Mitigation of MHD induced fast-ion redistribution in MAST and implications for MAST-Upgrade design

TL;DR

This study investigates how MHD activity causes fast-ion redistribution in MAST, identifies conditions suppressing this effect, and proposes a neutral-beam modification to improve plasma performance in future upgrades.

Contribution

It provides experimental analysis of fast-ion redistribution due to fishbone modes and suggests a beam geometry change to mitigate this in MAST-Upgrade.

Findings

High plasma density suppresses MHD activity and fast-ion redistribution.

Fast-ion redistribution is localized in the plasma core.

A beam geometry modification reduces fishbone mode drive.

Abstract

The phenomenon of redistribution of neutral beam fast-ions due to MHD activity in plasma has been observed on many tokamaks and more recently has been a focus of research on MAST (Turnyanskiy M. et al, 2011 Nucl. Fusion 53 053016). n=1 fishbone modes are observed to cause a large decrease in the neutron emission rate corresponding to a significant perturbation of the fast-ion population in the plasma. Theoretical work on fishbone modes states that the fast-ion distribution itself acts as the source of free energy driving the modes that cause the redistribution. Therefore a series of experiments have been carried out on MAST to investigate a range of plasma density levels at two neutral beam power levels to determine the region within this parameter space in which MHD activity and consequent fast-ion redistribution is suppressed. Analysis of these experiments shows complete suppression of MHD activity at high density with increasing activity and fast-ion redistribution at lower densities and higher NB power accompanied by strong evidence for localisation of the redistribution effect to a specific region in the plasma core. The results also indicate correlations between the form of the modelled fast-ion distribution function, the amplitude and growth rate of the fishbone modes, and the magnitude of the redistribution effect. The same analysis has been carried out on models of MAST-Upgrade baseline plasma scenarios to determine whether significant fast-ion redistribution is likely to occur in that device. A simple change to the neutral-beam injector geometry is proposed which is shown to have a significant mitigating effect in terms of the fishbone mode drive and is therefore expected to allow effective plasma heating and current drive over a wider range of plasma conditions in MAST-Upgrade.