Optical design of a weak-focusing accelerator with hills and valleys

A trend of proton beam therapy (PBT) systems is downsizing their footprints. We have proposed a new weak-focusing accelerator with cotangential beam trajectories, which is downsized compared with existing Hitachi PBT accelerators by utilizing a superconducting magnet. The larger main magnetic field by the superconducting magnet, however, requires septum magnets to generate larger magnetic field for beam extraction. For relaxing the specification of the septum magnets, we consider an azimuthally varying field (AVF) weak-focusing accelerator. By locating the septum magnets at one of the valleys and extracting the beam from the valley, the specification is relaxed with keeping its footprint. In this study, we designed the optics of the accelerator by the transfer matrix method. The magnetic field in the valleys is smaller than the magnetic field averaged along the maximum energy trajectory by 0.2 T. (author)