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

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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 ∼30%, and by ∼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, , while the power threshold at the minimum density scales as . The H-mode power threshold in JET-ILW is 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 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 E r well and thus may not be correlated with the formation of the edge transport barrier in JET.