Category Archives: superconducting magnets

A method for fast field settling in cryogen-free superconducting magnets for NMR

Cryogen-free magnets are around for EPR spectroscopy for a while already, however, in recent years they also become more popular for NMR spectroscopy (solids and solutions). This article greatly demonstrate the potential of the technology.

Kryukov, Eugeny, Yury Bugoslavsky, Angel Joaquin Perez Linde, Thomas Holubar, Stephen Burgess, David Marlow, and Jeremy Good. “A Method for Fast Field Settling in Cryogen-Free Superconducting Magnets for NMR.” Solid State Nuclear Magnetic Resonance 109 (October 2020): 101684.

https://doi.org/10.1016/j.ssnmr.2020.101684.

We propose a fast algorithm to energise a cryogen free magnet to a highly persistent state. A decay rate as low as 0.021 ppm/h can be achieved in less than an hour after reaching the target field. The decay rate drops further to 0.0004 ppm/h in the following 48 h. This procedure can be applied at different values of target field, which makes it feasible to use a single magnet for study of various NMR lines at different fields. The mechanism of establishing a highly stable magnetic field can be understood on the basis of the magnetic properties of the superconducting wire, which were studied using a vibrating sample magnetometer. The results confirm the high quality of the superconducting wire and joints.

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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