Developing fuel sources for a steady-state multi-purpose pellet launching system

Like for any power plant, refuelling will be one key issue in a future fusion reactor. It has to be safe and efficient; due to the operational principle, it also needs to occur in steady state. From previous investigations, it became clear that injection of pellets - mm-size solid bodies produced from frozen fuel – is the only possible and technically realistic solution for this task. Beyond their capability for fuelling, pellets have been found potentially useful as well for other purposes, e.g. to control edge localised modes. Therefore, efforts are required to develop a pellet launching system (PLS) capable to meet all basic reactor requirements. To foster this kind of technology evolution and to enhance the PLS applied at ASDEX Upgrade (AUG), a collaborative investigation between IPP and ORNL has been initiated. This aims to replace the existing pellet source able to deliver to a centrifuge injector up to 130 stored pellets produced in a batch process with repetition rates up to 80 Hz by a steady state source. The new source is based on the ORNL extruder types and capable of delivering either in true steady state or at least from a reservoir with sufficient solid fuel to produce up to 2000 pellets. Experiments extruding ice with different square cross sections showed a maximum throughput of about 300 mm³/s for Protium (H) and 215 mm³/s for Deuterium (D) with a 1.9 mm square cross section. Ice temperatures of 9 K for H and 16 K for D were found to be optimum. For D, extrusion rates correspond to a 1.9 mm pellet delivery rate of about 25 Hz. Doping the ice with up to 2% Neon was achieved at the expense of moderately reduced extrusion speeds. A new scheme has been designed to inject pellets in parallel from multiple sources for further enhancing the delivery rate.