Impact of a pulsed supersonic deuterium gas jet on the ELM behaviour in ASDEX Upgrade

The possibility for pacing of type-I edge localized modes (ELMs) in H-mode plasmas by intermittent gas injection was investigated in ASDEX Upgrade as a possible alternative to, and in comparison with, ELM control by pellets. A Laval nozzle type molecular deuterium injector was used, delivering 1.7 ms long jets with up to about 1020D per pulse at a supersonic flow velocity of 2.2 km s−1. With a repetition rate of 2 Hz and a fast rise time of ≈25 µs, comparable to typical ELM rise times, the injector seemed to be well-suited for single ELM trigger tests. When applied to H-mode discharges with a moderate type-I ELM frequency of 40–60 Hz, no prompt (<0.5 ms) ELM triggering could be achieved, in contrast to the experience with pellets. There was, however, clear evidence for a delayed effect in the form of an inverse correlation of the gas pulse amplitude with the time interval between the gas pulse and the next ELM. The apparent lack of prompt ELM triggering seems to be due to a self-blocking of the gas jet by an extremely fast formation of a high density plasma layer in the separatrix vicinity, while the delayed effect may be simply caused by the jet-induced axisymmetric edge profile modification, similar to the delayed ELM cascade observed after a prompt ELM in case of large pellet injection. The delayed trigger effect observed might still be useful for ELM control in future machines, but the related high gas fuelling at elevated pulse frequency could make it unattractive in view of overall plasma performance.