Development of advanced pellet injector systems for plasma fueling

Two types of solid hydrogen pellet injection systems have been developed, and plasma refueling experiments have been performed using these pellet injectors. One is an in-situ pipe-gun type pellet injector, which has the simplest design of all pellet injectors. This in-situ pipe-gun injector has 10 injection barrels, each of which can independently inject cylindrical solid hydrogen pellets (3.4 and 3.8 mm in diameter and length, respectively) at velocities up to 1,200 m/s. The other is a repetitive pellet injector with a screw extruder, which can form a 3.0 mmφ solid hydrogen rod continuously at extrusion rates up to 55 mm/s. This extruder allows consecutive pellet injection up to 11 Hz without time limit. Both of these pellet injectors employ compact cryo-coolers to solidify hydrogen; therefore, they can be operated using only electrical input instead of a complicated liquid helium supply system. In particular, using a combination of the repetitive pellet injector with cryo-coolers provides a steady-state capability with minimum maintenance. (author)