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

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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