A technique for producing large dual-layer pellets in support of disruption mitigation experiments

A special single-shot pellet injection system that produces and accelerates large cryogenic pellets (∼16mm diameter and composed of D₂ or Ne) to relatively high speeds (>300 and 600 m/s, respectively) was previously developed at the Oak Ridge National Laboratory. Subsequently, a similar system was installed on DIII-D and used successfully in disruption mitigation experiments. To circumvent some operational issues with injecting the large Ne pellets, a technique has been developed in which a relatively thin layer (0.1 to 1.0 mm) of D₂ is frozen on the inner wall of the pipe-gun barrel, followed by filling the core with solid Ne. A fast solenoid valve operating with a light gas (H₂ or He) at relatively high pressure (∼70 bar) provides the force necessary to break away the dual-layer pellet and accelerate it. The technique and the initial laboratory tests are described, as well as the implementation and operational issues for fusion experiments.