EX / P 3-27 1 Edge-and Divertor Plasma Behavior in High Power High Performance Double-null Plasmas on DIII -

We identify significant opportunities and challenges to reducing divertor heat flux in high power, high performance near-double null divertor (DND) plasmas on DIII-D, while still maintaining a sufficiently low enough density that allows for the application of ECH heating. For these DNDs, the scaling of the peak heat flux at the outer target (q⊥) ∝ [PSOL x IP] 0.92 for PSOL = 8 -19 MW and IP = 1.0–1.4 MA, and is consistent with standard ITPA scaling for single-null H-mode plasmas. Two divertor heat flux reduction methods were studied. First, the puff-and-pump radiating divertor at lower power input (≤11 MW) was effective in reducing divertor heat flux with modest degradation in core confinement. For these plasmas, argon is more effective than nitrogen as the seed impurity, because the former leads to less fuel dilution for similar divertor heat flux reduction. However, puff-and-pump was less effective in reducing q⊥ at high power (≥ 14 MW) and H98 (≥ 1.5) due to an improvement in confinement time when deuterium gas puff was applied. Our analysis to-date indicates that this improved energy confinement arises from a complex interplay between pedestal density and temperature profiles and particle transport, resulting in improved edge stability. Second, q⊥ for these high performance DNDs could be lowered by ≈35% when an open divertor was closed on the common flux side of the outer divertor target (“semi-slot”). However, plasma interaction with graphite tiles near the slot opening was a significant source for impurity contamination of the main plasma.