Frequency sweeping Alfvén instabilities driven by super-Alfvénic beams in the spherical tokamak START

Alfvén instabilities with rapidly decreasing frequency are observed in experiments with super-Alfvénic (up to Vbeam/VA @ 3) NBI in the spherical tokamak START (Culham Science Centre, UK). The unstable modes appear as high frequency, f0 @ 70-400 kHz, `burstsʹ detected by magnetic pick-up coils and soft X ray cameras, and `sweepʹ down by a factor of two in frequency in a single 0.2-0.3 ms burst. A strong correlation between the initial frequency of the frequency sweeping modes f0 and the parameter VAbT-3/4, where VA is the Alfvén velocity and bT the toroidal beta, is found.