Nb₃Alインサート・コイルの臨界電流性能評価

抄録

Nb₃Al superconductor has the potential capability of producing a higher magnetic field than that by Nb₃Sn and is less sensitive against strain, which will simplify the fabrication of a large coil. From this point of view, development work for a Nb₃Al conductor (cable-in-conduit conductor with stainless steel conduit) was started at the Japan Atomic Energy Research Institute and the Nb₃Al Insert was successfully developed and tested in 2002. In the tests, a nominal current of 46 kA was achieved in a 13 T magnetic field and critical currents were measured in terms of magnetic field and temperature in order to clarify the strain state of the cable, which determines the overall performance of the conductor. From the results, the thermal strain of the Nb₃Al conductor was estimated to be around −0.4%, corresponding to a 10% reduction in the critical current. This is much smaller than that observed in the same type of Nb₃Sn conductor (−0.7% strain and 50% reduction). In addition, there was no effect of the electromagnetic forces on critical current observed in the Nb₃Al conductor during coil charge while an evident reduction has been reported for Nb₃Sn conductors. The higher rigidity of the Nb₃Al strands and smaller sensitivity of the critical current against strain can account for these characteristics. Furthermore, although the insert was fabricated using a react-and-wind method, this did not produce any observable effect on the strain of the cable. These results demonstrated the excellent performance of the Nb₃Al conductor to be used for a high-field, large coils that will experience large electromagnetic forces.