Concept for a multi-purpose EU-DEMO pellet launching system

Pellets - mm-size solid bodies produced from frozen fuel - are mainly destined for fuelling purposes in the fusion reactor EU-DEMO. However, pellets have been proven capable tools for further tasks too, e.g. ELM frequency control and the efficient delivery of seeding gases have been already demonstrated. Here, a concept is presented for a single pellet launcher based on a stop-cylinder centrifuge accelerator equipped with multiple pellet sources. The sources can deliver pellets with different sizes and composition, finally combined into one single compound pellet train. Thus, the pellet launching system (PLS) is capable to control simultaneously different plasma parameters with a minimized cross talk between these different actuations. Currently, a new PLS is under development for the new large superconducting tokamak device JT-60SA which can be potentially regarded as a prototype for the envisaged EU-DEMO system. Its initial configuration will be capable to control plasma density and ELM pacing simultaneously; optionally a source for doped pellets can be added. Status and recent achievements of this systems are reported. At the full metal wall mid-size tokamak ASDEX Upgrade (AUG), work is ongoing developing a versatile control strategy and corresponding tools. Capable to inject pellets with high speed through a guiding tube from the torus inboard side, AUG represents a fully reactor relevant configuration. While all demonstrations have been performed with a single pellet source hence bound to actuation prioritizing one actuation parameter, all tools developed can be expanded straightforwardly to multi-purpose control. For example, one novel tool optimizes real-time feedback pellet flux control taking into account the discrete nature of the pellets. As well, a solution is worked to handle the issue of missed-out pellets. Such failed pellets are considered unavoidable due to the fragile nature of the solid fuel but identified as significant hazard for reactor burn control.