First campaign with alternative divertor configurations in ASDEX Upgrade

Abstract After a major hardware extension ASDEX Upgrade has now established and characterized a variety of alternative divertor configurations (ADCs) with heating powers reaching up to 20 MW and plasma currents up to 1 MA. These high performance conditions in configurations with arbitrarily small field line incidence angles were made possible due to the high precision in the tile alignment of 300 μ m as well as a careful optimization of the error fields caused by the current feeds. The formation of an X-point radiator (XPR) was observed in a low-field-side snowflake minus (LFS SF − ) configuration at a very low impurity concentration determined by the intrinsic sources only, i.e. without additional seeding of impurities. Compared to a typical lower single-null (SN) configuration the edge localized modes (ELMs) were found to be substantially smaller in size and higher in frequency and therefore barely detectable. While the primary strike line is fully detached the secondary one in the far-SOL shows heat fluxes of a similar magnitude as the primary strike line in a previous upper SN phase. These fluxes might be reduced by installing configuration-optimized baffles and/or by increasing the plasma current and thereby reducing λ q . In fact significantly smaller far-SOL heat fluxes were found when increasing the plasma current from 800 to 1000 kA. According to the divertor Langmuir probes the peak heat flux is then by a factor of two smaller than the ones in the SN reference.