Recent experiments on Alfvén eigenmodes in MAST

The developments of advanced tokamak scenarios as well as the employment of a new neutral beam injection (NBI) source with higher power and beam energy up to ≈65 keV have significantly broadened the frequency range and the variety of Alfvén eigenmodes (AEs) excited by the super-Alfvénic NBI on the spherical tokamak MAST. During recent experiments on MAST, several distinct classes of beam-driven AEs have been identified, with different modes being most unstable in different MAST scenarios. In MAST discharges with elevated monotonic q(r)-profiles and NBI power ⩾3 MW, chirping modes starting in the frequency range ⩽150 kHz decreased in frequency down to ≈20 kHz as q(0) decreased and then smoothly transformed to long-living modes with a weakly-varying frequency and a n = 1 kink-mode structure. The bolometer data suggest that the long-living modes can be responsible for fast ion losses on MAST, while the charge-exchange data show that a coupling between these modes and other low-frequency modes can cause a collapse of toroidal plasma rotation with a subsequent disruption. In MAST discharges with reversed magnetic shear, Alfvén cascade eigenmodes in the frequency range 40–180 kHz were observed at a moderate NBI power ⩽2 MW allowing an additional assessment of q(r)-profile evolution in time. A robust reproducible scenario was found on MAST, in which the instability of high-frequency modes in the range 0.4–3.8 MHz and typically with negative toroidal mode numbers was dominating the spectrum of beam-driven AEs. Since the highest frequency of such modes is close to the on-axis ion cyclotron frequency and the polarization study of these modes show a significant parallel perturbed magnetic field, these modes are identified as compressional Alfvén eigenmodes. For investigating the AE spectrum in plasmas with high β, an active AE antenna has been installed on MAST. First measurements of stable AE modes in MAST have been performed successfully and are described here.