Study on the Magnetic Field Homogeneity of HTS Bulk Magnets Including the Degraded HTS Bulk for Developing the Compact NMR Relaxometry

We have been studying the compact magnet for NMR device that consists of a stacked higherature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling method. It was difficult to trap the uniform magnetic field above 4.7 T (200-MHz-class NMR magnet) and field homogeneity under 0.01 ppm/cm ³ at liquid nitrogen temperature (77.4 K) because of the low $Jc$- $B$ characteristics of present HTS bulks. On the other hand, the strength and homogeneity of the magnetic field required for NMR relaxometry device are 1.5 T and 150 ppm/cm ³, respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous work, we obtained the trapped magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk magnet with 80-mm height, and 150-ppm/cm ³ field homogeneity was obtained using the fabricated field compensation methods. However, the asymmetric problem of the magnetic field uniformity occurs by the different superconducting characteristics of HTS bulks when we produce the compact NMR magnet by stacking a large number of HTS bulks. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the degraded HTS bulk was investigated using 3-D FEM analysis. We examined the effects of the stacking position of degraded HTS bulk using various $Jc$- $B$ characteristics.