Development of an NMR spectrometer operated beyond 1 GHz. Superfluid helium cooling system and some of the typical cooling cases

The pressurized superfluid cooling system has been completed a final adjustment with the 920 MHz NMR in 2002. The superfluid cooling system used this time had been using for the 920 MHz NMR from 2002 to 2007. The first cooling system was able to automatically control the cooling of the superconducting coil to 1.55 K +/- 5 mK, and it was possible to operate remotely via the internet. However, in case of the 1020 MHz NMR system, the heat load is increased to approximately four times more than originally planned value, was forced to change of the components during the cooling operation. Along with it, always should be cooling operation in a transient state, operating procedures were also changed significantly. Measurement system that shows an abnormal behavior by aging was used for state diagnosis only. Therefore, the cooling system was operated manually under the supervision. Fortunately, the cooling system had many measurement points for during test operation, so there was no trouble in determining the transient state. As a result, temperature of the superconducting magnets for 1020 MHz NMR had been kept to 1.72 K +/- 20 mK. (author)