JET-P(99)34

Frequency-Sweeping Alfvén Instabilities Driven by Super-Alfvénic Beams in the Spherical Tokamak START

Alfvé;n instabilities with rapidly decreasing frequency are observed in experiments with super-Alfvénic (up to Vbeam/VA ≅ 3) neutral beam injection in the spherical tokamak START (Culham Science Centre, UK). The unstable modes appear as high-frequency, ƒ0 ≅ 70÷ 400kHz, "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 0.2–0.3ms burst. A strong correlation between the initial frequency of the frequency-sweeping modes ƒ0 and the parameter VAβT–3/4, where VA is the Alfvén velocity and βT the toroidal beta, is found.
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JETP99034 111.75 Kb