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We had successfully operated the 1020-MHz (24.0 T) nuclear magnetic resonance (NMR) spectrometer for 8 months, including the cooling down, shimming, and the 6-month operation for NMR measurements. The NMR superconducting magnet, which had consisted of NbTi and Nb3Sn coils, was upgraded by replacing the innermost Nb3Sn coil with a (Bi,Pb)2Sr2Ca2Cu3O10(Bi-2223) one. The Bi-2223 innermost coil was connected in series with Nb3Sn and NbTi coils. The magnet was cooled with pressurized superfluid helium (He II). The steady-state operating temperature was 1.72 K. The magnet was power-supply driven, i.e., the transport current was always supplied by a power supply. A small normal transition and a power outage occurred during the 8-month operation; however, the magnet quench was avoided.
Nishijima et al. (Mon,) studied this question.