Excitation Test of Superconducting Magnet in 230-MeV Isochronous Cyclotron for Proton Therapy

We develop a superconducting isochronous cyclotron for proton therapy with a yoke weight of approximately 65 tons, which is less than one-third of the Sumitomo's normal-conducting 230 MeV cyclotron. Two NbTi superconducting coils with an axial gap are conduction-cooled by four 4 K Gifford-McMahon cryocoolers. The average field designed is 3.9 T at the extraction radius. Since superconducting magnets for treatment devices are required to have sufficient margin for critical temperature and to recover early from quenching, they were evaluated through excitation and quench tests. We achieved the maximum magnetic field generation without quenching, and measured the actual current dependence of inductance, which is in agreement with the computational results. A forced quench test was performed to test the protection circuit. The magnet was successfully protected, and the recovery time of the magnetic field generation was 17 hours.