Separating divertor closure effects in the SOL and pedestal*

Comparison between an open divertor and a more-closed divertor in DIII-D demonstrates detachment at a 20–30% lower pedestal density (n_e,ped) in the closed divertor, due to a combination of decreased fueling of the pedestal and increased dissipation in the scrape off layer (SOL) in the closed divertor, both resulting from increased neutral trapping in the divertor. Predicting whether the relationship between divertor closure and detachment will hold for an opaque SOL, in which the contribution of ionizing neutrals to fueling the pedestal is lessened, requires separating out the different mechanisms contributing to the density difference at detachment. A series of experiments on DIII-D characterizes matched discharges using various divertor configurations to isolate the effects of divertor closure. We identify detachment onset as the rollover in peak ion saturation current as a function of n_e,ped, and support interpretation with DIII-D’s diagnostic suite. We use high resolution edge Thomson scattering measurements and power balance to determine density profiles and to find separatrix location and density, n_e,sep. These experiments show detachment at 10–20% lower n_e,sep in the closed divertor than the open, supported by SOLPS simulations showing increased neutral trapping, and hence increased dissipation, in the closed divertor. We also find a difference in n_e,ped/n_e,sep: for matched n_e,sep, the closed divertor has 10–20% lower n_e,ped, consistent with modeling showing a smaller ionization fraction inside the separatrix in this case. Changing the ionization profile by changing closure also allows us to investigate the relative contributions of ionization and radial transport to the density pedestal profile. Understanding how these pieces fit together will help us develop predictive models of pedestal density and detached divertors compatible with a high performance core.