Performance improvements and optimization of new high-temperature superconducting coil with circle vacancies at edge portions

In general, Bi-2223/Ag tape wound on a high-temperature superconducting (HTS) coil has a critical current and n-value, which depend on magnetic field magnitudes and angles at constant temperature. The fitting equations of the critical current and n-value of the tape having parameters of magnetic field magnitude and angle are derived by measuring critical currents and n-values at various magnetic field magnitudes and angles at a temperature of 77 K. Employing these fitting equations, we established a model which enables us to analyze the transport-current performance of an HTS coil. This analysis clarified that in the edge portions of the coil, relatively large electric fields are generated, preventing the transport-current performance from undergoing further improvements. This paper proposes a new HTS coil whose edge portions have circular vacancies caused by dividing the edge portions and by shifting these divided small coils. The analysis of the proposed coil found that the coil's critical current, i.e., the transport-current performance, has largely improved and that the proposed coil is optimized. This optimum coil results in improvements in the stored energy and central magnetic field of 65% and 33.6%, respectively, compared with those of a normal rectangular coil having the same tape length.