Abstract
In addition to the relaxation of the pedestal, edge localised modes (ELMs) introduce changes to the divertor and scrape-off layer (SOL) conditions. Their impact on the inter-ELM pedestal recovery is investigated, with emphasis on the electron density (ne) evolution. The typical ELM cycle occurring in an exemplary ASDEX Upgrade discharge interval at moderate applied gas puff 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, while the 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 a high recycling regime with large ne in front of the plate, which is accompanied by high main chamber neutral fluxes. On similar timescales, the inner target fully detaches and the high field side high density region (HFSHD) is formed reaching up to the high field side midplane. This state evolves again to the pre-ELM state, when the main chamber neutral fluxes are reduced later in the ELM cycle. Neither the timescale of the appearance of the HFSHD nor the increase of the main chamber neutral fluxes fit the timescale of the ne pedestal, which is faster. It is found that during the ne pedestal recovery, the magnetic activity at the low field side midplane is strongly reduced indicating a lower level of fluctuations. A rough estimation of the particle flux across the pedestal suggests that the particle flux is reduced in this period. In conclusion, the evolution of the ne pedestal is determined by a combination of neutral fluxes, HFSHD and reduced particle flux across the pedestal. A reduced particle flux explains the fast, experimentally observed re-establishment of the ne pedestal best, whereas neutrals and HFSHD impact on the evolution of the SOL and separatrix conditions.
Highlights
At the edge of magnetically confined plasmas in the high confinement mode (H-mode) regime [1], which is the intended operational regime of future fusion devices like ITER [2], an edge transport barrier (ETB) occurs
Previous work at ASDEX Upgrade has identified the evolution of the outer divertor into a high recycling regime after an edge localised modes (ELMs) crash [10, 11], that is connected to large plasma densities in front of the target and high Da line radiation (Da)
A large asymmetry is found in the post-ELM phase in figure 7(b), when the inner divertor target is fully detached and the high field side high density region (HFSHD) is present, which in this case reaches up to the midplane causing the strong difference of the high field side (HFS) and low field side (LFS) scrape-off layer (SOL) ne profiles
Summary
At the edge of magnetically confined plasmas in the high confinement mode (H-mode) regime [1], which is the intended operational regime of future fusion devices like ITER [2], an edge transport barrier (ETB) occurs This ETB is connected to steep gradients in the density and temperature profiles, known as pedestal, which are limited by MHD instabilities called edge localised modes (ELMs) [3, 4]. Previous work at ASDEX Upgrade has identified the evolution of the outer divertor into a high recycling regime after an ELM crash [10, 11], that is connected to large plasma densities in front of the target and high Da line radiation (Da). This paper characterises and connects the evolution of the divertor, SOL and pedestal for a typical ELM cycle at ASDEX Upgrade and emphasises their impact on the initial ne pedestal recovery phase. This rough approximation suggests that the particle flux across the edge is reduced during the recovery of the electron density gradient ( ne) and increases afterwards (section 5)
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