On the magnetic field stability of cryogen-free magnets for magnetic resonance applications

Liquid cryogen-free magnets are typically used in gyrotrons. However, they are often thought to be not suitable for NMR due to vibrations of the cold-head. The authors in this study show, that with careful design the field instabilities can be brought to an acceptable level for solid-state NMR spectroscopy or MRI.

Kryukov, Eugeny, Angel Joaquin Perez Linde, Seema Raghunathan, Stephen Burgess, Paul Jonsen, and Jeremy Good. “On the Magnetic Field Stability of Cryogen-Free Magnets for Magnetic Resonance Applications.” Solid State Nuclear Magnetic Resonance 105 (February 2020): 101639.

https://doi.org/10.1016/j.ssnmr.2019.101639

The temporal magnetic field variation associated with Cold Head operation in cryogen-free magnets was studied. Three different mechanisms for such variations were tested separately and rated by their importance. It was found that mechanical displacement of the magnet inside the cryostat is the main issue of magnetic field perturbation. In a cryostat with the Gifford- McMahon type of cryocooler, motion of the displacer with magnetic material inside also produces significant field modulation. The temperature variation of the magnet, although noticeable, leads to smaller field distortions compared to the previous two factors. It was shown that the temporal magnetic field variation could be reduced down to below 20 ppb level that could be acceptable for MRI and MAS NMR applications. It was also shown that a single cryogen-free magnet could be easily used at different fields on a day-to-day basis without compromising the field stability unlike magnets housed in a liquid helium reservoir.

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