Experimental and analytical studies on non-uniformity properties of HTS bulk annuli for high magnetic performance applications

In designing and manufacturing of a new type of compact nuclear magnetic resonance/magnetic resonance imaging magnet, which consists of a stacked high-temperature superconducting (HTS) bulk annuli, the spatial homogeneity and the temporal stability of the trapped magnetic fields are the key issues. On the other hand, there is an asymmetry problem for superconducting properties including the critical current density along the axial (thickness) direction in HTS bulks due to its manufacturing process using a seed crystal located at the top surface. Therefore, in this study, we find the nonuniformity of the critical current density of the HTS bulk along the axial direction, which probably comes from the quench-melt-growth process using a seed crystal. The measured axial positions of the maximum magnetic field were slightly shifted to the bottom direction lower than the axial center position. The critical current density and/or superconducting properties of HTS bulks are probably decreased in proportion to the distance from a seed crystal. Therefore, the superconducting properties along the thickness direction in the HTS bulk were represented as a variable J_c-B and electrical conductivity.