Evaluation of Temperature Dependence of Magnetic Field Distributions of Bulk Superconductor Annuli

Industrial applications of melt-processed bulk superconductors with Tesla-class magnetic field are still tremendously limited by not only field strength but also field uniformity. Here, the authors report the method of generating a homogeneous field using bulk superconductor annuli. Two annuli with their c-axes along the vertical direction, whose dimensions are 80-mm outer and 50-mm inner diameter and 20-mm thickness, without a nonmagnetic metal ring and epoxy resin for additional mechanical strength, respectively, were vertically constructed with various gaps. Their field uniformity was simulated with changing parameters, difference of critical current density, gaps, and all. Field distributions in the composition with optimized gap (~13 mm), in terms of field uniformity, were measured. As a result, the field strength of 0.92 T and about 2% homogeneous volume in 20-mm-diameter spherical volume were achieved. Furthermore, by a decrease in operating temperature by a cryocooler, field strength was over four times stronger at 50 K than at 77 K. It is clarified that the strong and homogeneous magnetic field can be generated by optimizing the composition and operating temperature.