Development of a Nb₃Al Conductor to be Applied to a Fusion Reactor and Its Application to a Large Superconducting Coil

Abstract

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.