ELM Control at the L-H Transition Achieved by Pellet Pacing in the All-metal Wall Tokamaks ASDEX Upgrade and JET
On ITER pellets are foreseen for ELM control and fuelling. More importantly ELM control in particular control of the first ELM needs to be demonstrated already in the non-nuclear phase of ITER during operation in H or He. Whilst D pellets have been established as ELM control technique in the stationary phase with D target plasmas in devices with C as plasma facing component, the question of other isotopes and non-stationary phases are less well known. Here, we report on new pellet triggering experiments in ASDEX Upgrade and JET mimicking specific ITER operating scenarios. Both machines are equipped with a full metal wall, where recent investigations have shown that pellet triggering and pacing become more intricate. On both machines ELM triggering by D pellets injected into D plasmas during extended ELM free phases, often following the LÆ H transition has been demonstrated. In both devices the pellets are found to induce ELMs under conditions far from the stability boundary for type-I ELMs. Furthermore, on ASDEX Upgrade this study was conducted during LÆH transitions in the current ramp-up phase as foreseen for ITER. In addition, the pellet ELM trigger potential was proven at ASDEX Upgrade with a correct isotopic compilation for the non-nuclear phase on ITER, namely H pellets into either He or H plasmas. Results from this study are encouraging since they demonstrated the pellets potential to provoke ELMs even under conditions quite far from the stability boundaries attributed to the occurrence of spontaneous ELMs. However, with the change from C to an all-metal plasma facing components recently examples have been found on both machines where pellets failed to establish ELM control under conditions where this would be expected and needed. Consequently, a major task of future investigations in this field will be to shed more light on the underlying physics of pellet ELM triggering process, before sound predictions for ITER are possible.