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 V-beam/V-A congruent to 3) NBI in the spherical tokamak START (Culham Science Centre, UK). The unstable modes appear as high frequency, f(0) congruent to 70-400 kHz, 'bursts' detected by magnetic pick-up coils and soft X ray cameras, and 'sweep' 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 f(0) and the parameter VA beta(T)(-3/4), where V-A is the Alfven velocity and pr the toroidal beta, is found.