Low-Z Shell Pellet Experiments on DIII-D

Small (o.d. = 1.8 mm, t = 0.37 mm) polystyrene shells filled with either pressurized argon gas or boron powder have been fired into DIII-D plasmas for disruption mitigation experiments. The pellet shells were observed to burn up at ρ≅0.5, roughly consistent with ablation rate calculations. Pellet slowing from 350 m/s down to 100 m/s was observed, which is not well-understood at present. Negligible plasma current contraction or MHD onset were seen as a result of the shell burn-up in the plasma edge, consistent with calculations. The pellet payloads were observed to ionize rapidly in the pellet vicinity (<1 cm radius) and rapid (<15 ms) mixing through the plasma core was observed.