Recent Progress in Developing Nb₃Sn Conductors With Increased Specific Heat

A possible approach to reducing Nb3Sn magnet training is to increase the energy margin of Nb3Sn conductors by enhancing their specific heat (C-p). We have been developing Nb3Sn conductors with increased C-p by incorporating substances with high C-p at 2-10 K based on a conductor design that is compatible with standard Nb3Sn strand production. In the past couple of years our efforts have been mainly focused on improving strand design (e.g., position of high-C-p filaments, thickness of the Cu tube for the high-C-p filaments, ratio of the Cu powder to the high-C-p substance, filament spacing, etc.) in order to obtain good strand drawability and to reduce degradation after rolling, which is needed for production of Rutherford cables. We also tried a new high-C-p substance, Gd2O2S, and verified that it has much higher C-p over the whole magnetic field range than the Gd2O3 we used before. With that development work we can now produce high-C-p strands with good drawability and low levels of degradation after rolling. This paper reports our findings and the current status of the development of high-C-p Nb3Sn conductors.