Design of YBCO insert coil for a cryogen-free 22 T superconducting magnet

A YBCO insert coil has been developed for upgrading a cryogen-free 18 T superconducting magnet installed in the High Field Laboratory for Superconducting Materials (HFLSM) at Tohoku University to a 22 T superconducting magnet. The YBCO insert coil is designed to generate 6.5 T at 200 A in 15.5 T outer LTS coils. The YBCO insert coil is composed of a stack of 50 single pancake coils wound with YBCO-coated conductors (0.23 mm × 4 mm). The inner and outer diameters of the YBCO insert coil are 96 mm and 178 mm, respectively, and the total conductor length is about 3 km. The maximum hoop stress of the YBCO insert coil was estimated to be 310 MPa when the central magnetic field was 22 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. Thermal runaway may cause burnout of the YBCO insert coil, and therefore, it is important to calculate the voltage-current characteristics of the coil from the superconducting properties of the YBCO-coated conductors. The coil should have no damaged area in the windings because a damaged area would generate heat locally, eventually resulting in thermal runaway. Therefore, a demonstration coil with almost the same size as the pancake coils used for the YBCO insert coil was fabricated and tested in conduction cooling conditions in order to evaluate whether the coil could withstand the thermal stresses and electromagnetic force.