Stellarator impurity flux driven by electric fields tangent to magnetic surfaces

TL;DR

This paper demonstrates that electric fields tangent to magnetic surfaces significantly influence impurity flux in stellarators, especially for highly charged impurities, challenging previous assumptions that radial electric fields dominate impurity transport.

Contribution

It provides an analytical calculation showing the impact of tangential electric fields on impurity flux, revealing their importance in impurity control strategies.

Findings

Tangential electric fields can strongly affect impurity flux.

Impurity expulsion can occur due to tangential electric fields.

Radial electric field dependence is restored when including tangential components.

Abstract

The control of impurity accumulation is one of the main challenges for future stellarator fusion reactors. The standard argument to explain this accumulation relies on the, in principle, large inward pinch in the neoclassical impurity flux caused by the typically negative radial electric field in stellarators. This simplified interpretation was proven to be flawed by Helander et al. [Phys. Rev. Lett. 118, 155002 (2017)], who showed that in a relevant regime (low-collisionality main ions and collisional impurities) the radial electric field does not drive impurity transport. In that reference, the effect of the component of the electric field that is tangent to the magnetic surface was not included. In this Letter, an analytical calculation of the neoclassical radial impurity flux incorporating such effect is given, showing that it can be very strong for highly charged impurities and that, once it is taken into account, the dependence of the impurity flux on the radial electric field reappears. Realistic examples are provided in which the inclusion of the tangential electric field leads to impurity expulsion.