Speaker
Description
The final cooling stage of the Muon Collider concept relies on ultra-high-field solenoids exceeding 40 T, magnetic fields that can only be realised with high-temperature superconductors (HTS). The need for compact, cost-efficient magnets necessitates extremely compact winding packs, driving the overall operating current densities to 700 A/mm² and beyond. The resulting Lorentz forces and stored-energy densities make the mechanical and electromagnetic behaviour of the individual coils, the basic building blocks of our solenoid, a central challenge. The pancake coils, consisting of tightly wound, fully soldered non-insulated Rare-Earth Barium Copper Oxide (REBCO) coated conductors, require a robust test methodology to monitor fabrication quality and characterise their electromagnetic properties.
As a first step, single non-insulated, fully soldered REBCO pancake coils (Øin = 60 mm, Øout = 100 mm) are produced and characterised. These are wound on an automated hot-winding setup developed in-house from 4 mm wide commercially available HTS tape (SST, Faraday Factory, Fujikura). Experiments at 77 K in self-field provide a simple yet conclusive tool to characterise the electromagnetic properties, detect fabrication faults and qualify joints. Targeted parameters are the radial (turn-to-turn) resistance, the coil inductance, the joint resistance, and the magnetic field in the centre and on the coil surface.
In this work, a dedicated, 3D-printable test setup combined with a modular software environment has been developed. The code drives an arbitrary number of digital multimeters in parallel, controls the power supplies, and executes user-scripted measurement sequences. As characterisation at 77 K alone is insufficient, the software also supports our new 4.2 K test setup, which is currently under commissioning. Furthermore, we report on the experimental results recorded using our new setup on multiple pancake coils based on HTS tapes from various manufacturers. The data validate our winding and soldering process through consistently low radial resistance, ideal charging behaviour, the expected inductance and inconspicuous sectoral radial voltages, all matching our simulation results. This provides the experimental confidence needed to move forward towards the 10 T sub-scale prototype and, ultimately, the final-cooling solenoid of the Muon Collider.
This work has been partially supported by the CERN High Field Magnet Programme.
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