Properties of internal transport barrier formation in JT-60U

The dependence of the thermal diffusivity on the heat flux has been investigated in JT-60U plasmas by varying the neutral beam power. In positive magnetic shear (PS) plasmas, the ion thermal diffusivity (χ i ) in the core region generally increases with the heating power, as with the L mode at low heating powers. However, as a result of the intensive central heating, a weak internal transport barrier (ITB) is formed, and the χ i value in the core region starts to decrease. Corresponding to a further increase in the heating power, a strong ITB is formed and the χ i value is reduced substantially. In the case of reversed magnetic shear (RS) plasmas, on the other hand, no power degradation of the χ i value is observed in any of the heating regimes. The electron thermal diffusivity (χ e ) is correlated with the χ i value in PS and RS plasmas. Furthermore, the dependence of the ion thermal diffusivity on the radial electric field ( E r ) shear has also been investigated. By considering the non-locality in the relation between the E r shear and the χ i value, a threshold in the effective E r shear to change the state from a weak to a strong ITB is identified.