Performance of Cryogen-Free Superconducting Magnet in Isochronous Cyclotron for Proton Therapy

A superconducting azimuthally varying field (AVF) cyclotron, SC230, was developed for proton therapy by Sumitomo Heavy Industries, Ltd. in 2021. This isochronous cyclotron is compact owing to the high magnetic field generated by NbTi superconducting coils. The average magnetic field is 3.9 T at the outer region, and the extraction radius is 0.6 m. Its yoke weight is 65 t. Currently, it is the most compact isochronous cyclotron for this purpose. The coils are cryogen-free and cooled by conduction cooling using four 4 K-GM cryocoolers. The system is highly safe and easy to maintain, which reduces the effects of unstable helium supply. Thus, cryogen-free magnets have various advantages; however, in isochronous cyclotrons, the heat inputs to the coils during beam operation are larger owing to the leakage radio frequency (RF) and beam loss. The magnet is designed to have a sufficient temperature margin for stable operation. The coil temperature during the RF excitation and its dependence on the beam loss were measured to verify the stability of operations. Subsequently, it was confirmed that the coil cools sufficiently at the critical temperature.