Role of electric field curvature in the formation of edge transport barrier in the JT-60U tokamak

Based on the high-resolution spectroscopic measurements of the radial electric field (E _r) in the JT-60U tokamak, we have identified the mechanisms centered around the role of the E _r-curvature on the edge transport barrier (ETB) formation. In addition, we recognized a new role of the E _r-shear for the expansion of pedestal width, compensating an unfavorable effect of the E _r-curvature having its sign dependence on the turbulence transport. We found a new interesting phenomenon of very slow out-of-phase oscillations between the E _r and ion temperature gradient in the frequency range of 30-50 Hz, preceding the slow L-H transition, which can demonstrate a causality test of the ETB formation in association with the non-uniformity E _r-effects. A critical condition for the E _r-bifurcations is also discussed.