Heat Treatment Optimization of Rutherford Cables for a 15-T Nb$_3$Sn Dipole Demonstrator

Fermi National Accelerator Laboratory has been developing a 15-T Nb$_3$Sn dipole demonstrator for a future very high energy pp collider based on an optimized 60-mm aperture four-layer “cos-theta” coil. To increase the magnet efficiency, the coil was graded by using two cables with same 15 mm width and different thicknesses made of two different restacked rod process (RRP) wires. Due to the nonuniform field distribution in dipole coils, the maximum field in the inner coil will reach 15-16 T, whereas the maximum field in the outer coil is 12-13 T. In preparation for the 15-T dipole coil reaction, heat treatment studies were performed on strands extracted from these cables with the goal of achieving the best coil performance in the corresponding magnetic fields. In particular, the effect of maximum temperature and time on the cable critical current was studied to take into account actual variations of these parameters during coil reaction. In parallel and in collaboration with Oxford Instruments-Superconducting Technology, development was performed on optimizing Nb$_3$Sn RRP wire design and layout.