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ELM triggering by local pellet perturbations in type-I ELMy H-mode plasma at JET

The pellet pacing ELM control concept developed at the mid-sized tokamak ASDEX Upgrade is considered for the larger machines JET and ITER as well. By driving up the ELM frequency, the ELM size can be reduced and eventually suppressed below a dangerous level. An according pellet injection system for JET is under construction; the ITER design includes one as well. However, it is still to be proven whether the concept will also work for bigger machine sizes. A step forward in this mission was achieved by re-analysing previous JET experiments dedicated to pellet particle fuelling. Although the experiments were performed in a parameter regime unsuitable for pellet ELM pacing both with respect to pellet frequency and size they demonstrate that prompt triggering of ELMs takes place at JET as well. Prompt triggering, an indispensable premise for useful pacing, obviously can be achieved already with a pellet size sufficiently small to avoid unbearable side-effects by particle fuelling. An ELM released by the local perturbation imposed by the ablating pellet in the plasma edge region is detected when only a minor fraction (less than 1%) of the fuelling size pellets (containing about 4 × 1021 D) was ablated and deposited. It is thus very likely that JET will allow for investigations on the operational technique and the underlying physics of pellet pacing once the new system becomes operational.