Edge Radial Electric Field Structure and Its Connection to H-Mode Confinement in Alcator C-Mod

High-resolution charge-exchange recombination spectroscopic measurements of B 5+ ions have enabled the first spatially resolved calculations of the radial electric field (E r) in the Alcator C-Mod pedestal region [E. offer new challenges for theory and simulation and provide for important comparisons with other devices. Qualitatively, the field structure observed on C-Mod is similar to that on other tokamaks. However, the narrow high confinement mode (H-mode) E r well widths (5mm) observed on C-Mod suggest a scaling with machine size, while the observed depths (up to 300kV/m) are unprecedented. Due to the strong ion-electron thermal coupling in the C-Mod pedestal it is possible to infer information about the main ion population in this region. The results indicate that in H-mode the main ion pressure gradient is the dominant contributor to the E r well and that the main ions have significant edge flow. C-Mod H-mode data show a clear correlation between deeper E r wells, higher confinement plasmas, and higher electron temperature pedestal heights. However, Improved L-mode (I-mode) plasmas exhibit energy confinement equivalent to that observed in similar H-mode discharges, but with significantly shallower E r wells. I-mode plasmas are characterized by H-mode like energy barriers, but with L-mode like particle barriers. The decoupling of energy and particle barrier formation makes the I-mode an interesting regime for fusion research and provides for a low collisionality pedestal without edge localized modes (ELMs).