Development of advanced Nb3Al superconductors for a fusion demo plant

A Nb3Al superconductor inherently has the outstanding features of a high critical field and an excellent strain tolerance in critical current performance. The Japan Atomic Energy Research Institute developed the world's first large Nb3Al coil usin-jelly-roll processed Nb3Al strands. This coil was successfully operated up to the nominal current and field of 46 kA and 13 T, respectively. The test results demonstrated that a Nb3Al conductor is suitable for application in high field, large magnets, such as the toroidal field (TF) coils in a fusion reactor. In parallel with this work, a Nb3Al strand having a high critical current at high field has been developed by the National Institute of Materials Science. This strand cannot incorporate enough copper stabilizer, resulting in poor stability, since it is heat-treated at a much higher temperature than the melting temperature of copper. In order to find a solution to this issue, we performed an analytical study, and it showed that the externally incorporated copper after the high temperature heat treatment is effective for stabilization when the electric conductance and heat transfer coefficient between the Nb3Al strand and the external copper are more than 10 MS m(-1) and 10 kW m(- 2) K- 1, respectively. Since they seem to be attained by using present conductor technologies, the development of a TF coil operated at a high field around 16 T seems promising.