The role of MHD in causing impurity peaking in JET Hybrid plasmas

TL;DR

This paper investigates how magnetohydrodynamic (MHD) instabilities influence impurity peaking in JET Hybrid plasmas, revealing that certain MHD activities can both remove and concentrate tungsten impurities, affecting plasma performance.

Contribution

It provides a statistical survey of MHD instabilities in JET Hybrid discharges and proposes models for how n>1 islands affect tungsten impurity transport.

Findings

n>1 tearing activity enhances tungsten peaking

Core n=1 instabilities can remove or degrade impurities

MHD instabilities influence impurity transport mechanisms

Abstract

In Hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is found that n>1 tearing activity can significantly enhance the rate of on-axis peaking of tungsten impurities, which in turn significantly degrades discharge performance. Core n=1 instabilities can be beneficial in removing tungsten impurities from the plasma core (e.g. sawteeth or fishbones), but can conversely also degrade core confinement (particularly in combination with simultaneous n=3 activity). The nature of MHD instabilities in JET Hybrid discharges, with both its previous Carbon wall and subsequent JET-ILW, is surveyed statistically and the character of the instabilities is examined. Possible qualitative models for how the n>1 islands can enhance on-axis tungsten transport accumulation processes are presented.