'Nonlocal' transport in current ramp-up experiments in the FT-2 tokamak

Attention is drawn to the effect of fast propagation of a heat pulse across the plasma column in a tokamak. The high accuracy of the multiphase Thomson scattering diagnostics used in current ramp-up experiments made it possible to reveal that the electron temperature in the center of the plasma column rises over a time comparable to the skin time. The decrease in the thermal diffusivity coefficient χe(r) and rapid suppression of small-scale plasma-density turbulence, which has been studied in the FT-2 tokamak with the help of small-angle collective scattering of CO2-laser radiation, are addressed. The suppression of fluctuations over a wide frequency range immediately after the beginning of the current rise allows one to suggest that this fact is related to the rapid change in the radial electric field Er(r) and the velocity shear of the poloidal ExB rotation of the plasma. To explain the effect of nonlocal transport, it is suggested that the mechanism responsible for the change in Er(r) is related to an abrupt increase in E(parallel) during the rapid current rise. The model used takes into account the fact that the effective electron and ion collision frequencies differ significantly under the conditions of Ohmic discharges in the FT-2 tokamak, when ve*∼1 and vi* > or approx. 10