2017 年 52 巻 1 号 p. 11-17
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).