Seven-Year Operation of 25 T Cryogen-Free Superconducting Magnet as a User Magnet at HFLSM, IMR, Tohoku University

A 25 T cryogen-free superconducting magnet (25 T-CSM) was installed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University in 2015. The 25T-CSM consists of low-temperature superconducting (LTS) outsert coils and a high-temperature superconducting (HTS) coil; high strength CuNb/Nb3Sn and NbTi Rutherford cables, and Bi2Sr2Ca2Cu3Oy (Bi2223) tape were adopted for the LTS and HTS coil, respectively. The Bi2223 coil was a stack of impregnated double pancake (DP) coils. The first Bi2223 coil generated 10.6 T in a background field of 14.0 T by the LTS coils in 2015. Because of the coil stiffness insufficiency, it had an electrical instability. The new Bi2223 coil, made by improving the stiffness of DPs, successfully generated 11.1 T (25.1 T in total) in 2020. Both Bi2223 coils experienced accidental quenches during the operations as a user magnet. However, the magnet's quench detection and protection system protected the coils from the damage including the LTS coils. The 25T-CSM has been operated as a multipurpose user magnet for more than 1100 days in seven years, meaning that almost 100% of the available days were utilized for user experiments.