Charge exchange recombination spectroscopy measurements with high spatial and temporal resolution have made the evaluation of the radial electric field (Er) in the JT-60U tokamak peripheral region possible with a better signal-to-noise ratio. In a very low density (<1 × 1019 m−3) L-mode phase, the gradient in Er (or, equivalently, in poloidal rotation, Vθ) was found to start increasing prior to an increase in the ion temperature (Ti). As the Er values in the region r/a > 0.94 started to become negative, down to −40 kV m−1 (similar to Vϕ), the steepening of the Ti profile in the same region accelerated. Even though the numerical value of Vϕ was considerably smaller than that of Vθ, its contribution to shear could be significant: while for r/a ⩾ 0.90 Vϕ became increasingly negative; it simultaneously became increasingly positive for r/a < 0.90. Clean bifurcations in the edge Er values between weak negative and strong negative ones were observed in the later ELM-free H-mode phase, with essentially the same confinement properties corresponding to two very different E × B shearing rates. The 'normal' ELMy H-mode took place only after the bifurcations. The results indicate that energy confinement improvement may not necessarily be connected to E × B shear suppression of turbulence.
Edge turbulence reduction at the L-H transition in DIII-D