Spectroscopic measurements and modelling of plasma flows in the HFS SOL of ASDEX Upgrade

Knowledge on scrape-off layer flows on the high-field side is essential in understanding migration of eroded impurities  from the main chamber wall to the inner divertor. In this work, HFS SOL flows have been measured spectroscopically in ASDEX Upgrade, and the physical phenomena behind the observations have been studied by SOLPS 5.0 modelling.
Indirect flow measurements were performed during L-mode discharges with the inner divertor ranging from high-recycling to detached conditions by Doppler spectroscopy of injected N+ ions. The results indicated reversal of the toroidal near-SOL flow in high-recycling conditions, while the reversal disappeared after detachment of the inner divertor. The poloidal flow component was always directed away from the inner divertor close to the separatrix, suggesting a dominant contribution of the Er×B drift.
The behaviour of the D+ flow in the HFS SOL was studied by a density and power scan in SOLPS 5.0. In the near SOL, the results of the D+ flows were consistent with the experimental observations of the N+ flows, showing reversal in high-recycling conditions. The near SOL flow showed strong correlation with the poloidal pressure asymmetries between the HFS and LFS, driven mainly by the ÑB×B drift above the X point on the HFS and emphasized by localized ionization in the near SOL in high-recycling conditions. In the far SOL, the flow direction was strongly affected by radial transport of the plasma flowing on the inner flux surfaces.