Development of hydrogen pellet acceleration system using a two-stage light gas gun

Hydrogen pellets without sabots were accelerated to high speeds using a two-stage light gas gun for Heliotron-E. The primary application of this technology is plasma fueling of fusion devices. Conventional pellet injectors have limited pellet speeds in the range of 1-2 km/s. Higher velocities are desirable for more flexible density profile control and for deep penetration into a high-temperature plasma. By developing a new fast valve with high conductance to accelerate the piston and other components, a 2-mm-diam hydrogen pellet with a velocity of 3.2 km/s has been successfully accelerated without a sabot. The pipe gun technique for freezing hydrogen operation is simulated with the code MYKE. Development of new pistons instead of the plastic piston for the repetitive two-stage pellet injector is being carried out using metal and ceramic, as the surface of the plastic piston tends to wear out through repetitive operation and carbon powder produced from the plastic piston may cause trouble with the fusion device