Injection of high-speed solid D₂ pellets using a “Direct-Line-of-Sight” (DLS) guide tube

Extensive investigations, within the EUROfusion Work Package "Tritium, Fuelling and Vacuum", indicate that sufficiently deep fuel deposition inside H-mode plasmas of the EU-DEMO tokamak requires injection of fuel pellets from the High Field Side (HFS) at speeds ≳ 1 km/s. To implement this, two different approaches are being pursued: one makes use of “conventional” curved guide tubes, featuring large bend radii (≳ 6 m), to transport 1 km/s pellets to the HFS while trying to preserve their mass and integrity; the other explores the feasibility of injecting high-speed (≳ 2 km/s) pellets from the HFS, along “Direct-Line-of-Sight” (DLS) paths. This paper focuses on the latter approach. Recent tests with an existing ENEA-ORNL high-speed injector have confirmed that the trajectories of free-flight pellets, travelling under vacuum at speeds up to 2.4 km/s, spread within an angle ≲ 0.68°. Despite their small scatter cone, free-flight pellets may require too much cut off volume of the Breeding Blanket (BB), due to the large distance between the injector and the plasma. The introduction of a straight DLS guiding tube transporting the high-speed pellets, to avoid significant loss of BB material, has been investigated. The existing ENEA-ORNL injector has been modified to accommodate a 10 mm i.d. DLS guide tube, and intact pellets have been consistently delivered downstream of the guide at speeds up to 2.6 km/s, with remarkably reduced scatter cone, thus showing the viability of this innovative approach.