19th IAEA Fusion Energy Conference

Properties of Internal Transport Barrier Formation in JT-60U

Y. Sakamoto¹⁾, T. Suzuki¹⁾, S. Ide¹⁾, Y. Koide¹⁾, H. Takenaga¹⁾, Y. Kamada¹⁾, T. Fujita¹⁾, T. Fukuda¹⁾, T. Takizuka¹⁾, H. Shirai¹⁾, N. Oyama¹⁾, Y. Miura¹⁾, K. W. Hill²⁾, G. Rewoldt²⁾ and the JT-60 Team¹⁾
1) Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, 801-1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311-0193, Japan
2) Princeton Plasma Physics Laboratory, Princeton, New Jersey, U.S.A

Abstract.
The dependence of the ion thermal diffusivity (χ_i) on the radial electric field (E_r) shear has been investigated in JT-60U plasmas. In positive magnetic shear (PS) plasmas, χ _i 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 E_r shear, a weak internal transport barrier (ITB) is formed, and χ_i in the core region starts to decrease. Corresponding to a further increase of the heating power, a strong ITB is formed and χ_i is reduced substantially. In the case of reversed magnetic shear (RS) plasmas, on the other hand, no power degradation of χ_i is observed in any of the heating regimes. The electron thermal diffusivity (χ_e) is strongly correlated with χ_i in PS and RS plasmas. There exists a threshold in the effective E_r shear to change the state from a weak to a strong ITB. It is found that the threshold of the effective E_r 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.