L-H power threshold studies in JET with Be/W and C wall

TitleL-H power threshold studies in JET with Be/W and C wall
Publication TypeJournal Article
Year of Publication2014
AuthorsC.F Maggi, E. Delabie, T.M Biewer, M. Groth, N.C Hawkes, M. Lehnen, E. de la Luna, K. McCormick, C. Reux, F. Rimini, E.R Solano, Y. Andrew, C. Bourdelle, V. Bobkov, M. Brix, G. Calabro, A. Czarnecka, J. Flanagan, E. Lerche, S. Marsen, I. Nunes, D. Van Eester, M.F Stamp
JournalNuclear Fusion
Date PublishedFeb
ISBN Number0029-5515; 1741-4326

A comparison of the L-H power threshold (P-thr) in JET with all carbon, JET-C, and beryllium/tungsten wall (the ITER-like choice), JET-ILW, has been carried out in experiments with slow input power ramps and matched plasma shapes, divertor configuration and I-P/B-T pairs. The low density dependence of the L-H power threshold, namely an increase below a minimum density n(e,min), which was first observed in JET with the MkII-GB divertor and C wall and subsequently not observed with the current MkII-HD geometry, is observed again with JET-ILW. At plasma densities above n(e,min), P-thr is reduced by similar to 30%, and by similar to 40% when the radiation from the bulk plasma is subtracted (P-sep), with JET-ILW compared to JET-C. At the L-H transition the electron temperature at the edge, where the pedestal later develops, is also lower with JET-ILW, for a given edge density. With JET-ILW the minimum density is found to increase roughly linearly with magnetic field, n(e,min) similar to B-T(4/5), while the power threshold at the minimum density scales as P-sep,P-min similar to B-T(5/2). The H-mode power threshold in JET-ILWis found to be sensitive both to variations in main plasma shape (P-sep decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of P-sep and Z(eff) or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Z(eff) and/or subdivertor neutral pressure in the L-H transition physics. Depending on the chosen divertor configuration, P-thr can be up to a factor of two lower than the ITPA scaling law for densities above n(e,min). A shallow edge radial electric field well is observed at the L-H transition. The edge impurity ion poloidal velocity remains low, close to its L-mode values, <= 5 kms(-1) +/- 2-3 kms(-1), at the L-H transition and throughout the H-mode phase, with no measureable increase within the experimental uncertainties. The edge toroidal rotation profile does not contribute to the depth of the negative E-r well and thus may not be correlated with the formation of the edge transport barrier in JET.








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