Design Study of a Low Field Dipole for the FCC-ee Booster Ring

The FCC-ee booster dipoles are low-field irondominated cycled H-type magnets. They consist of an 11-meterlong laminated steel yoke with anodized aluminium busbars instead of coils. The magnet is required to operate at fields down to 6.5mT for 20 GeV injection. At such low field levels parasitic effects become significant, for example, the Earth's magnetic field, yoke hysteresis, and stray fields from cables. This poses a unique set of design challenges that are answered in this paper. The hysteretic effect is modelled, and the cross-section of the magnet is optimised to obtain the required field quality across the full cycle from 6.5mT to 59 mT. The predictive power of the hysteresis model is validated by the test of a similar magnet from CERN stock. Finally, magnetic measurements from a short prototype magnet are reported. The test results confirm that the proposed magnet meets the design requirements, demonstrating the feasibility of dipoles for 20 GeV booster injection energy.