Simplified eight-shot pneumatic pellet injector for plasma fueling applications on the Princeton Beta Experiment and on the Advanced Toroidal Facility

Plasma fueling via injection of solid hydrogenic pellets has expanded the operating range for tokamaks and stellarators to higher densities than attainable with gas puffing. Pellet injection has also resulted in improved plasma energy confinement in tokamak discharges for which the pellet or pellets penetrate deep into the plasma core. The eight-shot pneumatic pellet injector described herein has been developed for use on the Princeton Beta Experiment and on the Advanced Toroidal Facility for routine plasma fueling and for confinement optimization studies. The injector is based upon the so-called ‘‘pipe-gun’’ concept, which generates deuterium and hydrogen pellets by direct condensation in the gun barrel tubes, segments of which are cooled below the hydrogen triple-point temperature by contact with a liquid-helium-cooled block. Control of the pellet length is achieved both by regulating the deuterium fill pressure and by establishing temperature gradients along the barrel tubes. This injector features eight independent gun barrel assemblies mounted around the perimeter of a single cold block, each coupled to an ORNL-designed fast propellant valve. Thus, the injector is capable of injecting arbitrarily programmable sequences of up to eight pellets of sizes ranging from 1 to 3 mm at speeds up to 1500 m/s.