Power requirements for superior H-mode confinement on Alcator C-Mod: Experiments in support of ITER

J.W. Hughes, M.L. Reinke, J.L. Terry, D. Brunner, M. Greenwald, A.E. Hubbard, B. Labombard, B. Lipschultz, Y. Ma, S. Wolfe, S.J. Wukitch, A. Loarte

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Power requirements for maintaining sufficiently high confinement (i.e. normalized energy confinement time H ≥ 1) in H-mode and its relation to H-mode threshold power scaling, P , are of critical importance to ITER. In order to better characterize these power requirements, recent experiments on the Alcator C-Mod tokamak have investigated H-mode properties, including the edge pedestal and global confinement, over a range of input powers near and above P . In addition, we have examined the compatibility of impurity seeding with high performance operation, and the influence of plasma radiation and its spatial distribution on performance. Experiments were performed at 5.4 T at ITER relevant densities, utilizing bulk metal plasma facing surfaces and an ion cyclotron range of frequency waves for auxiliary heating. Input power was scanned both in stationary enhanced D (EDA) H-modes with no large edge localized modes (ELMs) and in ELMy H-modes in order to relate the resulting pedestal and confinement to the amount of power flowing into the scrape-off layer, P , and also to the divertor targets. In both EDA and ELMy H-mode, energy confinement is generally good, with H near unity. As P is reduced to levels approaching that in L-mode, pedestal temperature diminishes significantly and normalized confinement time drops. By seeding with low-Z impurities, such as Ne and N , high total radiated power fractions are possible, along with substantial reductions in divertor heat flux (>4×), all while maintaining H ∼ 1. When the power radiated from the confined versus unconfined plasma is examined, pedestal and confinement properties are clearly seen to be an increasing function of P , helping to unify the results with those from unseeded H-modes. This provides increased confidence that the power flow across the separatrix is the correct physics basis for ITER extrapolation. The experiments show that P /P of one or greater is likely to lead to H ≥ 1 operation, and also that such a condition can be made compatible with a low-Z radiative impurity solution for reducing divertor heat loads to levels acceptable for ITER.
Original languageEnglish
JournalNuclear fusion
Issue number8
Publication statusPublished - 1 Aug 2011

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