The study of the statistical properties of electric potential oscillations in the T-10 tokamak

Highlights in fusion plasma performance have been obtained under conditions where E x B shear stabilization mechanisms are likely play a key role. The oscillating shear flows, zonal flows and Geodesic Acoustic Modes (GAMs) are widely considered now as a turbulence stabilization mechanism. The study of the properties of plasma electric potential oscillations may explain the turbulence - zonal flow interplay. Low-frequency (f < 50 kHz) electric potential oscillations were investigated on the T-10 tokamak using Heavy Ion Beam Probe (HIBP), Correlation Reflectometry (CR) and Langmuir probes (LP). Regimes with Ohmic heating and with on- and off-axis ECRH were studied (B 2.2 - 2.5 T, 1_p = 180 - 330 kA, n_e = (1.3 - 2.5) x 10¹⁹ m^-3. It was shown that GAMs might have a complex structure, not similar to conventional periodical oscillations with a single frequency. GAMs have an intermittent character presenting the stochastic sequence of the wave packages with a 'lifetime' in a range of 0.5 - 2 ms. GAMs are more pronounced in ECRH plasmas. At the outer one third of the plasma column GAMs may be characterized not only by single, but also by a couple of closed but separated frequencies in a narrow interval 22 - 27 kHz. Each frequency peak is characterized by high potential/density correlation (coh > 0.6) and constant cross-phase, - π/ 2 for the main GAM peak, similar to observed in JIPP T-IIU, and + π/ 2 for the satellite. The low-frequency MHD mode m = 3 is pronounced at 7 kHz. The long- term correlation between potential (HIBP) and density (CR) oscillations were studied by the CR antenna shifted toroidally by one quarter of the torus similar to the duo-HIBP experiments in CHS. The result of the correlation measurements of the two diagnostics shows the high coherency (coh > 0.4 ) for GAMs and low-frequency MHD modes, suggesting the global character of the modes. The MHD mode m = 3 is also pronounced in core plasma potential and density (HIBP) and in floating potential at the plasma edge (LP) by quasicoherent oscillations with f = 7 kHz with high coherency between plasma potential (HIBP) and floating potential (LP). It also exhibits the stochastic character with ∼ 1-2 ms time-scale. Sawtooth oscillations presents the common mechanism modulating potential oscillations for both GAM and MHD mode m = 3. (author)