Development of Single-domain Annular REBCO Bulk Superconductors for Compact Cryogen-free NMR

In order to develop a compact cryogen-free 4.7-T (200 MHz) NMR magnet, we investigated methods of obtaining a homogeneous magnetic field distribution inside REBCO bulks. In this study we successfully fabricated large single-domain annular GdBCO bulk superconductors 60 mm in diameter and up to 60 mm thick. We also fabricated a large EuBCO bulk 60 mm in diameter, and evaluated the performance of these annular bulks in terms of the NMR magnet. It was found that enhancing Jc-B, increasing bulk thickness and employing an Eu element showing low magnetic permeability were effective in increasing the homogeneity of the magnetic field distribution. Furthermore, it was confirmed that a homogeneous area of magnetic distribution was obtained even at a higher field magnetization of 7 T (300 MHz) at 40 K for stacked GdBCO bulk superconductors 60 mm in diameter and 60 mm thick. Finally, several issues relating to further increasing the trapped magnetic field were discussed.