Development of cryogenic cooling system using a double G-M cryo-cooler for NMR spectrometer

The sensitivity of nuclear magnetic resonance (NMR) spectrometer is limited by thermal noise. To increase the signal-noise-ratio (SNR), cryogenically cooled probes with cryo-coolers have been used. In order to cool RF coils in the probe below 10K, Gifford- McMahon/Joule-Thomson (G-M/J-T) cryo-coolers are used. But, the G-M/J-T cryo-coolers aren't suitable for a long time operation. On the other hand, G-M cryo-coolers have high reliability. We designed a cryogenically cooled probe system using a double G-M cryo-cooler with an RF coil and preamplifiers that could be cooled below 5 and 50K, respectively. This probe system consists of a probe unit, a transfer unit and a cryo-cooling unit. The cryo-cooling unit consists of two G-M cryo-coolers, two counter-flow heat exchangers, helium gas lines, etc.,. Compressed helium gas is cooled by the cooling stages of G-M cryo-coolers and two heat exchangers. The cooled helium gas is supplied to the probe unit through the 2.5m long transfer unit which consists of a 45mm diameter flexible vacuum vessel containing four helium gas lines. Cold stages for the RF coil and the preamplifiers in the probe unit are cooled by the cryogenically cooled helium gas. Experimental results showed that the cold stage temperature for the RF coil and for preamplifiers after 12 hours was 5.29 and 40.3K, respectively.