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Abstract:
Recent ASDEX Upgrade experiments integrated benign type II ELMs (tolerable peak heat loads on target plates) with high performance. In both conventional and advanced H-modes, the operation window with type II ELMs was extended towards q95 > 3.5 in close to double null configurations at sufficient high edge pedestal density above 50% of the Greenwald density. Type I ELMs are suppressed at almost constant pedestal parameters presumably due to a change in edge stability provided by higher edge magnetic shear, and at the same confinement level. Since conventional reactor designs are optimised at q95 around 3 operation with type II ELMs has to compensate the required higher q-value by advanced performance. This was achieved in advanced H-mode scenarios integrating high βN > 3.5, improved confinement via density peaking ( H98 - P = 1.3), and densities of 90% of Greenwald density with type II ELMs. Another way to mitigate ELMs is active type I ELM control by means of hydrogen or impurity injection. Using small hydrogen pellets we demonstrated an enhancement of the ELM frequency to almost the pellet rate of 20 Hz and a considerable reduction of the energy loss/ELM by more than a factor of 3.
