Investigating Pellet Physics for ELM Pacing and Particle Fuelling in ITER

To get deeper insight into the MHD activity triggered by pellets, a study was carried out, including a wide range of plasma scenarios with and without ELMs based on data from AUG and JET. All presently available experimental evidences are compatible with the hypothesis that the helical perturbation caused by the pellet cloud creates a non-linear trigger at any time during an ELM cycle where the plasma is most of the time in a linearly stable but nonlinearly unstable state. This suggests that pacing can be established beyond the range of present proof of- principle demonstrations under conditions required for ITER, i.e. at a range of ELM rate enhancement that suppresses the ELM impact below the hazard level. It seems that there is also quite some headroom to reduce the associated fuelling contribution coming with pacing. The demonstration of these features requires a dedicated approach at a sufficiently large tokamak that is currently under way at JET. A new pellet launcher was built and integrated into the JET system that is being converted into a multi-injector multi-track system. Being the first of its kind it also becomes the first truly ITER like pellet injection system. Once put fully into operation, it will allow for pacing as well as fuelling investigations.
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