Reduction of ion thermal transport due to non-uniformity effects of the radial electric field at the edge of the JT-60U tokamak

Non-uniformity effects of the edge radial electric field, Er, on edge transport barrier (ETB) formation have been identified with high-spatiotemporal resolution spectroscopic measurement. We identified the decisive importance of Er-curvature (i.e. 2nd derivative of Er) on ETB formation corresponding to a local peak value in the pedestal region of the normalized ion temperature gradient, LTi−1≡−∇Ti/Ti. On the other hand, the Er-shear (i.e. 1st derivative of Er) was discovered to promote the expansion of pedestal width, compensating an unfavorable effect of the Er-curvature having its sign dependence on the transport reduction/enhancement-predicted by a theoretical model as LTi−1∝(Er′Er′−ErEr″)0.5. A significant reduction of the inferred ion thermal transport coefficient in the pedestal region, χi,ped., due to the non-uniformity effects of Er was also confirmed quantitatively in the H-mode plasma at the initial ELM-free phase. When the inhomogeneity of radial electric field became strong enough, the inferred χi,ped. value was close to its neoclassical one at the later ELM-free H-phase until 1st type-I ELM onset. Nevertheless, we observed the further reduction of electron density fluctuations (in the range up to 300 kHz) and Dα signals in association with the increment of electric field inhomogeneity. These suggest that the electron turbulent transport is suppressed further.