Wide-Density Pedestal Enables Simultaneous High Performance and Benign Edge in DIII-D Hybrid Scenarios

Recent hybrid scenario experiments with higher density (f_GW ~ 0.7) and squareness (ζ ~ -0.1) showed simultaneously high performance (β_N ~ 3.5, H_98y2 ~ 1.3) and the mitigation of type-I edge localized modes (ELMs). A wide density pedestal width, with the separatrix electron density above 3e19 m^-3, couple together the high-performance core with type-I ELM mitigation. BOUT++ simulations show that plasmas with a lower pedestal density gradient and higher scrape-off layer (SOL) density are unstable to a local resistive ballooning mode (RBM) near the plasma edge. The instability from this RBM leads to the high frequency (f_ELM ≥ 500 Hz) ELMs, with oscillations in the plasma stored energy of <1% with each ELM, observed in these plasmas. At higher squareness, the reduced density gradient and stronger shaping help to stabilize the unstable peeling-ballooning mode. Experimental scans of the gas fueling rate and plasma squareness show that a higher SOL density, while needed for the high-frequency ELMs, is not sufficient for the mitigation of type-I ELMs. At lower squareness, regular type-I ELMs still occur interspersed with high-frequency ELMs. These experimental results are consistent with squareness scans in BOUT++, showing improved stability of the peeling-ballooning mode at higher squareness. The wide pedestal density profile is also helpful for maintaining higher pedestal temperatures during partial divertor detachment. Using N₂ puffing in the divertor private flux region, a reduction in the divertor heat flux by over an order of magnitude was achieved with pedestal temperatures of T_e,ped ~ 1 keV. In the core region, the higher density (n_e ~ 7e19 m^-3) is helpful for improving stability to tearing modes and decreasing heat diffusivity at lower injected torque compared to past lower density (n_e ~ 4e19 m^-3) hybrid plasmas.