Edge energy transport barrier and turbulence in the I-mode regime on Alcator C-Mod

• Main Authors: Hubbard, Amanda E. (Contributor), Whyte, Dennis G. (Contributor), Churchill, Randy Michael (Contributor), Cziegler, Istvan (Contributor), Golfinopoulos, Theodore (Contributor), Hughes, Jerry W. (Contributor), Rice, John E. (Contributor), Bespamyatnov, I. (Author), Greenwald, Martin J. (Contributor), Howard, Nathaniel Thomas (Contributor), Lipschultz, Bruce (Contributor), Marmar, Earl S. (Contributor), Reinke, Matthew Logan (Contributor), Rowan, W. L. (Author), Terry, James L. (Contributor), Alcator C-Mod Group (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science (Contributor), Massachusetts Institute of Technology. Department of Nuclear Science and Engineering (Contributor), Massachusetts Institute of Technology. Department of Physics (Contributor), Massachusetts Institute of Technology. Plasma Science and Fusion Center (Contributor), Dominguez, A. (Contributor)
• Format: Article
• Language: English
• Published: American Institute of Physics, 2011-09-16T19:47:23Z.
• Subjects: Article
• Online Access: Get fulltext

 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 × ∇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 Ip. 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 χeff, 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 (Δf/f ∼ 0.45) at low q95 and high Tped, up to 1 keV. The Er 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 Vpol shear typical of H-modes.