Development of a Nb_3AI Conductor to be Applied to a Fusion Reactor and Its Application to a Large Superconducting Coil

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.