Sustained High Beta Operation With Internal Transport Barriers on DIII-D

Sustained high beta, high confinement operation with $\beta_N\sim\,$4, $\beta_T\sim\,$3-8\% and $H_{89}\sim\,$2.5 for $\sim$2~s has been achieved in discharges with internal transport barriers (ITBs) and negative central magnetic shear (NCS) on DIII-D. The minimum safety factor was maintained at $\sim$2, transiently leading to high bootstrap current fraction operation, $f_{BS}\sim\,$60\%. This combination of high beta and high confinement operation with moderate safety factor, $q_{95}\sim\,$5.5-3.5, also leads to high normalized fusion performance, with fusion gain factor spanning the anticipated ITER performance range. Transport barriers are observed in the ion temperature and rotation profiles, but not in the electron temperature profile, which is broad. These results address a critical issue for ITB operation in AT plasmas, obtaining sustained ITB profiles compatible with high beta limits. Previously, typical ITB operation has been limited to $\beta_N <\,$3 by pressure peaking. Pressure peaking in these discharges remains low due to a broad electron temperature profile.