Development of a Two-Stage Pneumatic Repeating Pellet Injector for the Refueling of Long-Pulse Magnetic Confinement Fusion Devices

Abstract

Next-step fusion devices, like the International Thermonuclear Experimental Reactor (ITER), and future fusion power plants will require a flexible plasma fueling system, including both gas puffing and high- and low-speed pellet injection. To sustain core plasma density, relatively large pellets penetrating beyond the separatrix will have to be provided at a repetition rate of ∼1 Hz for very long pulse operation. In the context of a cooperative agreement between the U.S. Department of Energy and the Euratom-ENEA Association, Oak Ridge National Laboratory (ORNL) has collaborated with ENEA Frascati to demonstrate the feasibility of a high-speed (2 to 3 km/s) repeating (∼1-Hz) pneumatic pellet injector for long-pulse operation. A test facility was assembled at ORNL that combined a Frascati repeating two-stage light-gas gun and an existing ORNL deuterium extruder, equipped with a pellet chambering mechanism/gun barrel assembly. It was operated in the course of three joint experimental campaigns between September 1993 and May 1995. The results of the first two campaigns appear in an earlier paper. Here, the results are reported of the third campaign, during which the original objectives of the collaboration were met. Both performance and reliability of the system were improved, with the facility’s being capable of delivering sequences of 2.7-mm deuterium pellets at a repetition rate of 1 Hz and velocities up to 2.5 km/s. The test facility was also briefly operated with neon pellets to explore the potential to produce fast “killer” pellets. Speeds of 1.7 km/s were easily achieved using a piston mass of 43 g. Higher speeds should be achievable with a system specifically designed for neon or other high-Z gases.