Modeling carbon pellet injection in DIII-D divertor for advanced material studies.

Injections of spherical carbon pellets into DIII-D L-mode plasma discharges were performed in order to investigate pellet dynamics and ablation in the plasma as part of the 2021 Frontiers in Science campaign. Reliable pellet ablation and transport models in high heat flux plasma environment are needed for aerospace and fusion applications. Several types of pellets, from 2 mm to 3.6 mm diameter porous carbon and 0.7 mm glassy carbon spheres, were injected using spring loaded injectors installed in DIII-D Divertor Material Evaluation System (DiMES). Pellet trajectories were tracked using fast imaging cameras in visible and IR light. The DIII-D spectroscopy diagnostics were used to monitor plasma carbon content in the core. The obtained experimental pellet trajectories and ablation rates were compared with simulations performed using DUSTT-UEDGE codes showing that the pellets experienced unexpected radial acceleration. Possible physical mechanism of the acceleration is discussed. The comparison allowed improving the models used in the codes for large pellets in order to reproduce the observed dynamics.