Investigation of Tungsten and Carbon Pellet Injection for Impurity Transport Studies on DIII-D

A newly developed spring-loaded DiMES pellet injector was used on the DIII-D tokamak to study impurity transport and particulate dynamics. The system injects pellets ranging from 0.5 to 20 mg at velocities of 3–10 m/s into the lower divertor plasma, with precise timing and comprehensive diagnostics. Tungsten-core pellets with carbon shells were specifically designed to introduce high-Z impurities into the plasma with controlled radial deposition, achieved by the shell protecting the W core until it ablates. SPRED extreme ultraviolet and complementary UV spectroscopy confirmed tungsten delivery to the plasma core during H-mode discharges. Carbon pellets—porous and glassy—enabled complementary studies of carbon ablation and transport in both L-mode and H-mode. High-speed visible and infrared imaging with intersecting views enabled the reconstruction of 3D trajectories of the ablating pellets, which were compared with modeling results from the DUSTT and UEDGE codes, showing good agreement. The results support the development of validated impurity transport models and highlight the injector's unique capability to study tungsten behavior under reactor-relevant conditions.