The Superconducting Toroid for the New International AXion Observatory (IAXO)

IAXO, the new International AXion Observatory, will feature the most ambitious detector for solar axions to date. Axions are hypothetical particles that were postulated to solve one of the puzzles arising in the standard model of particle physics, namely the strong CP (charge conjugation and parity) problem. This detector aims at achieving a sensitivity to the coupling between axions and photons of one order of magnitude beyond the limits of the current detector, the CERN Axion Solar Telescope (CAST). IAXO is equivalent to combining roughly 20000 CAST detectors into a single apparatus. The IAXO detector relies on a high-magnetic-field distributed over a very large volume to convert solar axions to detectable X-ray photons. Inspired by the ATLAS barrel and end-cap toroids, a large superconducting toroid is being designed. The toroid comprises eight, 1-m-wide and 21-m-long racetrack coils. The assembled toroid is sized 5.2 m in diameter and 25 m in length and its mass is about 250 tons. The useful field in the bores is 2.5 T while the peak magnetic field in the windings is 5.4 T. At the operational current of 12 kA the stored energy is 500 MJ. The racetrack type of coils are wound with a reinforced aluminum stabilized NbTi/Cu cable and are conduction cooled. The coils optimization is briefly described as well as new concepts for cryostat, cold mass, supporting structure, and the sun tracking system. Materials selection and sizing, conductor, thermal loads, the cryogenics system, and the electrical system are described. Lastly, quench simulations are reported to demonstrate the system´s safe quench protection scheme.