Various features of H-mode observed in the JT-60U tokamak have been intensively investigated with respect to the effective edge ion collisionality vi*eff, which takes the impurity contribution into consideration. Cumulated data exhibits vi*eff at the transition does vary, depending upon target density, magnetic field, equilibrium profile and the heating beam power. Scatters in the vi*eff have been categorized into three regions: low field, hot-ion mode and high- beta p mode regime. In all the regimes reduced turbulent density fluctuation was observed across the transition, which is a result of the decrease of coherence in the edge density fluctuation and increase in the radial shearing of the poloidal flow velocity. Reflectometric technique and fast-sample magnetic probes were used for the turbulence diagnostics in this work. It was also documented that turbulent density fluctuation inversely scales with shear in the poloidal flow velocity in the low-field H-mode. Particularly in high- beta p H-mode discharge, the poloidal rotation is in the ion diamagnetic direction, and enhanced magnetic turbulence was observed, which may invoke the theoretical considerations regarding the role of magnetic turbulence as a cause of the H-mode transition.
Influence of magnetic configuration on edge turbulence and transport in the H-1 Heliac