Acceleration of large size deuterium pellets to high speeds using a small two-stage pneumatic gun

The technology of two-stage pneumatic pellet injectors represents by far th e most reliable way to perform deep plasma fueling, with pipe gun devices c apable of routinely launching small or medium size (up to 4 mm) D-2 pellets at speeds in excess of 3 km/s, using rather small two-stage guns. It is st ill an open question, however, if scaling of the pellet size to the Interna tional Thermonuclear Experimental Reactor relevant values (6-8 mm) will or will not require a somewhat proportional increase in the physical size of t he two-stage gun. In order to investigate this question, an extensive study was carried out at ENEA Frascati, using numerical simulation codes. It cle arly indicated that a "compact'' two-stage gun may have the potential to ac celerate large size pellets at speeds up to 5 km/s. A low cost experiment w as also scheduled. A spare pipe-gun cryostat of the single-shot two-stage p neumatic injector, previously used for high-speed pellet fueling of the Fra scati tokamak upgrade, was modified in order to accommodate larger bore (up to 6 mm) launching barrels. In this article, we will mainly discuss the re sults of numerical simulations. A very early experimental campaign, carried out in 1996, will also be briefly reported, during which intact 6 mm D-2 p ellets were launched at speeds up to 2.5 km/s. (C) 1999 American Institute of Physics. [S0034-6748(99)01105-3].