Design of a Large Momentum Acceptance Gantry Based on AG-CCT for Lightweight Proton Therapy Facility

Superconducting (SC) gantry can be applied to proton therapy with significantly reduced footprint and weight. However, the relatively lower ramping limit of the SC magnetic field becomes a bottle-neck for fast energy change and beam delivery. The issue can be mitigated by designing a large momentum acceptance (LMA) beam optics. We present the design of an LMA gantry using AG-CCT SC magnets and symmetrical achromatic lattice. A fast degrader is combined in the design so that the gantry can rapidly switch energy during the treatment. The AG-CCT design process and beam transport simulation are all performed with our homemade integrated code CSPT, which has interfaces to Geant-4 and Opera, and can reach a maximum speed-up ratio of ~450 by applying parallel computation technique. The multi-particle simulation based on realistic field distribution proves that the gantry has a large momentum acceptance of ~20%. Due to its large momentum acceptance, the dispersion effect caused by the scanning magnet is not neglectable. A dispersion compensation method, accompanied by a compact nozzle layout, is proposed to achieve a scanning field of 25×25 cm² with a maximum beam energy spread of 5.2%.