Study on the Magnetic Field Homogeneity of Stacked HTS Bulk Magnets Including the Deteriorated HTS Bulk by Crack for Compact NMR Relaxometry

We have been studying the compact magnet for NMR device that consists of a stacked high temperature superconducting (HTS) GdBCO bulk annuli trapped by a field cooling (FC) 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/cm³ 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/cm³ respectively. Therefore, we have been investigating the development of the compact magnet for NMR relaxometry device. In our previous study, 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/cm³ field homogeneity was obtained using the fabricated field compensation methods on inner diameter of 20 mm HTS bulk magnet. However, it is expected that the asymmetric problem of the magnetic field uniformity will be occurred by the crack in the HTS bulks with long-term operation. Therefore, in this study, the field homogeneity of HTS bulk magnet by the arrangement of the HTS bulk with crack was investigated using 3-D FEM analysis. We examined the effects of the stacking position of HTS bulk with the crack and the allowable range of degradation of HTS bulk to achieve the target field homogeneity of 150 ppm/ cm³.