ELM Pacing Investigations at JET with the New Pellet Launcher
A new pellet injection system was installed at JET designed for both fuelling and ELM pacing. The purpose of the pacing section was to validate pellet ELM pacing as a suitable tool for ELM mitigation in ITER. Pellet pacing was confirmed at the large size scale of JET, and an enhancement of the intrinsic ELM frequency up to a factor of 4 was achieved. Investigations of the dynamics of triggered ELMs were performed with respect to their spontaneous counterparts. Triggered ELMs essentially show features also typical for spontaneous ELMs in several operational regimes, a strong hint for compatibility with other plasma control tools. Observations and modelling results indicate the ELM triggering occurs by the pellet ablation plasmoid evolving into the first ELM filament followed by a poloidal spread of the instability. An ELM obviously can be forced by a pellet due to the strong local perturbation imposed already under unusual onset conditions but then evolves like any ELM typical for this plasma regime. For tool optimization the pellet mass and hence the convective confinement losses imposed have to be minimized. In our experiments, a lower mass threshold was observed for the first time. It has been found that to reliably trigger an ELM the pellet needs to be sufficiently large (and fast) to penetrate to the top of the pedestal. Recent investigations are clear steps forward to validate the pellet pacing approach for ITER. Pellet pacing is a robust and reliable tool deployed in many tokamaks, however a full demonstration in an ITER relevant parameter regime has not been achieved yet.