Reduced transport andErshearing in improved confinement regimes in JT-60U

The global confinement and the local transport properties of improved core confinement plasmas in JT-60U were studied in connection with E r shear formation. In the improved core confinement mode with internal transport barriers (ITBs), these are roughly classified into `parabolic type' ITBs and `box type' ITBs. The parabolic type ITB has a reduced thermal diffusivity χ in the core region; however, the E r shear, dE r /dr, is not as strong. The box type ITB has a very strong E r shear at the thin ITB layer and χ decreases to the level of neoclassical transport there. The estimated E × B shearing rate, ω E × B , becomes almost the same as the linear growth rate of the drift microinstability, γ L , at the ITB layer in the box type ITB. Experiments with hot ion mode plasmas during the repetitive L-H-L transition showed that the thermal diffusivity clearly depends on the E r shear and the strong E r shear contributes to the reduced thermal diffusivity.