Pellet injector systems for plasma refueling on LHD

Hydrogen pellet injection is one of the leading candidates for a refueling to magnetic confined fusion plasmas, since it can provide particles directly to core plasma. In order to investigate appropriateness of the pellet injection as a plasma refueling method, two types of solid hydrogen pellet injector systems have been developed and plasma experiments with pellet injection have been carried out on LHD (Large Helical Device). One is an in-situ pipe gun type pellet injector, which is the simplest of all pellet injector concepts. The in-situ pipe gun injector has 10 independent controlled barrels and each barrel can inject 3 mmφ x 3 mml pellets with a velocity of ∼1200 m/s. The other is a repetitive pellet injector with a screw extruder. The screw extruder can form 2.5 mmφ solid hydrogen rod continuously at a extruding rates of ∼35 mm/s and, therefore, the repetitive pellet injector can inject pellets in infinitum with repetitive rate of 10 Hz. Common feature of these injectors is employing a compact cryocooler to solidify hydrogen and, therefore, these injectors can be operated with just electricity instead of liquid helium supply system. (author)