Spatio-temporal evolutions of ion heat flux and radial electric field during internal transport barrier formation on JT-60U

Abstract

Spatio-temporal structures of ion heat flux and radial electric field are investigated on JT-60U to understand a mechanism of the internal transport barrier (ITB) formation. Contrary to the conventional argument that the ITB formation is due to the × flow shear suppressions on turbulence, we observe that the × shearing rate during the ITB formation is not enough for turbulence suppressions with & ~ 1. Another finding is that the ITB is formed simultaneously with abrupt decreases of ion heat flux in global region (like the report in [S. V. Neudatchine et. al., Plasma Phys. Control. Fusion 41, L39 (1999)]), and the gradient-flux relations are non-diffusively developed step-by-steps. These observations suggest the importance of the change in the long-radial fluctuations, which is previously observed in JT-60U [R. Nazikian et. al., Phys. Rev. Lett. 94, 135002 (2005)]. Therefore, the model of effects by non-uniformity of the radial electric field, under the condition where the scale separation is violated, is employed to explain such fluctuation quenching. The consistent results are obtained that the non-uniformity of radial electric field effectively works on the fluctuations with long radial wavelength and increases the temperature gradient. It seems natural that the suppression of the long-radial fluctuations causes to drive the global transport improvement to form the ITB.