Dynamics of ion internal transport barrier in LHD heliotron and JT-60U tokamak plasmas

Abstract

Dynamics of ion internal transport barrier (ITB) formation and impurity transport both in the Large Helical Device (LHD) heliotron and in the JT-60U tokamak are described. Significant differences between heliotron and tokamak plasmas are observed. The location of the ITB moves outwards during the ITB formation regardless of the sign of magnetic shear in JT-60U, and the ITB becomes more localized in plasmas with negative magnetic shear. In LHD, a low Te/Ti ratio (<1) of the target plasma with high power heating is found to be necessary to achieve the ITB plasma and the ITB location tends to expand outwards or inwards depending on the condition of the target plasmas. Associated with the formation of the ITB, the carbon density tends to be peaked due to inward convection in JT-60U while the carbon density becomes hollow due to outward convection in LHD. The outward convection observed in LHD contradicts the prediction by neoclassical theory.