Studies of D₂ pellet ELM triggering in ITER-relevant conditions with modeling and experiments in DIII-D

Pacing ELMs by injecting pellets in order to reduce peak ELM heat fluxes in ITER is an attractive alternative to RMP ELM suppression in the case that the latter is determined to be unworkable or ineffective. In DIII-D, experiments have been performed using tangentially injected pellets off-midplane to mimic the shallow penetration of pellets in the hot, dense ITER pedestal. Pellets injected with the tangential trajectory did not successfully trigger ELMs, but neither did perpendicular midplane injected pellets in similar plasmas, suggesting that the trajectory was not solely responsible. In separate experiments in low collisionality DIII-D plasmas, pellets injected at the midplane were also unable to trigger ELMs, where this same size pellet had successfully demonstrated ELM triggering in previous discharges with higher collisionality. Initial indications are that pellet particle deposition profiles were not adequate to trigger ELMs in the experimental plasma conditions, and this idea is being tested with modeling of pellet ablation.