Large tangential electric fields in plasmas close to temperature screening

TL;DR

This paper investigates how tangential electric fields and electron kinetic effects in low-collisionality stellarator plasmas influence impurity transport, revealing conditions where impurity screening can occur and highlighting the importance of tangential electric fields.

Contribution

It demonstrates that electron kinetic effects can reduce the radial electric field, enabling impurity screening, and emphasizes the significance of tangential electric fields and magnetic drifts in impurity transport modeling.

Findings

Electron kinetic effects can make the radial electric field small or negative.

Impurity screening can occur in very low collisionality plasmas like those in LHD.

Tangential electric fields, influenced by magnetic drifts, significantly impact impurity transport.

Abstract

Low-collisionality stellarator plasmas usually display a large negative radial electric field that has been expected to cause accumulation of impurities due to their high charge number. In this paper, two combined effects that can potentially modify this scenario are discussed. First, it is shown that, in low collisionality plasmas, the kinetic contribution of the electrons to the radial electric field can make it negative but small, bringing the plasma close to impurity temperature screening (i.e., to a situation in which the ion temperature gradient is the main drive of impurity transport and causes outward flux); in plasmas of very low collisionality, such as those of the Large Helical Device displaying impurity hole, screening may actually occur. Second, the component of the electric field that is tangent to the flux surface (in other words, the variation of the electrostatic potential on the flux surface), although smaller than the radial component, has recently been suggested to be an additional relevant drive for radial impurity transport. Here, it is explained that, especially when the radial electric field is small, the tangential magnetic drift has to be kept in order to correctly compute the tangential electric field, that can be larger than previously expected. This can have a strong impact on impurity transport, as we illustrate by means of simulations using the newly-developed code KNOSOS (KiNetic Orbit-averaging-SOlver for Stellarators).