Spatial structure of density fluctuations and geodesic acoustic mode in T-10 tokamak

Investigation of turbulence properties could reveals the nature of anomalous transport in tokamak. This paper presents results of systematic investigations of poloidal and toroidal structure of small-scale density fluctuations as well as the properties of Geodesic Acoustic Modes (GAM) in T-10 tokamak. It was found that total density fluctuations amplitude is strongly non uniform in poloidal direction. Turbulence amplitude had a maximum at low magnetic field side and minimum at high magnetic field side in accordance with theory predictions. The asymmetry of turbulence amplitude increased with the rise of total heating power. Poloidal dependence of the amplitude of different spectra components are showed in this paper. Improved reflectometry system allowed to made long distance toroidal correlations without parasitic cross-talk between different antennae arrays. It was found that quasi-coherent oscillations had a toroidal correlation length about 12.5 m whereas the length of stochastic low frequency fluctuations was 2.5 m. It was found some evidence of oblique propagation of the quasi-coherent oscillation at small angle (about 0.1 degree) with respect to magnetic field line. Experiments with joint turbulence observations by different diagnostics were made in T-10 tokamak. Strong correlation at GAM frequency was found between fluctuations of plasma potential, measured by heavy ion beam probe, and density fluctuations, measured by correlation reflectometry. GAM amplitude in density fluctuations spectra was high at the top antennae positions and negligibly small at equatorial plane in accordance with theory predictions. It was reported previously that density perturbations by GAM exist at local electron densities lower than 2 x 10¹⁹ m^-3 in Ohmic discharges. Recent experiments with ECR heating have shown presence of GAM in density fluctuation spectra at density about (3 - 3.5) x 10¹⁹ m^-3. Effect of the GAM on the turbulence amplitude at LFS and HFS is also discussed. (author)