Introduction. Pellet injection is a well proven technology representing to date the most efficient method for core fuelling of fusion plasmas. It is believed that, due to favourable drift effects across the plasma column, injection from the inboard torus side via curved guide tubes can provide high fuelling efficiency, in spite of restrictions imposed to the projectiles speed by such a scheme. Extensive tests carried out at ORNL suggest that the speed will need to be limited to a few hundreds of m/s, for reliable delivery of intact pellets ~m. Simulation codes, however, indicate that the fuelling efficiency increases with the injection speed ~([2]), while recent tests at Asdex Upgrade demonstrated that D2 ice pellets can survive at velocities of~ 1 km/s inside a guiding system, provided this is very carefully designed ~([3]). Moreover, when injection from the inboard torus side is impractical due to tight space (as will be for instance the case of Ignitor and JT60-SA), pellets have to be necessarily injected from the outboard side at speeds ≥ 3 km/s, in order to achieve sufficient fuel penetration in spite of the unfavourable drift. ENEA and ORNL are therefore collaborating on the development of a high-speed four-barrel "pipe gun" injector for the Ignitor experiment (fig. 1), designed to launch D2 ice pellets (1.9, 2.6, 3.2 and 4.4 mm in size) at speeds up to 4 km/s, using two-stage light gas guns~([4]).
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