Testing of Stacked Pancake Coils for a Cryogen-Free 25-T Superconducting Magnet

A cryogen-free 25-T superconducting magnet with a 52-mm room temperature bore consists of an 11-T REBa2Cu3O7-delta (REBCO) insert coil and outer 14-T low-temperature superconducting (LTS) coils. The REBCO insert coil is composed of a stack of 56 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 263 mm, respectively, and the total conductor length is about 14 km. The REBCO insert coil is cooled by circulating helium gas using both single-stage and two-stage Gifford-McMahon cryocoolers, and the LTS coils are cooled by two Gifford-McMahon/Joule-Thomson cryocoolers. The maximum hoop stress of the REBCO insert coil was estimated to be 387 MPa when the central magnetic field was 25 T. If thermal runaway occurs in a conduction-cooled system, the REBCO coils will almost certainly be burned out. Therefore, the coil should have no damaged area in the winding because a damaged area would generate heat locally, eventually resulting in thermal runaway. In order to avoid the degradation of the coil due to thermal stress, the coil was divided into all winding parts in the radial direction. In practice, the coil was divided by inserting a polyimide tape coated with a fluorine resin, which has low adhesive strength, into the epoxy resin at every turn. Before fabricating and testing the actual REBCO insert coil, two kinds of coils were fabricated and tested in order to evaluate the validity of the winding method. Two stacked pancake coils wound with REBCO-coated conductors were fabricated and tested in liquid helium with a 4-T background magnetic field in order to evaluate whether the coil could withstand the hoop stress of over 387 MPa. Moreover, we fabricated and tested a demonstration coil which was composed of a stack of four single pancake coils with the same size as the pancake coils used for the REBCO insert coil in order to evaluate whether the coil could withstand the thermal stress and the electromagnetic force.