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Abstract:
Particle deposition deep inside the hot target plasma column by cryogenic hydrogen pellet injection is required for efficient particle refueling of fusion devices such as tokamaks. As the ablation plasmoid is subject to a strong outward drift in hot plasmas, pellet launch from the tokamak inboard side is more useful than from the outboard. The depth of the pellet particle deposition depends on density and temperature of the target plasma, and on the pellet mass and velocity. Plasma operation determines density and temperature values, the maximum affordable density perturbation limits the pellet mass. Consequently, the pellet speed remains the only technically variable parameter allowing improvement of the refueling performance. To achieve this an inboard high-speed pellet injection system based on looping type geometry was designed and built at the midsize tokamak ASDEX Upgrade, and first fueling studies had validated the potential for the required injection velocity increase. Throughout the last two years experimental efforts focused on careful step-by-step optimization of the different system hardware components and the operational procedures. Introducing amongst other features a well pumped, rectangular guide track section, the feasibility for the inboard launch scheme up to an injection velocity of 1 km/s was successfully demonstrated. Detailed off-line tests have confirmed that pellets can withstand controlled mechanical and thermal impact even at this high speed, albeit for the sacrifice of increasing and significant mass losses. In a first application to plasma refueling deep penetration into hot target plasmas and hence, high fueling performance was achieved by deeper pellet born particle deposition and hence enhanced particle sustainment times.
