Design of Multiple REBCO Sector Coils for Improving Focusing Force in Skeleton Cyclotron for Cancer Therapy

We have proposed a high-temperature superconducting (HTS) air-core cyclotron for cancer therapy, called the Skeleton Cyclotron. The design of a full-scale Skeleton Cyclotron is currently underway. The magnetic field in the acceleration plane of the cyclotron requires both an isochronous field and an azimuthally varying field (AVF). Particles deviating from the orbit and the median plane are focused and defocused, respectively. In conventional cyclotrons using copper wire and iron cores, the iron poles are machined into a spiral shape to ensure a focusing force. Our proposed Skeleton Cyclotron is an air-core design, consisting of several circular-shaped main coils and triangular-shaped sector coils wound with REBCO tape. In this design, the iron sectors are replaced by triangular HTS sector coils. The axial focusing force for accelerated particles is generated by the flutter produced by these triangular-shaped sector coils. However, winding REBCO wire into the spiral sector-type coil with negative bend curvatures is technically challenging. Therefore, we considered the use of multiple straight sector coils wound with REBCO tape. Moreover, by independently adjusting the operating currents in the sector coils, we can freely manipulate the generated magnetic field, enabling the acceleration of various types of beams within a single cyclotron. In this study, we designed multiple sector coils, evaluated the reproduction of the AVF for variable energy, and conducted orbital simulations of accelerating particles.