Properties of internal transport barrier formation in JT-60U

The dependence of the ion thermal diffusivity (χi) on the radial electric field (Er) shear has been investigated in JT-60U plasmas. In positive magnetic shear (PS) plasmas, χi in the core region generally increases with the heating power, similar to the L mode at low heating power. However, as a result of the intensive central heating, which is relevant to the enhancement of the Eγ shear, a weak internal transport barrier (ITB) is formed, and χi in the core region starts to decrease. Corresponding to a further increase of the heating power, a strong ITB is formed and χi is reduced substantially. In the case of reversed magnetic shear (RS) plasmas, on the other hand, no power degradation of χi is observed in any of heating regimes. The electron thermal diffusivity (χe) is strongly correlated with χi in PS and RS plasmas. There exists a threshold in the effective Eγ shear to change the state from a weak to a strong ITB. It is found that the threshold of the effective Eγ shear in the case of a PS plasma depends on the poloidal magnetic field at the ITB. There are multiple levels of reduced transport in the strong ITB for RS plasmas. (author)