Characterizing Up-Down Asymmetries in Main Chamber Fueling in DIII-D Plasmas

Previous studies on DIII-D revealed large in-out asymmetries in edge particle fueling. Lyman-α (Ly-α) emissivity measurements from the LLAMA diagnostic showed that, with favorable B×∇B drift direction, the ionization rate in the lower main chamber is typically an order of magnitude larger at the high-field-side (HFS) compared to the low-field-side (LFS), while fueling is more balanced with unfavorable drift direction. This work extends that understanding using the newly commissioned ALPACA diagnostic [1], which provides Ly-α emissivity profiles in the upper chamber. Initial results from a density scan in lower-single-null H-modes indicate that fueling from the plasma crown can contribute significantly: at high density (n_e,PED = 8×10¹⁹m^-3), the peak Ly-α brightness at the LFS is comparable between the upper and lower chambers, though still ~5x lower than at the HFS. At low density (n_e,PED = 5×10¹⁹m^-3), the LFS fueling remains up-down symmetric, but the HFS fueling becomes dominant, with the Ly-α brightness being ~25x higher. This contribution will also examine how the up-down asymmetry in main-chamber fueling changes as the plasma shape approaches double null shape. Since fueling in current tokamaks is dominated by edge neutrals, understanding of poloidal asymmetries in neutral density is critical for predicting pedestal performance.

[1] Horvath et al., RSI 95, 083523 (2024)

Work supported by US DOE under DE-FC02-04ER54698, DE-AC02-09CH11466, DE-SC0014264, DE-AC52-07NA27344 and DE-SC0019302.