A Framework for the Magnetic Design of Large Bore, Curved AG-CCT Magnets for Proton Therapy Facilities

The canted-cosine-theta (CCT) magnet is a promising candidate for shrinking the volume of the gantry for radiotherapy facilities. The magnets are often demanded to have large bores and small bending radii in the gantry design, which would lead to complex harmonic coupling and non-negligible fringe-field effects. To address these issues, a design framework is proposed in this contribution. The winding coefficients of the CCT coils are firstly determined by an improved backcasting algorithm to cancel out the harmonics. A toroidal shielding iron is mounted outside of the coil afterward, which can not only lower the stray field but also significantly eliminate the overshooting of the fringe field. The excitation efficiency of the magnet is enhanced by the shield without introducing any high-order harmonics. This framework is validated on an AG-CCT magnet being developed for our superconducting proton therapy gantry. The field derivatives and particle tracking results meet our optical design value well.