Investigation of the specific plasma potential oscillations with frequencies near 20 KHz by heavy ion beam probing in T-10

Full text: Investigations of the specific oscillations with frequencies 15-30 kHz on the T- 10 tokamak (R = 150 cm, a = 30 cm) with Heavy Ion Beam Probe (HIBP) diagnostics were conducted in regimes with off-axis ECRH (B = 2.33 T, I = 220 kA, rECRH = 12cm). Previous experiments in OH regimes have shown that '20 kHz' modes are mainly the potential fluctuations. These oscillations are seen on the signals of HIBP, Langmuir probes and reflectometry. They should cause the fluctuations of the poloidal rotation, i.e. the torsional plasma oscillations with m=0, called as the zonal flows. The HIBP sample volume was localized at r = 24 - 29 cm. It was observed that both in OH and ECRH regimes, the power spectrum of potential oscillations has a form of solitary quasi-monochromatic peak with the contrast range of 3-5. The frequency of '20 kHz' mode is varied in the region of observations; it diminishes to the plasma edge from 20 kHz at 22-24 cm till 13-15 kHz at 28- 29 cm. Increase of the frequency was observed, when ECRH was turn-on. The frequency change is correlated with change of the electron temperature Te, measured by 2nd ECE harmonic on the nearest chord 24 cm. Analysis have shown that frequency of the '20 kHz' mode is varied with local Te: f20 ∼ Te0.5, which is similar to a theoretically predicted dependency for Geodisic Acoustic Modes: fGAM cs/R ∼Te0.5, where cs is a sound speed. The absolute frequencies are close to GAM values within a factor of unity (1.3 - 1.5). (author)