Evaluation of strain applied to strands in a 13 T–46 kA Nb3Al cable-in-conduit conductor

A 13 T–46 kA Nb 3 Al coil, a Nb 3 Al insert, has been developed to demonstrate the applicability of the react-and-wind technique to toroidal field (TF) coils of a fusion reactor, such as ITER. It is estimated that a conductor is subjected to ≈0.4% bending strain after heat treatment when the react-and-wind method is applied to the TF coil fabrication. Therefore, 0.4% bending strain was artificially applied to the Nb 3 Al insert conductor after the heat treatment. Since a stainless steel conduit was used in this conductor, the strands are subjected to the compressive strains due to thermal stress. The effective strains applied to the strands are estimated by comparing the critical current test results of the Nb 3 Al insert at 10–13 T and calculation. Those due to thermal stress and conductor bending were evaluated to be ≈−0.4% and ≈0%, respectively. The effective strain by the conductor bending is sufficiently small so as not to affect the critical current performance of the Nb 3 Al conductor. In addition, the evaluated strain of the Nb 3 Al conductor is compared with those of same scale Nb 3 Sn conductors, in which there is an unexpected strain, which is proportional to the electromagnetic force. Such strain was not observed in the Nb 3 Al conductor. One of the explanations is higher rigidity of the Nb 3 Al strand than the Nb 3 Sn one. This shows that a Nb 3 Al conductor is suitable for application to large magnets, such as the TF coil of the fusion machine, which experiences large electromagnetic force in the conductor.