Density limit studies on DIII-D

We have studied the processes limiting plasma density and successfully achieved discharges with density approximately 50% above the empirical Greenwald scaling with H-mode energy confinement. This was accomplished by density profile control enabled through pellet injection and divertor pumping. Several density-limiting mechanisms, including MARFEs and pellet loss processes, were successfully avoided. Finally we have looked in detail at the first and most common density limit process in DIII-D, total divertor detachment, and found that the local upstream separatrix density (n_e^sep,det) at detachment onset (partial detachment) increases with the scrape-off layer heating power, P_heat, i.e. n_e^sep,det approximately P_heat^0.8. This is in marked contrast to the line-average density at detachment which is insensitive to the heating power. Our data are in reasonable agreement with the Borass model, which predicted that the upstream density at detachment would increase as P_heat^0.7.