Development of hybrid (high beta) plasmas for D-T operation in JET

A key aim of the 2021 JET deuterium-tritium (D-T) experiments was to demonstrate steady high fusion power (10-15MW) with the ITER-like Be/W first wall. Plasmas were developed using D, repeated with T to investigate and mitigate isotope effects, and run with D-T to maximise fusion power. Compared with high current (q95~3) 'baseline' plasmas, the JET 'hybrid' scenario has reduced current (2.3MA at q95~4.5-5) and increased q0 (>=1) to avoid deleterious MHD modes and access favourable confinement properties at high poloidal beta (>1). This candidate approach for ITER had never previously been tested using T or D-T fuel. In this presentation the process of 'hybrid' D-T scenario development will be explained for key phases from current ramp-up to termination, all of which are sensitive to isotope effects and impurities from the wall. For example, in the ohmic current ramp, used to pre-form the q-profile, an increase in central impurity radiation with main ion isotope mass was anticipated from previous mixed H-D experiments a and predictive modelling, allowing mitigation actions to be rapidly implemented for T and D-T. During the early H-mode phase, prevention of impurity influxes at the edge pedestal was the primary method for core radiation control using a combination of screening and ELM flushing. This was more challenging for T & D-T plasmas compared with D, and fine adjustment of heating and gas fuelling was needed to avoid excessive edge radiation and to establish regular ELMs with H98 1. After careful adaptation for D-T, high fusion power was achieved, broadly consistent with previous modelling predictions b given the available heating power. This led to a record fusion energy for a plasma with nD=nT of ~46 MJ.