Local Stress Due to Screening Current of No-Insulation Non-Circular REBCO Coil in Half-Scale Coil System of Skeleton Cyclotron

Compact cyclotrons are required for the production of radiopharmaceuticals used in targeted alpha-particle therapy, which is a promising treatment for patients with intractable cancers. To realize a compact cyclotron, we have proposed the Skeleton Cyclotron—an air-core compact cyclotron using high-temperature superconducting (HTS) technology. This cyclotron consists of circular and non-circular coils, all wound with REBCO tape using a no-insulation (NI) winding technique. To verify the feasibility of superconducting technology in NI coil systems with such an air-core cyclotron structure, we have fabricated a half-scale coil system of the Skeleton Cyclotron called the Ultra-Baby Cyclotron, which has a diameter of 0.7 m (the full-scale Skeleton Cyclotron has a diameter of 1.6 m). Experiments are currently in progress. In previous experiments, the strain gauge readings showed large values. The cause is currently under investigation, and one possible explanation may be screening currents. In this study, numerical simulations of the additional stresses due to screening currents in the non-circular coils of the Ultra-Baby Cyclotron reproduce the stresses observed experimentally.