In recent years, Benchtop NMR using permanent magnets has a global market even it signal have insufficient sensitivity and the spectral separation capability is poor due to the limited magnetic field strength of the permanent magnet. The economic and physical constraints of conventional superconducting magnets, as pointed out by Longstaffe et al. [1], limited the possibilities of NMR. As a result, Benchtop NMR shows great potential for the future NMR application.
On the other hand, we have focused on bulk superconductors and sought a way to achieve Benchtop NMR with superconducting magnets[2,3]. Our established superconducting bulk magnet, we have achieved a spatial resolution of 50 μm meter and showed it will be an MRI apparatus for small object (less 10 mm). Our ultimate goal is to realize high-resolution NMR using a portable superconducting bulk magnet. The superconducting bulk was designed to be kept below the critical temperature (EuBCO Tc : 92 K) of 50 K or less by conduction cooling, with a magnetic field strength of 4.75 T (1H resonance frequency: 202 MHz) and a room temperature bore with an inner diameter of 23 mm. The magnet consists of eight vertically stacked annular single domain c-axis oriented Eu-Ba-Cu-O crystals has energized to magnetic field strength of 4.75 T using field cooling method by using conventional superconducting magnets for NMR. The magnet that provides the magnetic field homogeneity required for high-resolution NMR is a bulk magnet with an outer diameter of 64 mm, an inner diameter of 32 mm, and a height of 26 mm at the top and bottom ends, and an outer diameter of 64 mm, an inner diameter of 40 mm, and a height of 82 mm at the middle. In addition, in order to keep about precisely control the homogeneity of the NMR magnet where the magnetic field is applied, a superconducting cylinder with tape wire used for the superconducting coil wrapped around the cylinder is inserted into the bore of the bulk magnet described above. We developed the shim coils, sample spinning system, and D2 locking system required for high-resolution NMR for a room temperature bore of a bulk magnet. As a result, a spectral resolution of less than about 1 Hz was obtained, which can be used for high-resolution NMR using a second-order multichannel shim coil and 20 Hz sample spinning.
Superconducting bulk magnets that enable high-resolution NMR have been developed, but in order to exceed the benchtop NMR of permanent magnets, portability must be demonstrated. Permanent magnets do not lose their magnetic field without a power supply, but bulk superconducting magnets need to be constantly cooled by a power supply. Fortunately, the refrigerator that cools the magnet is operated by a compressor that operates at 100V and less than 15A and is air-cooled, and it can be transported with the magnetic field attached if it has a large-capacity battery. For this reason, we conducted a test of transporting the refrigerator while it was still cold below 50 K, using a large-capacity lithium-ion battery, which has recently become popular, as the power source. We report on the results of this test, as it showed the ability to meet the specifications of NMR even after transportation.
[1] J. Giberson, J Scicluna, N. Legge, J. Longstaffe “Chapter three—developments in benchtop NMR spectroscopy 2015–2020”, Annual Reports on NMR Spectroscopy ed G A Webb (New York: Academic), (2021) 153–246
[2] K. Ogawa, T. Nakamura, Y. Terada, K. Kose and T. Haishi, “Development of a magnetic resonance microscope using high Tc bulk superconducting magnet”, Appl. Phys. Lett, 98 (2011) 234101
[3] T. Nakamura, D. Tamada, Y. Yanagi, Y. Itoh, T. Nemoto, H. Utumi and K. Kose, “Development of a superconducting bulk magnet for NMR and MRI”, J. Magn. Reson. 259 (2015) 68-75
Keywords: Superconducting Magnet, Bulk Superconductor, REBCO, Benchtop NMR