Field Stability Analysis of 25 T Cryogen-Free Superconducting Magnet and Upgrade Plans for 30 T System at HFLSM, IMR, Tohoku University

Magnetic field stability and hysteresis can be serious problems in a high-field high-temperature superconducting magnet, because of the large shielding current effect. Accordingly, the field stability of a 25 T cryogen-free superconducting magnet (25T-CSM) was investigated. This particular system can generate a central field of 24.6 T in a 52-mm room-temperature bore. It was found that the magnetic field relaxation rate of the 25T-CSM with a Bi2Sr2Ca2Cu3O, (Bi2223) insert was due to the flux creep effect, and was small enough to allow nuclear magnetic resonance (NMR) experiments. Based on these experiences in the development of the 25T-CSM, primitive designs for a 30 T cryogen-free superconducting magnet (30T-CSM) were suggested based on upgrading the present Bi2223 insert to a REBa2 Cu3Oy (RE123, RE: rare earth and Yttrium) insert incorporating two co-wound RE123 tapes. It was confirmed that the prototype test coil could be operated at 4.2 K under large electromagnetic stresses of up to 540 MPa without experiencing degradation.