Effect of partially reacting Nb3Sn before magnet winding on the strand critical current

Nb3Sn is currently the material most commonly foreseen for the development of high field superconducting magnets. This is done by either the wind and react technique, usually Z used for compact magnets like accelerator magnets, or the react and wind technique, more common on large scale magnets. In both cases, a thorough understanding of the Cu-Sn system diffusion and phase formation processes is necessary to optimize the Nb3Sn reaction cycle. Attention has to be paid to both the superconducting performance and the prevention of thermally induced damage of the final produced conductor. The formation of the eta and epsilon phases of the Cu-Sn phase diagram was investigated as a function of time and temperature. Wherever possible, the activation energies and diffusion coefficients were calculated. The feasibility of winding partly reacted cables to reduce the manufacturing time was also explored. Nb3Sn strands have been partially reacted to convert the Sri to the eta and epsilon phases of the Cu-Sn phase diagram, and then plastically strained to figure out a cabling and/or a winding degradation. After completion of the reaction cycle, the critical current was measured and compared with that obtained with an uninterrupted cycle.