Neutral and plasma dynamics in the high field side SOL with the outer divertor detachment in DIII-D and ASDEX Upgrade

Experimental measurements in the DIII-D tokamak identify a region of high neutral hydrogenic emission region close to the separatrix in the high field side (HFS) scrape off-layer (SOL) with multiple diagnostics measuring hydrogen lines (Lyman and Balmer alpha), pointing to a strong ionization of neutrals in the SOL before reaching the separatrix. This emission front is enhanced with the detachment of the inner leg but reduced with detachment onset at the outer leg in H-mode, the same trend observed using electron density measurements at the inner midplane (IMP) in ASDEX-Upgrade (AUG) tokamak L-modes, where the density in the SOL is much larger than the measurements in the edge at the outer midplane (OMP)[1]. These similarities indicate that DIII-D, despite the differences with AUG in divertor configuration and wall material, presents a similar feature: a very high electron density in the HFS SOL close to the separatrix, responsible for this emission front. These observations in DIII-D and AUG point to a common feature in tokamaks, in a region poorly diagnosed in most tokamaks and crucial to understand the edge fueling from divertor recycling.



Work supported by US DOE under DE-FC02-04ER54698, DE-SC0014264, DE-AC02-09CH11466, DE-AC05-00OR22725, DE-AC52-07NA27344, DE-NA0003525.