Measurement and control of the fast ion redistribution on MAST

TL;DR

This study quantifies fast ion redistribution in MAST, linking it to energetic particle modes and demonstrating that off-axis neutral beam injection can restore classical diffusion behavior.

Contribution

It provides the first quantification of fast ion redistribution in MAST using a scanning neutron camera and links the phenomenon to n=1 energetic particle modes.

Findings

Fast ion diffusivity correlates with n=1 mode amplitude.

Off-axis neutral beam injection mitigates mode growth.

Fast ion behavior can be restored to classical diffusion.

Abstract

Previous experiments on MAST and other tokamaks have indicated that the level of fast ion redistribution can exceed that expected from classical diffusion and that this level increases with beam power. In this paper we present a quantification of this effect in MAST plasmas using a recently commissioned scanning neutron camera. The observed fast ion diffusivity correlates with the amplitude of n=1 energetic particle modes, indicating that they are the probable cause of the non-classical fast ion diffusion in MAST. Finally, it will be shown that broadening the fast ion pressure profile by the application of neutral beam injection at an off-axis location can mitigate the growth of these modes and result in the classical fast ion behaviour