Gas puff fueled H-mode discharges with good energy confinement above the Greenwald density limit on DIII-D

Tokamak discharges with electron densities as high as 1.4 times the Greenwald density, and good energy confinement, H-ITER89P = 1.9, were obtained with D-2 gas puffing on DIII-D. The divertor configuration of these discharges, low triangularity with pumping of the private flux region, was important in avoiding a transition to the L-mode or Type III ELM regimes in which energy confinement was reduced. Although these discharges show a decrease in H-mode pedestal energy at high density through reduction in the edge pressure gradient, peaking of the density profile compensated for the effect of this decrease on the overall stored energy. Spontaneous density profile peaking occurred under conditions which enhance the neoclassical Ware pinch. The high density phase was terminated by an internal MHD event that has the characteristics of a neoclassical tearing mode. (C) 2001 Elsevier Science B.V. All rights reserved.