Simulations of molecular and impurity transport effects on divertor detachment*

Divertor detachment is an important process in high power fusion reactors by which the power loads onto material surfaces may be reduced. Achieving this state typically requires a combination of power and momentum loss processes, from a complex combination of impurity species radiation (energy loss), and hydrogenic reactions (energy and momentum loss). In this work the SD1D and Hermes models built on BOUT++ have been extended to simulate multiple ion species, and applied to study both the role of molecular species in detachment, and the transport of impurities. We find that molecular processes contribute significantly to a strong rise of photon emission intensity (e.g. H-alpha) around detachment, in qualitative agreement with experimental results on TCV [Verhaegh et al 2020]. The different molecular reaction channels are decomposed, finding that for MAST-U like conditions the negative ion contribution only becomes significant in deep detachment. Multiple charge states of injected neon impurity are then studied, and used to test the impact of the boundary condition [Tskhakaya & Kuhn 2005] and parallel transport processes on the divertor solution. By evolving a separate ion temperature for each charge state, these simulations enable the validity of the common ion temperature approximation to be investigated. The aim of this work is to motivate future efforts to validate impurity transport models and atomic data against experiment.