Improved Ohmic confinement induced by multipulse turbulent heating in the Hefei Tokamak-6M

The improved Ohmic confinement phase (H-mode) has been observed during the turbulent heating (TH) pulse on the Hefei Tokamak-6M (HT-6M) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190]. The electron temperature and density profiles become steeper, and a more negative radial electric field E-r well is developed across the normal Ohmic phase (L-mode) to H-mode transition, at the plasma edge. The edge toroidal and poloidal velocities upsilon(phi) and upsilon(theta) and the main ion pressure gradient del P-i are substantially enhanced after the TH pulse. It is found that across the transition, upsilon(theta), upsilon(phi) and del P-i all play significant roles for regulating the E-r profile, and the negative well shape of E-r in the H-mode is dominantly maintained by the poloidal rotation. The time evolution indicates that prior to the transition upsilon(theta) plays a key role in inducing the rapid variations of the E-r and its shear. The density fluctuation suppression is independent of the sign of the E-r shear (E-r') and E-r curvature (E-r") and consistent with the theoretical models of Shaing et al. and Zhang and Mahajan, while the E-r" sign has an appreciable effect on the suppression of the plasma potential fluctuations. This fact reveals that the dependence of these two fluctuation suppressions on the E-r shear are different, suggesting that the existing L-->H transition theories which consider only a single fluctuating field should be improved. (C) 1998 American Institute of Physics. [S1070-664X(98)01006-4]