Reduced transport and Er shearing in improved confinement regimes in JT-60U

The global confinement and the local transport propertiesof improved core confinement plasmas in JT-60Uwere studied in connection with Er 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 Er shear, dEr/dr, is not as strong.The box type ITB has a very strong Er shearat the thin ITB layer and χ decreases to the level of neoclassical transport there.The estimated E × B shearing rate, ωE × B,becomes almost the same as the linear growth rate of the drift microinstability, γL, at the ITB layer in the box type ITB.Experiments with hot ion mode plasmas during the repetitive L-H-L transitionshowed that the thermal diffusivity clearly depends on the Er shear and the strong Er shear contributes to the reduced thermal diffusivity.