Study on the shape optimization of the HTS bulk magnets with active compensation for compact NMR relaxometry magnets

Compact NMR magnets consisted of the stacked HTS bulk annuli trapped by a field cooling (FC) method have been suggested and 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 the conventional NMR device. It is relatively easy to generate a magnetic field over 1.5 T at 77.4 K using the stacked HTS bulk annuli, but it is still hard to obtain 150 ppm/cm³ field homogeneity with the conventional superconducting magnet (SCM) as the magnetizing magnets. In this paper, to improve the trapped magnetic field homogeneity and to obtain the enlarged sample space of HTS bulk magnet for compact NMR relaxometry, the HTS bulk magnet with 10 mm gap length in the center region of HTS bulk magnet (we call it 'split coil-shaped HTS bulks') were proposed and studied as the functions of size and shape of HTS bulk. It was studied using 3D FEM based electromagnetic analysis. The improved field homogeneity was obtained by using hybrid models consisted with the HTS bulk and attached small size HTS coil.