First Experiments with Cryogenic Shell Pellets for Disruption Mitigation

Experiments on DIII-D utilized a novel disruption mitigation scheme using cryogenic shell pellets (CSPs), which consist of solid neon encapsulated by a solid deuterium shell. These pellets are designed to deliver radiative impurities to the plasma core, where most of the energy resides, in order to maximize assimilation, induce a slow thermal quench, and eliminate existing runaway electron seeds through collisions and stochastization of magnetic flux surfaces. Initial results injecting a 3 mm CSP containing 9 Torr-L of neon and 12 Torr-L of deuterium into an H-mode plasma show assimilation fractions exceeding 60%, compared to < 20% typically observed with shattered pellet injection (SPI). The cooling duration (i.e. the time between the pellet’s arrival and the end of the thermal quench) is similar to that of a 7 mm SPI with 45% neon content (144 Torr-L Ne, 176.5 Torr-L D₂). However, current quench durations, which are a proxy for the amount of radiated energy, are longer for CSP than for the 45% neon SPI, suggesting that less energy is radiated during the mitigation process. Further analysis of the existing results is underway, along with additional tests using CSPs with lower intrinsic neon content to reduce edge radiation.