The 920 MHz NMR magnet which successfully generated its designed magnetic field on April 2001 was installed at Tsukuba Magnet Laboratory (TML) of the National Institute for Materials Science (NIMS). The magnet again generated 920 MHz without a quench, and has been in continuous operation for over one and a half years up to the present time. The cryostat, which houses the superconducting magnet, employs a pressurized superfluid helium cooling process. Its operating temperature is below 1.55 K. The consumption rates of liquid helium and liquid nitrogen are 0.98 L/h and 0.76 L/h, respectively. Liquid helium is manually supplied every week and liquid nitrogen is automatically supplied every week. Although the cryostat has experienced earthquakes more than 70 times and power failure 3 times, there has been no serious trouble related to the cryostat and the cooling operation is still continuing without interruption.
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Ito et al. (2004) studied this question.