Density limit studies on DIII-D

We have studied the processes limiting plasma density and successfully achi eved discharges with density similar to 50% above the empirical Greenwald s caring with H-mode energy confinement. This was accomplished by density pro file control, enabled through pellet injection and divertor pumping. Severa l density-limiting mechanisms, including MARFEs and pellet loss processes, were successfully avoided. Finally we have looked in detail at the first an d most common density limit process in DIII-D, total divertor detachment, a nd found that the local upstream separatrix density (n(e)(sep.det)) at deta chment onset (partial detachment) increases with the scrape-off layer heati ng power, P-heat, i.e. n(e)(sep.det) similar to P-heat(0.8). This is in mar ked 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 incre ase as P-heat(0.7). (C) 1999 Elsevier Science B.V. All rights reserved.