Edge energy transport barrier and turbulence in the I-mode regime on Alcator C-Mod
- Plasma Science and Fusion Center, MIT, Cambridge, Massachusetts 02129 (United States)
- Fusion Research Center, University of Texas at Austin, Austin, Texas 78712 (United States)
We report extended studies of the I-mode regime [Whyte et al., Nucl. Fusion 50, 105005 (2010)] obtained in the Alcator C-Mod tokamak [Marmar et al., Fusion Sci. Technol. 51(3), 3261 (2007)]. This regime, usually accessed with unfavorable ion B x {nabla}B drift, features an edge thermal transport barrier without a strong particle transport barrier. Steady I-modes have now been obtained with favorable B x {nabla}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 {approx} 1.5-3 times higher, increasing with I{sub p}. 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 {chi}{sub eff}, by typically a factor of 3, between L- and I-mode. The decrease correlates with a drop in mid-frequency fluctuations (f{approx} 50-150 kHz) observed on both density and magnetics diagnostics. Edge fluctuations at higher frequencies often increase above L-mode levels, peaking at f{approx} 250 kHz. This weakly coherent mode is clearest and has narrowest width ({Delta}f/f{approx} 0.45) at low q{sub 95} and high T{sub ped}, up to 1 keV. The E{sub r} 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{sub pol} shear typical of H-modes.
- OSTI ID:
- 21537878
- Journal Information:
- Physics of Plasmas, Vol. 18, Issue 5; Other Information: DOI: 10.1063/1.3582135; (c) 2011 American Institute of Physics; ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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