A fuel pellet injector for the microwave tokamak experiment

Unlike other fueling systems for magnetically confined fusion plasmas, a pellet injector can efficiently deliver fuel gas to the plasma core, enhancing plasma confinement. We installed a new pellet injector on the microwave tokamak experiment (MTX, formerly Alcator‐C) to provide a plasma with a high core density for experiments both with and without ultrahigh‐power microwave heating. Its four‐barrel pellet generator is the first to be designed and built at Lawrence Livermore National Laboratory. Based on ‘‘pipe‐gun’’ technology developed at Oak Ridge National Laboratory, it incorporates our structural and thermal engineering innovations and a unique control system. The pellet guide section, differential vacuum‐pumping stages, and fast‐opening propellant valves are reused parts of the Impurity Study Experiment (ISX) pellet injector built by ORNL. We tailored designs of all other systems and components to the MTX. Our injector launches pellets of frozen hydrogen or deuterium into the MTX, either singly or in timed bursts of up to four pellets at velocities of up to 1 000 m/s. Pellet diameters at formation range from 1.02 to 2.08 mm. A diagnostic stage measures pellet velocities and allows us to photograph the pellets’ flight. We are striving to improve the injector’s performance, but its operation is already very consistent and reliable.