Introduction
Cryogen-free systems are widely used for Electron Paramagnetic Resonance and Dynamic Nuclear Polarisation experiments. Besides the low maintenance costs (especially nowadays with skyrocketing prices for liquid helium due to the ongoing helium supply crisis), the cryogen-free technology presents two other major innovations for NMR. First, the magnet is much more compact. The probe can be inserted from the top of the magnet, that eliminates need to bring the magnet up onto the long legs. Second, the magnetic field in cryogen-free magnets is always ready to be changed. That is opening the possibility to use the same magnet at different fields.
Method
We made a 400 MHz MAS NMR system and measured numbers of liquid and solid state samples to demonstrate the performance of the system. The temporal magnetic field distortion generated by the mechanical nature of the cryogen-free cold head is the reason why this technology is still rarely used for high resolution NMR.
Results
We showed that these distortions can be made smaller than one part per billion with respect to the main magnetic field. That makes the cryogen-free magnets acceptable for solid state MAS NMR and possibly for liquid state NMR as well.
We also showed that the field settling process can be completed within an hour after the field change. That makes possible to change the field in the magnet in a day to day basis without compromising the measurements resolution.
Conclusion
We showed that the cryogen-free magnets have a good potential to replace the conventional magnets with the field up to 750 MHz in near future.