RADIATIVE DIVERTOR EXPERIMENTS IN DIII-D WITH D-2 INJECTION

Deuterium gas injected into ELMing H mode divertor discharges in the D III-D tokamak typically reduced the total power at the divertor target similar to 2 times and the peak heat flux similar to 3 to 5 times wit h modest (<10%) degradation in plasma energy confinement. The paramete r range for the discharges investigated was: I-p= 1.0-2.0 MA, q(95) ap proximate to 2.9-6.0 and total input power less than or similar to 20 MW. Most of this reduction in heat flux occurred at the sudden formati on of a high density, highly radiating region located between the outb oard divertor separatrix strike point and the X point. This divertor b ehaviour is associated with a 'partially detached' divertor plasma con dition, which is referred to in this paper as the partially detached d ivertor (PDD) regime. With the onset of the PDD, typically at a line a veraged density of 0.6 to 0.7 times the Greenwald density limit, an ab rupt reduction in plasma electron pressure (greater than or similar to 4 times) was observed at the outboard divertor separatrix strike poin t; at the same time, however, only a modest (less than or similar to 3 0%) change in the electron pressure was observed upstream near the out board midplane separatrix. The data suggest that significant plasma mo mentum loss occurred between the high density, highly radiative region and the (downstream) divertor separatrix target. Plasma performance s howed little degradation with the onset of the PDD regime. Deuterium i njection made only modest changes in the temperature and density profi le shapes near the midplane separatrix of the main plasma. The PDD app roach is shown to be compatible with discharges operating at low safet y factor (i.e. q(95) congruent to 2.9) and to be effective in signific antly reducing toroidal asymmetry in observed divertor plasma properti es (e.g., heat flux). The potential for operating in a steady state ha s been demonstrated using feedback control of the neutral pressure out side the main plasma.