A Benchtop Cryogen-Free 23.5-T/25-mm-RT-Bore Magnet for 1-GHz microcoil NMR Spectroscopy

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Project Summary In this project we propose to develop a benchtop cryogen-free 23.5-T high-temperature superconducting (HTS) magnet for 1-GHz microcoil nuclear magnetic resonance (NMR) spectroscopy. Higher-field magnet offers better resolution and sensitivity, enabling analysis of larger molecules like complex proteins, however currently available ≥1-GHz NMR magnets are very expensive and require a vast installation site, limiting ≥1-GHz NMR spectroscopy to a few labs in the world. A benchtop microcoil NMR magnet by definition is compact and thus its cost will be less by nearly an order of magnitude than that of the standard NMR magnet, and placeable on a workbench. Also, LHe-free operation enables the user to be independent from a cooling source in short supply. As a preliminary work (R21GM129688), we have completed a 12.5-mm-cold-bore HTS REBCO magnet prototype and successfully operated it up to 25 T at 10 K cooled by a cryocooler only, without liquid helium, verifying our high-field REBCO magnet design with the proposed screening-current reduction method. Based on these preliminary results and pioneering design concept, we will first design and build a cryogen-free, shielded all-REBCO 23.5-T/25-mm-RT-bore magnet having a 5-gauss fringe field radius of 1.5 m, and then convert this non-NMR-field 23.5-T magnet to a benchtop 1-GHz microcoil NMR magnet having a high homogeneity of <0.1 ppm over a 5-mm-diameter, 10-mm-length cylindrical volume. We also intend to use an in-house built NMR probe to demonstrate the proposed magnet. This benchtop magnet will incorporate all the innovative design and operation concepts validated by the prototype magnet in our preliminary R21 program: 1) all-HTS composition and operation at above 4.2 K cooled only by a cryocooler, the first ever >4.2-K operation among all ultra-high- field superconducting magnets; 2) extremely-thin-copper-layered no-Insulation winding technique that makes a REBCO magnet very compact, mechanically robust, and self-protecting; 3) a single coil formation that leads, compared with the traditional multi-nested high-field NMR magnet, to simpler and more affordable manufacturing processes; 4) operational temperature-controlled screening-current reduction method which reduces peak stresses within the REBCO coil and field errors; and 5) cryogenic design for conduction-cooling operation. We intend to adopt a passive shielding by using iron to reduce the 5-gauss radius within 1.5 m. To achieve a target field homogeneity, we will adopt three—superconducting, ferromagnetic, and room-temperature—shimming technique which will be complemented by our 1.3-GHz/54-mm high-resolution NMR magnet, currently under development at the FBML, for which we are developing innovative field-shimming techniques. We envision this benchtop cryogen-free 1-GHz microcoil NMR magnet will become a very powerful and affordable research tool for the NMR based structural biology community who eagerly anticipates higher operating frequencies. We believe that the enabling technologies of our proposed benchtop magnet is poised to lead HTS magnet technology to liquid-helium-free >1-GHz NMR magnets.