Long term testing of superfluid-cooled 920 MHz NMR cryostat

Abstract

The design of the cryostat structure has been checked from the point of view of safety of the cryostat. 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 up to 21.20 T (901.2 MHz), and then operated in the persistent mode for four months. The magnetic field was successfully increased up to 21.6 T (920.3 MHz) in April 2001, and then operated in the persistent mode for four months. During the long term operation the shield temperature was measured. The minimum temperature changed periodically depending on the liquid nitrogen level of the 80 K shield.