Edge radial electric field structure and its connections to H-mode confinement in Alcator C-Mod plasmas

R.M. McDermott, B. Lipschultz, J.W. Hughes, P.J. Catto, A.E. Hubbard, I.H. Hutchinson, R.S. Granetz, M. Greenwald, B. Labombard, K. Marr, M.L. Reinke, J.E. Rice, D. Whyte

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High-resolution charge-exchange recombination spectroscopic measurements of B ions have enabled the first spatially resolved calculations of the radial electric field (E) in the Alcator C-Mod pedestal region [E. S. Marmar, Fusion Sci. Technol. 51, 261 (2006)]. These observations 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 well widths (5 mm) observed on C-Mod suggest a scaling with machine size, while the observed depths (up to 300 kV/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 Er well and that the main ions have significant edge flow. C-Mod H -mode data show a clear correlation between deeper E 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 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.
Original languageEnglish
Article number056103
Number of pages12
JournalPhysics of Plasmas
Issue number5
Early online date12 Mar 2009
Publication statusPublished - 2009

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