Reduction of AC losses in superconducting wire with very fine filaments by weakening flux-pinning strength

For use of superconducting wires in ac conditions, the ac loss is required to be a certain level. For this purpose, the diameter of superconducting filaments is reduced in order to decrease the pinning loss in filaments, and wires are tightly twisted in order to decrease the coupling loss in normal matrix. Recent developments in fabrication technique of ultrafine superconducting filaments have made superconducting wires for ac use possible. Further reduction in ac loss is desired to improve the efficiency of superconductors in ac equipments. Since the pinning loss is dominant in such wires, the reduction of this loss is important. When the filament diameter is reduced, the distance between filaments should be reduced so as to keep the mean current density constant. Reduction of the distance below a certain level causes coupling between filaments due to the proximity effect. The effective diameter of filaments becomes larger than the designed value, resulting in fairly large pinning losses. It is necessary to suppress the proximity coupling by a proper choice of the matrix material and optimizing the distance between filaments. The pinning loss inside the filament should also be reduced by changing the flux pinning strength. In this work, the authors verify that the pinning losses in superconducting wires are reduced by changing the flux pinning strength