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

C.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. MarsenI. Nunes, D. Van Eester, M.F. Stamp

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Abstract

A comparison of the L-H power threshold (Pthr) 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 IP/BTpairs. The low density dependence of the L-H power threshold, namely an increase below a minimum density ne,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 ne,min, Pthris reduced by ∼30%, and by ∼40% when the radiation from the bulk plasma is subtracted (Psep), 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, ne,min∼ BT4/5, while the power threshold at the minimum density scales as Psep,min∼ BT4/5. The H-mode power threshold in JET-ILW is found to be sensitive both to variations in main plasma shape (Psepdecreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of Psepand Zeffor subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Zeffand/or subdivertor neutral pressure in the L-H transition physics. Depending on the chosen divertor configuration, Pthrcan be up to a factor of two lower than the ITPA scaling law for densities above ne,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 km s-1± 2-3 km s-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 Erwell and thus may not be correlated with the formation of the edge transport barrier in JET. © 2014 IAEA, Vienna.
Original languageEnglish
Article number023007
Pages (from-to)-
JournalNuclear Fusion
Volume54
Issue number2
DOIs
Publication statusPublished - 1 Feb 2014
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Nuclear and High Energy Physics
  • Condensed Matter Physics

Cite this

Maggi, C. F., Delabie, E., Biewer, T. M., Groth, M., Hawkes, N. C., Lehnen, M., De La Luna, E., McCormick, K., Reux, C., Rimini, F., Solano, E. R., Andrew, Y., Bourdelle, C., Bobkov, V., Brix, M., Calabro, G., Czarnecka, A., Flanagan, J., Lerche, E., ... Stamp, M. F. (2014). L-H power threshold studies in JET with Be/W and C wall. Nuclear Fusion, 54(2), -. [023007]. https://doi.org/10.1088/0029-5515/54/2/023007