1 EX / 10-1 Effectiveness of high-frequency ELM pacing with deuterium and non-fuel pellets in DIII -

DIII-D studies of high-frequency ELM pacing by pellet injection were extended to ITER scenarios at low neutral beam torque and to the use of non-fuel materials, a technique that could potentially reduce the throughput to the pumping and fuel reprocessing systems. ELM peak heat flux mitigation was obtained with injection of D2 pellets at frequencies up to 90 Hz in low-torque ITER baseline scenario (IBS, q95=3.2, βN=1.7, ne,ped=7.5x10 m, Tinj<0.1 N-m). D2 pellet injection resulted in ELM pacing at frequencies 6-8x the natural ELM frequency (10 Hz). The resulting inner divertor peak heat flux was reduced by more than a factor of 10, with minor modification in the heat-flux footprint width. However, at the highest injection frequencies, changes of the pedestal parameters similar to gas puffing were observed, resulting in a 20% reduction of pedestal pressure. The feasibility of ELM pacing with non-fuel pellets was explored with the new Impurity Granule Injector, capable of injecting sub-millimeter granules of non-fuel materials (e.g. Li, C, B4C), at frequencies up to 200 Hz, with controllable speed (50-150 m/s) and selectable granule size. Experiments with Li granules 0.4-0.9 mm injected in plasmas at q95~4.6, ne,ped~4x10 m, βN=1.5 demonstrated full-shot ELM pacing, with a 3-5x increase of fELM over the natural ELM frequency, and a reduction of ELM peak heat flux ~1/fELM. However, in the lower q95~3.2, low-torque IBS, ELM pacing by Li granule injection did not translate into heat flux mitigation, with a fraction of paced ELMs showing peak heat flux similar to natural ELMs. The combined dataset provides a unique contribution to ELM control research, challenging the understanding of the relation between high frequency ELM pacing and heat flux mitigation.