Ongoing Efforts at Internal-Tin Nb3Sn Strand With Higher Jcn and Lower Hysteresis Loss

Western Superconducting Technologies Co. Ltd. (WST) has been keeping in the research and development of internal-tin Nb 3 Sn strand for higher performance level, for the future fusion reactor after ITER. Deformation optimization, Cu splits, and Sn spacers were applied to prepare Nb 3 Sn strand with higher $J_{{\rm{cn}}}$ and lower hysteresis loss, and the effect of these three factors on performance was investigated and discussed. Compared with the Nb 3 Sn strand for ITER, strand distortion was obviously reduced by deformation optimization and $J_{{\rm{cn}}}$ was improved, but at the same time hysteresis level was increased. To reduce the level of hysteresis loss, Cu splits were used to replace a few Nb filaments in each subelement. Cu splits could break the bridging Nb 3 Sn filament rings and as a sequence hysteresis loss was dramatically reduced along with the reduction of $J_{{\rm{cn}}}$ , which is attributed to low reaction degree of Nb filaments in the outermost subelements. Therefore, Sn spacers between the outermost subelements and the Ta barrier were added, and a 37-subelement structure was used to shorten the diffusion path of Sn. The reaction degree of the filaments was enhanced as well as $J_{{\rm{cn}}}$ . Finally, it could be concluded that a 37-subelement structure with Cu splits is effective to reduce the level of hysteresis loss and raise the level of J cn .