Experimental Study on the Critical Current Properties of Flexible Nb3Al Superconducting Wires in Cryocooler System

In recent years, Jelly-rolled Nb/Al composite monofilament Nb
₃
Al superconducting wires with an outer diameter of 30 μm, and multi-stranded Nb
₃
Al wires were successfully fabricated by NIMS in Japan. So, it is necessary to measure the temperature dependence of the critical current of Nb
₃
Al wires in cryocooler system for development of superconducting applications cooled by conduction cooling method. The single and multi-stranded ultrafine Nb
₃
Al superconducting sample wires with different outer diameter were prepared to evaluate the critical current properties. The critical currents of sample wires were evaluated by the current sweep method and constant current method. In this study, the sample wires were soldered to the copper wire to prevent burnout of the sample wire during critical current measurements. Therefore, the amount of current shunted to the copper wire was estimated to accurately evaluate the critical current of the superconducting wire. Also, the critical current of Nb
₃
Al wires due to bending strain (bending radius of 5 to 15 mm) was experimentally investigated. From the critical current measurement results of Nb
₃
Al wires with different cross-sectional structures, it was confirmed that the superconducting properties of the wires were improved by reducing the wire diameter. It was shown that the critical current density of the developed Nb
₃
Al wire is lower than that of REBCO wire, but higher than that of commercial NbTi and Nb
₃
Sn wires.