Radiation control in deuterium, tritium and deuterium-tritium JET baseline plasmas – part I

The achievement of a steady ELMy H-mode phase with high ion temperature, but without a gradual rise in plasma radiation, has been a crucial point to establish high plasma performance scenarios in JET ITER-like-wall plasmas. Indeed, radiation events, due to the release of high Z impurities, such as Nickel and Copper, and W sputtered from the divertor, can strongly reduce the power crossing the plasma separatrix and slow the ELMs dynamics, thus inducing H to L transition. In particular, in JET baseline plasmas, because of the outward neo-classical transport [A.R. Field et al 2021 Plasma Phys. Control. Fusion 63 095013], plasma impurities are mainly localized in the mantle region, as detected by a real-time surrogate model for bolometer tomography based on machine learning [D.R. Ferreira et al 2021 Fusion Engineering and Design 164], and the consequent excessive radiation in this region is the main cause of plasma termination in recent Deuterium, Tritium and Deuterium -Tritium operations. To guarantee impurity accumulation being flushed by the ELMs, ELM control schemes, which ensure a throughput of particles, either via gas fueling and via pellets, have been exploited. In this work, the staged approach strategy towards radiation control, which allowed to sustain for more than 10 s Tritium and Deuterium-Tritium baseline discharges without radiation issues, is presented.