The discovery of high Tc superconductors has led to the development of novel applications of bulk materials for practical industries, as well to the application of the wiring for metallic low-temperature superconductors. Uniform REBCO bulk materials have been successfully manufactured with the size reaching more than 60 mm in diameter, and these cylindrical superconducting bulk magnets have created applications for compact cryogen-free NMR magnets. In the latest research, a newly designed magnet composed of eight-stacked bulk magnets has been developed. Furthermore, a GdBCO tape was wound around the copper pipe, and this was inserted into the bore of the magnet. The NMR spectrum resolution of 1 Hz at the full width at half maximum was observed on this magnet. This proved that the magnetic field homogeneity required for the high-resolution NMR has been accomplished using a cylindrical superconducting bulk magnet. In the near future, the mechanical strength of the bulk superconductor itself will be improved, leading to the common use of 400 MHz (9.4 T) NMR magnets.
Compact NMR magnets comprised of stacked HTS bulk annuli trapped using a field cooling (FC) method and cooled by liquid nitrogen have been developed. The strength and homogeneity of the magnetic field required for the NMR relaxometry device are 1.5 T and 150 ppm/cm3, respectively, and these values are much lower than a conventional NMR device. The target magnetic-field strength of over 1.5 T at liquid nitrogen temperature can easily be obtained using stacked HTS bulk annuli. However, it is hard to obtain a target field homogeneity above 150 ppm/cm3 using a conventional superconducting magnet (SCM) as the magnetizing magnet. As a result, experimental and analytical studies have been conducted to improve the spatialfield homogeneity of HTS bulk magnets developed for NMR relaxometry. A method for re-applying the magnetic field, a field compensation method using several iron rings and hybrid model magnet comprised of HTS bulk and HTS coil, has been proposed and its effectiveness was confirmed. Finally, asymmetric problems of magnetic-field uniformity occurred owing to the different Jc–B characteristics of the HTS bulk annuli, and the degraded HTS bulk annuli were analyzed using the three-dimensional (3-D) finite element method (FEM).