I-mode: An H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod

D.G. Whytea, A.E. Hubbard, J.W. Hughes, B. Lipschultz, J.E. Rice, E.S. Marmar, M. Greenwald, I. Cziegler, A. Dominguez, T. Golfinopoulos, N. Howard, L. Lin, R.M. McDermottb, M. Porkolab, M.L. Reinke, J. Terry, N. Tsujii, S. Wolfe, S. Wukitch, Y. Lin

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An improved energy confinement regime, I-mode, is studied in Alcator C-Mod, a compact high-field divertor tokamak using ion cyclotron range of frequencies (ICRFs) auxiliary heating. I-mode features an edge energy transport barrier without an accompanying particle barrier, leading to several performance benefits. H-mode energy confinement is obtained without core impurity accumulation, resulting in reduced impurity radiation with a high-Z metal wall and ICRF heating. I-mode has a stationary temperature pedestal with edge localized modes typically absent, while plasma density is controlled using divertor cryopumping. I-mode is a confinement regime that appears distinct from both L-mode and H-mode, combining the most favourable elements of both. The I-mode regime is investigated predominately with ion δB drift away from the active X-point. The transition from L-mode to I-mode is primarily identified by the formation of a high temperature edge pedestal, while the edge density profile remains nearly identical to L-mode. Laser blowoff injection shows that I-mode core impurity confinement times are nearly identical with those in L-mode, despite the enhanced energy confinement. In addition, a weakly coherent edge MHD mode is apparent at high frequency ∼100-300 kHz which appears to increase particle transport in the edge. The I-mode regime has been obtained over a wide parameter space (B = 3-6 T, I = 0.7-1.3 MA, q95 = 2.5-5). In general, the I-mode exhibits the strongest edge temperature pedestal (T) and normalized energy confinement (H 1) at low q95 (( 4 MW). I-mode significantly expands the operational space of edge localized mode (ELM)-free, stationary pedestals in C-Mod to T 1 keV and low collisionality ? 0.1, as compared with EDA H-mode with Tped <0.6 keV, ? > 1. The I-mode global energy confinement has a relatively weak degradation with heating power; W ∼ IP leading to increasing H98 with heating power.
Original languageEnglish
Article number105005
JournalNuclear Fusion
Issue number10
Publication statusPublished - 19 Oct 2010

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