Magnetic Field Quality Evaluation of a 20-T Cryogen-Free Superconducting Magnet With a Bi2223 Insert

The magnetic field stability such as relaxation and a magnetic field hysteresis of a practical 20-T cryogen-free superconducting magnet (20 T-CSM) with a Bi
₂
Sr
₂
Ca
₂
Cu
₃
O
_y
(Bi2223) insert coil was investigated. The relaxation of a central magnetic field is basically related to the flux creep phenomena in the magnetization of the Bi2223 tapes. We found that the relaxation rate S of the magnet is more than two orders smaller than that of the Bi2223 tapes. The magnetic field stability of the 20 T-CSM becomes less than 2 G/h even at 19 T after a lapse of half a day from the charging. It is small enough for the solid-state NMR studies. However, the repeating excitation increases the central magnetic field beyond the saturated values under the relaxation.