Experimental investigation of ion-temperature-gradient-like turbulence characteristics in T-10 core plasmas with toroidal and poloidal correlation reflectometry

Abstract

The physical mechanisms of small scale density fluctuations in the frequency range5-400 kHz were investigated with correlation reflectometry in different types of ohmically heated discharges in the T-10 tokamak. A temporal formation of velocity shear in the central region of the plasma column during the discharge transition from saturated to improved ohmic confinement resulted in suppression of long wavelength quasi-coherent turbulence, while the amplitude of fluctuations with a broader frequency spectrum was not affected. The potential of correlation reflectometry was extended by simultaneous plasma probing from the low field side and high field side. Enhancement of quasi-coherent turbulence at the low field side by a factor of 5 was measured, while the other turbulence type was poloidally symmetric. A high long distance toroidal correlation of up to 40% was observed experimentally for quasi-coherent density fluctuations at a distance of 10 m after one turn around the tokamak major axis. Fluctuations propagate at an angle of about 0.5° with respect to the perpendicular to the magnetic field line, demonstrating a drift mechanism of turbulence. Plasma rotation was estimated from a radial force balance equation for ions with the radial electric field measured with a heavy ion beam probe. A comparison of the calculated plasma rotation with the measured turbulence rotation showed that the turbulence rotates in the ion diamagnetic drift direction in the plasma frame. All experimental observations are consistent with `toroidal' and `slab' ion temperature gradient turbulence as the underlying physical mechanism.