A three-barrel repeating pneumatic pellet injector for plasma fueling of the Joint European Torus

A three-barrel repeating pneumatic pellet injector has been developed for plasma fueling of the Joint European Torus (JET) tokamak. The versatile device consists of three independent machine-gun-like mechanisms that operate at cryogenic temperatures (10–20 K). Individual extruders provide a continuous supply of solid hydrogen isotope to each gun assembly, where a reciprocating breech-side cutter forms and chambers cylindrical pellets from the extrusion; the deuterium pellets are then accelerated in the gun barrels with compressed hydrogen gas (pressures up to 105 bar) to velocities <1.5 km/s. The injector features three nominal pellet sizes (2.7,4.0, and 6.0 mm) and repetitive operation (5, 2.5, and 1 Hz, respectively) for quasi-steady-state conditions (> 10 s). The design allows the gun barrels to be aligned mechanically for accurate aiming. A remote stand-alone control and data acquisition system is used for injector operation. The injector system has been installed on JET. The design features, operation, and performance characteristics of the injector are described.