Frequency-sweeping Alfven instabilities driven by super-Alfvenic beams in the spherical tokamak START

Alfven instabilities with rapidly decreasing frequency are observed in experiments with super-Alfvenic (up to Vbeam/VA ≅ 3) neutral beam injection in the spherical tokamak START (Culham Science Centre, UK). The unstable modes appear as high-frequency, f0 ≅ 70/400 kHz, 'bursts' detected by magnetic pick-up coils and soft X-ray cameras, and 'sweep' down by a factor of two infrequency 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 VAβT-3/4, where VA is the Alfven velocity and βT the toroidal beta, is found. (author)