Design of a REBCO Insert Coil for a Cryogen-Free 25-T Superconducting Magnet

A cryogen-free 25-T superconducting magnet with a 52-mm room-temperature bore consisting of an 11.5-T REBa₂Cu₃O_7-δ (REBCO) insert coil and outer 14-T low-temperature superconducting (LTS) coils is currently under development. The REBCO insert coil is composed of a stack of 68 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 276 mm, respectively; and the total conductor length is 17.6 km. The maximum hoop stress of the REBCO insert coil was estimated to be 455 MPa when the central magnetic field was 25.5 T. The REBCO insert coil is cooled by circulating helium gas using both single- and two-stage GM cryocoolers, and the LTS coils are cooled by Gifford-McMahon/Joule-Thomson cryocoolers. If thermal runaway occurs in a conduction-cooled system, the REBCO coil will almost certainly be burned out. Therefore, it is important to avoid thermal runaway of the REBCO insert coil under all operating conditions and protected operations. As operating conditions of the REBCO insert coil, quenching of the LTS coils should be assumed in the severest situation, since the current of the REBCO insert coil will increase after the LTS coils quench. From the results of quench analysis of the LTS coils and circuit simulations, it was confirmed that thermal runaway can be avoided by setting the intermediate potential of the REBCO coil to the ground potential and by using an external dump resistance of 11 Ω.