Spatio-temporal structure of the edge radial electric field during H-mode in JT-60U

Charge exchange recombination spectroscopy measurements with high spatial and temporal resolution have made the evaluation of the radial electric field (E r ) in the JT-60U tokamak peripheral region possible with a better signal-to-noise ratio. In a very low density (19 m−3) L-mode phase, the gradient in E r (or, equivalently, in poloidal rotation, V θ) was found to start increasing prior to an increase in the ion temperature (T i). As the E r values in the region r/a > 0.94 started to become negative, down to −40 kV m−1 (similar to V ϕ), the steepening of the T i profile in the same region accelerated. Even though the numerical value of V ϕ was considerably smaller than that of V θ, its contribution to shear could be significant: while for r/a ⩾ 0.90 V ϕ became increasingly negative; it simultaneously became increasingly positive for r/a < 0.90. Clean bifurcations in the edge E r 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.