Optimization of Two-Step Heat Treatments of Bronze-Route Conductors for High-Field Applications

Until now, the superconducting high field facility HOMER II of the Forschungszentrum Karlsruhe has been designed to reach magnetic field strengths of about 20 Tesla. To increase the field strength even further, two different methods are used. One is the building of a HTSC-insert coil, the other the manufacture of one or more (Nb,Ta,Ti) ₃Sn coils. These coils will operate in magnetic fields close to the upper critical field of the conductors, so special attention has to be focused on the heat treatment of the wires, which strongly influences the physical properties critical current and n-value but also upper critical magnetic field and maximum pinning force density. In this paper we will present results of two-step heat treatments on an alloyed bronze route Nb₃Sn conductor. Differences between one- and two-step heat treatments will be discussed as well as the optimum heat treatment for different background magnetic fields