  IAXO, the new International AXion Observatory, will feature the most
ambitious detector for solar axions to date. Axions are hypothetical particles
which 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). 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, one meter wide and twenty one meters 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
shortly 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.
