Effects of impurity seeding in DIII-D radiating mantle discharges

Impurity injection with noble gases has been used in DIII-D to increase radiation in the mantle region, with confinement enhancements above the ITERL-89P L mode scaling relation in both diverted and limited discharges. For discharges with an L mode edge, impurity injection produces a prompt increase in confinement and a more gradual increase in density. These changes occur at densities and radiated power fractions significantly lower than those observed in the TEXTOR tokamak device. ELMing H mode discharges with active pumping and high deuterium gas feed (puff and pump) exhibit an increase in density with no degradation in energy confinement after impurity injection, increasing to nearly the Greenwald density limit following a spontaneous transition several hundred milliseconds after impurity injection. The highest density phase of both L mode and ELMing H mode radiating mantle discharges is usually terminated after the onset of n=2 MHD activity, identified as an m/n=3/2 neoclassical tearing mode. A reduction in density fluctuations after impurity injection in the mantle region has been measured using beam emission spectroscopy in L mode discharges and is coincident with reductions in thermal diffusivity and increases in core toroidal rotation. The similarities and differences between these types of impurity seeded discharge will be presented. (author)