Comparison of initial poloidal impurity distributions for MGI, SPI, and dual SPI plasma shutdowns.

A comparison of the pre-thermal quench radiation profiles due to high-Z impurity injection by massive gas injection (MGI), shattered pellet injection (SPI), and dual SPI from different toroidal locations into the same plasma is presented. Two different injection locations on the low-field side either above or below the midplane for MGI mitigated plasmas confirm previously found results that injection above the midplane flows inboard towards the high-field side prior to the thermal quench while MGI injection below the midplane remains trapped and even flows in the opposite direction towards the X-point.[1] Two SPI systems on DIII-D allow for comparison of impurity flows along field lines pitched towards the high or low-field side. These comparisons show that unlike MGI, the impurities are deposited inside the pedestal and are more constrained to the field lines resulting in less poloidal spreading. Tomography reconstructions of radiated power measurements for dual SPIs fired at different toroidal locations show cooling of multiple flux tubes which could explain the observed difference in mitigation metrics compared to single SPI experiments.

[1] Eidietis, N. W, et al., PoP 24, 102504 (2017)