Fast-Ion Deuterium Alpha spectroscopic observations of the effects of fishbones in the Mega-Ampere Spherical Tokamak

TL;DR

This paper reports on FIDA spectroscopic measurements of fishbone modes in MAST, revealing significant fast-ion redistribution and energy effects, enhancing understanding of energetic particle behavior in spherical tokamaks.

Contribution

It provides the first detailed FIDA-based observations of fishbone-induced fast-ion redistribution in a spherical tokamak with reversed shear.

Findings

Fast-ion redistribution occurs in real and velocity space.

Fishbones cause up to 50% reduction in FIDA signal.

Fast ions retain energies up to 95% of injection energy.

Abstract

Using the recently-installed Fast-Ion Deuterium Alpha (FIDA) spectrometer, the effects of low-frequency (20-50 kHz) chirping energetic particle modes with toroidal mode number n \geq 1 on the fast-ion population in MAST plasmas are considered. Results from the FIDA diagnostic are presented and discussed in the light of the present theoretical understanding of these modes, known as fishbones, in plasmas with reversed shear. Measurements of the fast-ion population reveal strong redistribution of fast ions in both real and velocity space as a result of the fishbones. Time-resolved measurements throughout the evolution of a fishbone show radial redistribution of fast ions with energies up to 95% of the primary beam injection energy. Correlations between changes in the FIDA signal and the peak time derivative of the magnetic field perturbation are observed in a limited range of operating scenarios. The transient reduction in signal caused by a fishbone may in some cases reach 50% of the signal intensity before mode onset.