Fast, controlled stepping drive for D2 filament ejection

Centrifugal pellet injectors are required to refuel plasma machines. The pellet feed into the centrifuge should, if possible, be direct to keep the exit angle divergence small. The D2 filaments used are first stored in a cryostat and then rapidly transported to the intake region of the centrifuge. An intermittent drive for fast, controlled ejection of D2 filaments is described here. Mean filament speed of up to 0.6 m/s per step (1.2 mm) are achieved for the centrifugal pellet injector which refuels the ASDEX tokamak at Garching. The timing of the (81) step shifts can be synchronized with the rotor motion. The drive allows rates of up to 50 pellets per second. The drive method also seems to be suitable for direct feeding of other known centrifugal pellet injectors.