4.5 K Cooling System for a Cryogenically Cooled Probe for a 920 MHz NMR

The feasibility of closed-cycle 4.5 K cooling has been demonstrated for a cryogenically cooled probe for the world highest magnetic-field 920 MHz Nuclear Magnetic Resonance (NMR) operated at the National Institute for Materials Science (NIMS), by means of the following model experiments. The cooling system with a cooling capacity of 4.55 W comprises a Gifford-McMahon (GM) cryocooler and two-stage expansion Joule-Thomson (J-T) circuit. Supercritical helium is produced by the 1st J-T valve, which is then liquefied by the 2nd J-T valve. Helium mist cools a cooling stage for the radio frequency (RF) coils. The NMR sensitivity improvement in terms of the signal-to-noise (S/N) ratio of the 4.5 K probe is expected to be double of the conventionally used 20 K cryogenic probe, due to the reduction of the thermal noise of the RF coils.