Divertor, scrape-off layer and pedestal particle dynamics in the ELM cycle on ASDEX Upgrade

Abstract

In addition to the relaxation of the pedestal, edge localised modes(ELMs)introduce changes tothe divertor and scrape-off layer(SOL)conditions. Their impact on the inter-ELM pedestalrecovery is investigated, with emphasis on the electron density(ne)evolution. The typical ELMcycle occurring in an exemplary ASDEX Upgrade discharge interval at moderate applied gaspuff and heating power is characterised, utilising several divertor, SOL and pedestal diagnostics.In the studied discharge interval the inner divertor target is detached before the ELM crash, whilethe outer target is attached. The particles and power expelled by the ELM crash lead to a re-attachment of the inner target plasma. After the ELM crash, the outer divertor target moves into ahigh recycling regime with largenein front of the plate, which is accompanied by high mainchamber neutralfluxes. On similar timescales, the inner target fully detaches and the highfieldside high density region(HFSHD)is formed reaching up to the highfield side midplane. Thisstate evolves again to the pre-ELM state, when the main chamber neutralfluxes are reduced laterin the ELM cycle. Neither the timescale of the appearance of the HFSHD nor the increase of themain chamber neutralfluxesfit the timescale of thenepedestal, which is faster. It is found thatduring thenepedestal recovery, the magnetic activity at the lowfield side midplane is stronglyreduced indicating a lower level offluctuations. A rough estimation of the particleflux across thepedestal suggests that the particleflux is reduced in this period. In conclusion, the evolution ofthenepedestal is determined by a combination of neutralfluxes, HFSHD and reduced particle flux across the pedestal. A reduced particleflux explains the fast, experimentally observed re-establishment of thenepedestal best, whereas neutrals and HFSHD impact on the evolution ofthe SOL and separatrix conditions.