Observation of 1...20 khz fluctuations in the URAGAN-2M torsatron

First observations of plasma fluctuations in the frequency range of 1...20 kHz in various plasma conditions of the URAGAN-2M (U-2M) torsatron are presented. Observed fluctuations of the H_a emissivity are coherent with the line-averaged density fluctuations measured by the 140 GHz heterodyne interferometer. Bursts of 1...2 kHz, m=1 oscillations, localized in the central region of the plasma column, are always observed at the heating degradation stage of the crankshaft antenna operation in ''standard'' U-2M discharges (B0=0.35 T). For this ''standard'' discharges with low RF power 10...15 kHz plasma density fluctuations are strongly localized at the region of substantial H_a emissivity gradients. Transient burst of 6 kHz central fluctuations are also observed in the medium magnetic field (B₀=0.067 T) discharges. Strong m = 1 fluctuations in the frequency range of 10...20 kHz are observed in the plasma conditioning discharges with low temperature and magnetic field (B₀=0.01 T). These fluctuations are localized in the central part of the plasma. Direction of poloidal rotation of the mode which causes fluctuations is reversed during the discharge simultaneously with its substantial amplitude increase and frequency decrease from 20 to 10 kHz. The fluctuations observed in standard U-2M discharges have frequency below the geodesic acoustic mode (GAM) frequency and can be caused by drift plasma modes. The frequency of fluctuations in the plasma conditioning discharges is observed in the range near the GAM frequency or higher.