The Japan Atomic Energy Research Institute has been involved in developing Nb3Al conductors since the middle of 1980s based on the consideration that Nb3Al conductors are capable of producing a higher magnetic field than Nb3Sn conductors owing to the extremely high critical current density in high magnetic fields. At the beginning of the development work, a fabrication technique for Nb3Al strands using a Jelly-roll process was established. This process requires heat treatment at 750°C for 50 h instead of a temperature of more than 1,800°C as required by the conventional method. Using this technique, about 1 ton of strands was produced and a 150-m Nb3Al cable-in-conduit conductor was fabricated. For the next step, to demonstrate the applicability of the Nb3Al conductor to a large coil, a coil 1.5-m in diameter, called the Nb3Al Insert, was manufactured. A react-and-wind method was tried for the production process as it simplifies the fabrication of large coils such as an ITER-TF coil. Performance tests of the Nb3Al Insert were conducted in 2002. The Nb3Al Insert could be charged to the designed point of 13 T and 46 kA without showing any instability. Thus, the world's first large superconducting coil using a Nb3Al conductor was successfully developed, thus indicating the possibility of producing fusion magnets that can operate in higher magnetic fields than those used with Nb3Sn conductors.
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, althoug