Beta-dependence of energetic particle-driven instabilities in spherical tokamaks

Abstract

Extensive data on energetic particle-driven (EPD) modes in the tight-aspect ratio MAST and START experiments with super-Alfvénic neutral beam injection is reviewed. The dependence of toroidal Alfvén eigenmodes (TAEs) and frequency-sweeping 'chirping' modes on the thermal plasma β-value is investigated experimentally, and a decrease both in the mode amplitudes and in the number of unstable modes with increasing β is found, in line with theoretical expectations. 'Fishbone' modes in MAST are seen at higher β-values than TAEs or 'chirping' modes, and occasionally are found to trigger long-lasting modes, which may affect global MHD stability. In the light of the data analysis the possible importance of EPD instabilities in larger burning plasma spherical tokamaks, which would necessarily operate in high-β regimes, is discussed.