Continuous pellet fueling system for the W7-X stellarator

An international team has developed and built a continuous, high-speed pellet injection system to fuel W7-X plasmas in quasi steady-state conditions. The team includes researchers from ORNL, IPP, PPPL and NIFS. This system will also serve as a prototype for the ITER pellet fueling system. The W7-X continuous pellet fueling system is designed to inject 3 mm H₂ or D₂ pellets into W7-X at speeds up to 1000 m/sec with a repetition rate of up to 10 Hz. A twin-screw extruder cooled by three cryocoolers is used to produce a continuous flowing ribbon of solid H₂ or D₂ that can be cut into a pellet and fired on demand by a light gas gun. The pellet size and repetition rate will be controllable in real-time to maintain optimum plasma profiles for confinement. The W7-X continuous pellet system has been assembled and commissioned at ORNL and is now undergoing commissioning at W7-X where it will be used in the next experimental campaign. High pellet reliability with H₂ pellets was demonstrated as was the ability the adjust the speed from 200-1000 m/s and the pellet size from full to ½ the nominal 3 mm cylindrical size within a few seconds. Diagnostics to measure pellet mass, speed, and ablation emission have been implemented.