclassical impurity transport in LHD.

## Abstract

In toroidal magnetic confinement devices, such as tokamaks and stellarators, neoclassical transport is usually an order of magnitude larger than its classical counterpart. However, when a high-collisionality species is present in a stellarator optimized for low Pfirsch-Schl\"uter current, its classical transport can be comparable to the neoclassical transport. In this letter, we compare neoclassical and classical fluxes and transport coefficients calculated for Wendelstein 7-X (W7-X) and Large Helical Device (LHD) cases. In W7-X, we find that the classical transport of a collisional impurity is comparable to the neoclassical transport for all radii, while it is negligible in the LHD cases, except in the vicinity of radii where the neoclassical transport changes sign. In the LHD case, electrostatic potential variations on the flux-surface significantly enhance the neoclassical impurity transport, while the classical transport is largely insensitive to this effect in the cases studied.