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

• Published in: Nuclear fusion 2011-05, Vol.51 (5), p.053009
• Main Authors: Kamiya, K, Sakamoto, Y, Matsunaga, G, Kojima, A, Urano, H, Oyama, N, Koide, Y, Kamada, Y, Ida, K, Kurki-Suonio, T
• Format: Article
• Language: English
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/0029-5515/51/5/053009

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 ( 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.