Design and implementation of a high speed guiding system for inboard pellet refuelling

Cryogenic pellet injection from the magnetic high field side (HFS) of a tokamak is the currently favoured scheme to refuel steady state H-mode discharges. However, in currently operating tokamaks access to the HFS has to be gained via strongly curved guide tubes severely restricting injection speeds and leading to shallow plasma penetration where a large portion of the injected material is lost in the edge gradient layer within a few milliseconds after injection. Therefore, at ASDEX Upgrade an optimized pellet guiding system for speeds close to 1000 m/s has been designed and put into operation. The 17 m long track consists of: (i) a funnel installed to catch pellets scattered by the accelerator; (ii) a preliminary teflon tube section; and (iii) a guiding track transferring the pellets in a diagnostic port into the plasma. Using a looping geometry the system is designed to keep semi-permanent contact between pellet and track by steering the pellets in a controlled trajectory. Emphasis is also put on efficient evacuation of all sections to avoid pressure build-up in the system due to the Leidenfrost effect. In first proof-of-principle plasma discharges a scan of pellet injection at v=240–880 m/s and plasma parameter studies at v=560 m/s were completed.