Optimised ‘hybrid’ scenario H-mode plasmas for W radiation control in JET with the Be/W wall

Screening of high-Z (W) impurities from the confined plasma by the temperature gradient at the plasma periphery of fusion-grade H-mode plasmas was demonstrated for the first time in JET with the Be/W wall (Field et al 2023 Nucl. Fusion 63 016028). Additional experiments have been performed in JET during 2023, including in deuterium-tritium (DT) during the DTE3 campaign, to further optimise the impurity screening in such plasmas, as well as our bolometric measurements of the W impurity fluxes between and during edge-localised modes. A decrease in plasma current from 2.3 MA to 2.1 MA reduced the electron density and thereby increased the ion temperature at the H-mode pedestal top, resulting in stronger impurity screening behaviour. The scenario was then successfully transferred to operation in DT by increasing the toroidal field, in order to compensate the lower L/H-threshold power in DT compared to D plasmas. Here, results of detailed analysis and modelling of the neoclassical (NC) W transport in four pulses from these experiments are presented, two in D at 2.3 MA and 2.1 MA plasma current and a matched pulse pair at 2.1 MA in D and DT. Using the FACIT code (Fajardo et al 2023 Plasma Phys. Control. Fusion 65 035021) to model the NC W transport for these more recent pulses, the outward convection just inside the pedestal top found in our earlier study could not be reproduced. Possible reasons for this discrepancy between experimental observations and our modelling results are discussed, including potential deficiencies in our measurement technique and/or incompleteness of the NC transport modelling.