Bi-directional Alfv\'en Cyclotron Instabilities in the Mega-Amp Spherical Tokamak

TL;DR

This paper investigates Alfvén cyclotron instabilities driven by energetic beam ions in the MAST spherical tokamak, revealing mode behaviors across a range of magnetic fields and identifying counter-propagating modes at high fields.

Contribution

It provides new experimental insights into the excitation and characteristics of Alfvén cyclotron instabilities in spherical tokamaks with varying magnetic fields.

Findings

Discrete mode spectrum observed at high magnetic fields

Modes propagate counter to beam and plasma current

Instabilities occur over a wide range of toroidal magnetic fields

Abstract

Alfv\'en cyclotron instabilities excited by velocity gradients of energetic beam ions were investigated in MAST experiments with super-Alfv\'enic NBI over a wide range of toroidal magnetic fields from ~0.34 T to ~0.585 T. In MAST discharges with high magnetic field, a discrete spectrum of modes in the sub-cyclotron frequency range is excited toroidally propagating counter to the beam and plasma current (toroidal mode numbers n < 0).