Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

TL;DR

This paper investigates impurity control in high-temperature helical plasma devices, showing that under certain conditions, negative radial electric fields can lead to outward impurity flux, challenging previous assumptions.

Contribution

It demonstrates, through experimental data and neoclassical theory, that low-collisionality plasmas can have outward impurity flux despite negative radial electric fields.

Findings

Negative radial electric fields can coexist with outward impurity flux.

High ion temperature does not necessarily lead to high impurity content.

Experimental data supports the theoretical prediction of impurity behavior.

Abstract

Achieving impurity and helium ash control is a crucial issue in the path towards fusion-grade magnetic confinement devices, and this is particularly the case of helical reactors, whose low-collisionality ion-root operation scenarios usually display a negative radial electric field which is expected to cause inwards impurity pinch. In these work we discuss, based on experimental measurements and standard predictions of neoclassical theory, how plasmas of very low ion collisionality, similar to those observed in the impurity hole of the Large Helical Device, can be an exception to this general rule, and how a negative radial electric field can coexist with an outward impurity flux. This interpretation is supported by comparison with documented discharges available in the International Stellarator-Heliotron Profile Database, and it can be extrapolated to show that achievement of high ion temperature in the core of helical devices is not fundamentally incompatible with low core impurity content.