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 ν_φ and ν_θ and the main ion pressure gradient ∇P_i are substantially enhanced after the TH pulse. It is found that across the transition, ν_θ, ν_φ and ∇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 ν_θplays 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.