The effect of plasma shape on H-mode pedestal characteristics on DIII-D

The characteristics of the H-mode are studied in discharges with varying triangularity and squareness. The pressure at the top of the H-mode pedestal increases strongly with triangularity, primarily due to an increase in the margin by which the edge pressure gradient exceeds the ideal ballooning mode first stability limit. Two models are considered for how the edge may exceed the ballooning mode limit. In one model, access to the ballooning mode second stable regime allows the edge pressure gradient and associated bootstrap current to continue to increase until an edge localized, low toroidal mode number, ideal kink mode is destabilized. In the second model, the finite width of the H-mode transport barrier and diamagnetic effects raise the pressure gradient limit above the ballooning mode limit. We observe a weak inverse dependence of the width of the H-mode transport barrier, △, on triangularity relative to the previously obtained scaling △ ∝ (β_P^PE)^1/2. The energy loss for Type I ELMs increases with triangularity in proportion to the pedestal energy increase. At low density, the energy confinement of high-triangularity discharges is larger than discharges with low triangularity, as a result of an increase in the energy in the H-mode pedestal. At high density, both the change in pedestal pressure and the response of the density profile are found to play a role in setting the energy confinement. The highest energy confinement at high density was obtained in low-triangularity discharges.