Investigation of the plasma potential oscillations in the range of geodesic acoustic mode frequencies by heavy ion beam probing in tokamaks

Specific oscillations within a range of 20 kHz (“20 kHz-mode”) were investigated on the T-10 and TEXT tokamaks using Heavy Ion Beam Probe (HIBP) diagnostic. Regimes with ohmic heating on both machines, and with off-axis ECRH in T-10 were studied. It was shown that “20 kHz-modes are mainly the potential oscillations. The power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with a contrast range of (3–5). They are the manifestation of torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. It was shown that in TEXT the radial electric field oscillations exist in a limited radial range of 0.65 > ρ < 0.95. The frequency of “20 kHz-mode” is varied in the region of observation; it diminishes to the plasma edge. In T-10, after ECRH switch-on, the frequency increases, correlating with the growth of the electron temperature Te. In both machines the frequency of the “20 kHz-mode” varies with local Te: f ∼ Te1/2, which is consistent with theoretical scaling for geodesic acoustic modes (GAM): fGAM ∼ cs/R ∼ Te1/2, where cs is the speed of sound. The absolute frequencies are close to GAM values within a factor of unity.