Comparative analysis of core heat transport of JET high density H-mode plasmas in carbon wall and ITER-like wall

A consistent deterioration of global confinement in H-mode experiments has been observed in JET [1] following the replacement of all carbon plasma facing components (PFCs) with an all metal ('ITER-like') wall (ILW). This has been correlated to the observed degradation of the pedestal confinement, as lower electron temperature (T-e) values are routinely measured at the top of the edge barrier region. A comparative investigation of core heat transport in JET-ILW and JET-CW (carbon wall) discharges has been performed, to assess whether core confinement has also been affected by the wall change. The results presented here have been obtained by analysing a set of discharges consisting of high density JET-ILW H-mode plasmas and comparing them against their counterpart discharges in JET-CW having similar global operational parameters. The set contains 10 baseline (beta(N) = 1. 5 similar to 2) discharge-pairs with 2.7 T toroidal magnetic field, 2.5 MA plasma current, and 14 to 17 MW of neutral beam injection (NBI) heating. Based on a T-e profile analysis using high resolution Thomson scattering (HRTS) data, the T-e profile peaking (i.e. core T-e (rho = 0.3)/edge T-e (rho = 0.7)) is found to be similar, and weakly dependent on edge T-e, for both JET-ILW and JET-CW discharges. When ILW discharges are seeded with N-2, core and edge T-e both increase to maintain a similar peaking factor. The change in core confinement is addressed with interpretative TRANSP simulations. It is found that JET-ILW H-mode plasmas have higher NBI power deposition to electrons and lower NBI power deposition to ions as compared to the JET-CW counterparts. This is an effect of the lower electron temperature at the top of the pedestal. As a result, the core electron energy confinement time is reduced in JET-ILW discharges, but the core ion energy confinement time is not decreased. Overall, the core energy confinement is found to be the same in the JET-ILW discharges compared to the JET-CW counterparts.