Testing of superfluid-cooled 920 MHz NMR cryostat

Abstract

A 900 MHz class NMR magnet and cryostat was constructed and then successfully operated in December 1999. The measured heat leak to the superfluid bath was high compared with the design value of 0.26 W at 1.8 K. The excess heat leak was thought to have come from an unexpected 140 micron gap in the cold safety valve. A kind of impurity, which was deposited on the surface of the valve sheet during the precooling process, seemed to be the reason for the gap of 140 microns. The design of the cryostat structure has been checked from the point of view of safety of the cryostat. The size of the cold safety valve has been increased. Although the design value of the heat load has been increased, the heat leak to the superfluid bath has been reduced to 0.5 W at 1.7 K by purification of the magnet bath and careful control of the cold valve sheet surface. The magnet was energized again up to 21.20 T (901.2 MHz), and then operated in the persistent mode for four months. The magnet field was successfully increased up to 21.6 T (920.3 MHz) in April 2001.