Reduced transport and Er shearing 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 chi 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 chi decreases to the level of neoclassical transport there. The estimated ExB shearing rate, w(ExB), becomes almost the same as the linear growth rate of the drift microinstability, gamma(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 Er shear and the strong E-r shear contributes to the reduced thermal diffusivity.