Simulations of multiple shell-pellet injection into DIII-D

Dispersive shell pellet (DSP) injection, comprising a hollow shell filled with a dispersive payload, remains a viable alternative for disruption mitigation on ITER. DSP differs from SPI in its ability to cool the plasma from the inside out while maintaining edge flux-surfaces—potentially increasing the radiation fraction even with a lower-Z payload, which could reduce the electric field and current quench (CQ) rate. MHD modeling of single DSP injection into DIII-D was validated against experiments, reproducing several observed trends. But ablation calculations for DIII-D and ITER plasmas find that penetration of a DSP into the core of ITER will be challenging. New simulations model two DSPs fired simultaneously into DIII-D. A symmetric case is compared to cases with a difference of 5-10% in velocity. In the symmetric case, both payloads are released near mid-radius and an inside-out thermal quench (TQ) dominated by n=2 MHD instabilities ensues. When the speeds differ, the release of the first payload temporarily halts the ablation of the second shell through the first payload delivery region, resulting in release of the second payload deep in the core. The second pellet is therefore able to "piggyback" on the first—a potential strategy for DSP penetration into the core of ITER.