A '20 T at 20 K' Model Coil for the Muon Collider Target Solenoid and Fusion Applications

The development of high-field, large-bore magnets based on High-Temperature Superconductors (HTS) is a strategic challenge with impact across high-energy physics, fusion energy, and other scientific and societal applications. In particular, the target decay and capture solenoid for the Muon Collider and the central solenoid of next-generation fusion devices both demand solenoids capable of producing a 20 T peak field in a bore larger than 1 m, at 20 K, with stored energy exceeding 300 MJ (single coil). Such performance is well beyond present state-of-the-art, requiring a dedicated programme to validate conductor technologies, magnet architectures, and protection strategies. This paper presents the results of a feasibility study originated from discussions between CERN and Eni on a common solenoid concept serving as technology validation for both high-energy physics experiments and magnetic-confinement fusion devices such as tokamaks. The main result of the study, which brought together universities and research centers, is a "20@20" model coil concept. We report here the rationale for the selection of the target performance of the "20@20" model coil, and the highlights of the study, which was instrumental to identify high priority developments required towards its realization.