Investigation of the specific plasma potential oscillations with geodesic acoustic mode frequencies by heavy ion beam probing

Abstract

Investigation of the specific oscillations with frequencies 15-30 kHz on the T-10 tokamak (R= 150cm, a = 30 cm) with Heavy Ion Beam Probe (HIBP) diagnostic was conducted in regimes with off-axis ECRH (B = 2.33 T, Ip= 220 kA, r_ECRH=12 cm). 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 observation; it diminishes to the plasma edge from 20 kHz at 22-24 cm till 13-15 kHz at 28-29 cm. After ECRH switch-on, the frequency increases, correlating with growth of the electron temperature Te, measured by 2-nd ECE harmonic on the nearest chord 24 cm. Analysis have shown that frequency of the "20 kHz" mode is varied with local T_e: f_20 ~ T_e**1/2 that is similar to a theoretical scaling for Geodesic Acoustic Modes (GAM): f_GAM ~ c_s/R ~ T_e**1/2, where cs is a sound speed. The absolute frequencies are close to GAM values within a factor of unity (1.12-1.5).