Study on trapped field characteristics of HTS bulk annuli with iron rings for ferromagnetic shimming of a compact NMR magnet

Recently, the performance of high temperature superconducting (HTS) bulks such as critical current density, size, and mechanical strength has been improved rapidly. So, various applications with HTS bulks such as motors, bearings, and flywheels have been investigated by many research groups. A compact nuclear magnetic resonance (NMR) magnet is one of the new applications after the technique to enhance maximum trapped field of the HTS bulk more than 11.74 T, a corresponding ¹H NMR frequency of 500 MHz, has been developed. This new compact NMR magnet out of HTS bulks is far less expensive than those conventional NMR magnets and expected to be widely used in food and drug industry. In design and manufacture of those compact NMR magnets, spatial field homogeneity of large trapped magnetic field in HTS bulk annuli is essential. This paper presents the magnetic field distribution in single and three assembled HTS bulk annuli, measured by a 3-axis hall sensor, and experimental results of its spatial homogeneity improvement by mounting an iron ring inside or outside of the HTS bulk annuli.