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

Abstract

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 Er well is developed across the normal Ohmic phase (L-mode) to H-mode transition, at the plasma edge. The edge toroidal and poloidal velocities υφ and υθ and the main ion pressure gradient ∇Pi are substantially enhanced after the TH pulse. It is found that across the transition, υθ, υφ and ∇Pi all play significant roles for regulating the Er profile, and the negative well shape of Er in the H-mode is dominantly maintained by the poloidal rotation. The time evolution indicates that prior to the transition υθ plays a key role in inducing the rapid variations of the Er and its shear. The density fluctuation suppression is independent of the sign of the Er shear(Er′) and Er curvature (Er″) and consistent with the theoretical models of Shaing et al. and Zhang and Mahajan, while the Er″ 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 Er shear are different, suggesting that the existing L→H transition theories which consider only a single fluctuating field should be improved.