Evaluation of strain dependence of critical current of Nb3Al conductor for coils with react-and-wind method

The Nb3Al is one of the attractive superconducting materials for future fusion devices like DEMO because of its higher critical current density (Jc) than Nb3Sn at around 16 T. In cable-in-conduit (CIC) conductor like the ITER conductor, strands suffer thermal and bending strain. The Jc of Nb3Al strand is generally decreased by longitudinal strain. However, in previous work, no degradation of Jc by bending strain of 0.4% was observed for a Nb3Al D-shaped coil made by react-and-wind (R(and)W) method in the R(and)D campaign for the JT-60 superconducting modification (JT-60SA). The R(and)W method allows us to heat-treat conductor in small furnace and to wind into coil along with insulation after heat treatment like NbTi conductor. This method considerably reduces manufacturing cost and time schedule. This also indicates that large complicated coils can be fabricated by Nb3Al CIC conductor, if the bending limitation and the mechanisms are clarified. In this work, in order to clarify the bending limitation, a new experimental apparatus was developed to measure bending strain dependence of critical current (Ic) of a CIC conductor wound like a torsion coil spring. The CIC conductor sample consists of 54 Nb3Al strands and 27 Cu wires contained in a stainless steel conduit and formed in a spring-shape. The results of bending strain dependence of Ic shows that the no degradation of Ic up to 0.4% of bending strain and Ic at 0.55% of bending strain was 96% of Ic at 0%. The precise mechanism of new apparatus and FEM analysis result of CIC conductor sample will be presented. The application of these results to coil fabrication will be also discussed. (author)