Global confinement behavior with small shape changes in DIII-D negative triangularity discharges

DIII-D negative triangularity plasmas have exhibited H-mode confinement with an ELM-free boundary and a tolerance for high normalized beta operation (N ~3). In an experiment on DIII-D with a lower-single-null negative triangularity shape, the upper triangularity was altered shot-to-shot with the same heating trajectory. As the upper triangularity was relaxed from upper = -0.40 to -0.20, the edge pedestal pressure increased by ~70% from 0.9 kPa to 1.6 kPa as the plasma transitioned from an L-mode to an ELM'ing H-mode-like edge. The stored energy and confinement, and hence the H-factor remained nearly the same with H98Y2 ~ 1. This constancy of global confinement with varying pedestal height is counter to typical H-mode behavior. A study of kinetic profiles shows the lower stored energy of the pedestal is compensated with increased stored energy in the core as the triangularity becomes more negative. There is a small contribution due to increased plasma volume. The fast ion energy content remains constant at about 25%. The ability to vary the edge pedestal and confinement mode with a small change in shape yet retain good energy confinement would be an attractive feature in a reactor.