Investigation of 3D effects on heat fluxes in performance-optimized island divertor configurations at Wendelstein 7-X

The 3D effects on divertor heat loads have been investigated for performance-optimized island divertor configurations at Wendelstein 7-X with 3D modeling and IR camera measurements. A new high mirror configuration optimized for more stable island divertor operation due to reduced bootstrap currents and a more even heat load distribution between the main divertor targets has been investigated for the first time numerically and experimentally. Transport calculations with EMC3-EIRENE show a strong dependence of the heat flux distributions on the configurations and the details of the 3D island geometry. IR camera measurements confirm the predictions concerning the global heat load distributions for the standard configuration but show more even distributions between the main wetted divertor plates in the high mirror configurations. The local heat load profiles show offsets in their position of up to 5 cm to each other and averaged peak heat fluxes varying between 0.5 MWm−2 and 2.2 MWm−2 depending on the divertor module considered. These heat flux asymmetries complicate the local matching of profiles between experiment and 3D modeling. The 3D equilibrium of a high mirror high-performance scenario predicted by the HINT code has been investigated with EMC3-EIRENE for the first time to anticipate plasma response in higher performance scenarios. The island divertor is preserved for β=3% despite enhanced stochasticity. However, the islands are increased in size while the path lengths for parallel transport are reduced causing a substantial change in the divertor heat flux patterns.