Edge energy transport barrier and turbulence in the I-mode regime on alcator C-mod

A.E. Hubbard, D.G. Whyte, R.M. Churchill, I. Cziegler, A. Dominguez, T. Golfinopoulos, J.W. Hughes, J.E. Rice, M.J. Greenwald, N. Howard, B. Lipschultz, E.S. Marmar, M.L. Reinke, J.L. Terry, I. Bespamyatnov, W.L. Rowan

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We report extended studies of the I-mode regime Whyte, Nucl. Fusion 50, 105005 (2010) obtained in the Alcator C-Mod tokamak Marmar, Fusion Sci. Technol. 51(3), 3261 (2007). This regime, usually accessed with unfavorable ion B × ∇B drift, features an edge thermal transport barrier without a strong particle transport barrier. Steady I-modes have now been obtained with favorable B × ∇B drift, by using specific plasma shapes, as well as with unfavorable drift over a wider range of shapes and plasma parameters. With favorable drift, power thresholds are close to the standard scaling for L-H transitions, while with unfavorable drift they are ∼ 1.5-3 times higher, increasing with I. Global energy confinement in both drift configurations is comparable to H-mode scalings, while density profiles and impurity confinement are close to those in L-mode. Transport analysis of the edge region shows a decrease in edge , by typically a factor of 3, between L- and I-mode. The decrease correlates with a drop in mid-frequency fluctuations (f ∼ 50-150 kHz) observed on both density and magnetics diagnostics. Edge fluctuations at higher frequencies often increase above L-mode levels, peaking at f ∼ 250 kHz. This weakly coherent mode is clearest and has narrowest width (ff ∼ 0.45) at low q and high T , up to 1 keV. The E well in I-mode is intermediate between L- and H-mode and is dominated by the diamagnetic contribution in the impurity radial force balance, without the V shear typical of H-modes.
Original languageEnglish
JournalPhysics of Plasmas
Issue number5
Publication statusPublished - 1 May 2011

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