Description
A new divertor geometry enabling high shaping on DIII-D has been leveraged to achieve the highest pressure pedestals observed to date on the device. Peeling-ballooning model calculations suggested increased plasma triangularity and volume would allow the pedestal stability boundary to develop a broad channel of operation at high density and pressure simultaneously, leading to a state of a low collisionality pedestal top at high density. This state is rarely achieved in present devices and represents a path for significant gains in future devices, highlighting it as a key challenge for core-edge integration physics.
ELMy H-mode, QH-mode, and RMP ELM suppressed regimes were developed to access high pedestal pressure through the Super H-mode channel, with each scenario exploration leading to novel pedestal physics insights. ELMy H-modes and co-Ip QH-modes resulted in the highest pedestal pressure and core performance observed to date in DIII-D, yielding Pped of 37-40kPa, roughly 2-4 times typical ELMy H-mode values, and Ptau/IaB of 11-14, about twice typical values.
Counter-Ip QH-mode discharges reached record QH densities of ~1.4x1020m-3. QH-mode persisted as the density was raised, shifting from the peeling to ballooning stability boundary. This phenomenon is likely due to significantly decoupling of low-n peeling and intermediate/high-n ballooning instabilities as a result of the strong shaping.
RMP experiments established full ELM suppression in the SVR configuration at moderate plasma current (1.2–1.6 MA) with odd-parity n=3 fields. These discharges sustained high performance (βN up to 2.8, H98 up to 2, and E up to 0.4 s) with total pedestal pressures up to 26 kPa and extended suppression windows (q95 ≈ 4.2–5.3). Optimization of dRsep and coil configuration enabled stationary ELM-free operation close to double-null shapes, highly relevant for future pilot plant designs.
The pedestal width in these experiments for all three regimes did not conform to the EPED1 width scaling typical of H-modes in DIII-D and other devices and instead were found to be consistent with n=60 ballooning mode critical gradient as a proxy for the KBM.
This work was supported in part by the US Department of Energy under DE-FC02-04ER54698, DE-SC0014264, DE-AC02-09CH11466, DE-AC05-00OR22725, DE-SC0019302, DE-AC52-07NA27344, DE-SC0022270, and DE-FG02-04ER54761.