Optimization of High-Temperature Superconducting Coil Employing Edge Vacancies and Central Separation

In this paper, we propose a new high-temperature superconducting (HTS) coil with shorter HTS tape length. In general, the critical current and n-value of an HTS tape depend on the applied magnetic field magnitudes and angles. Therefore, we derived fitting equations for the critical current and n-value in terms of these parameters obtained from the measurements of a Bi-2223/Ag tape at 77 K. Relatively large electric fields were generated at the coil edges because of the large magnetic fields and angles. This phenomenon prevents improvement of transport-current performance of an HTS coil. To solve this problem, we created a new HTS coil with edge vacancies and central separation. Analysis results revealed that the coil critical current increased significantly. Furthermore, we found an optimum coil shape that results in the improvement of the central magnetic field and stored energy by 33% and 99.5%, respectively, compared with those of the normal coil, irrespective of the reduction in the total HTS tape length owing to edge vacancies.