Speaker
Description
The electrical and physical properties of $\mathrm{Nb_3 Sn}$ strands are strongly dependent on the heat treatment during which tin diffuses into niobium by solid-state diffusion. During diffusion, $\mathrm{Nb_3 Sn}$ grains grow at the Nb/bronze interface. The shape and size of the grain depend on the temperature of the last step of the heat treatment, its duration and the size of the Nb filaments. The volume of reacted $\mathrm{Nb_3 Sn}$ together with the grains’ structure influence the non-copper critical current density $j_c$ and the magnetization. Therefore, an optimization of the heat treatment with respect to $j_c$ and hysteresis loss is important when working on the design of superconducting cables. This contribution presents the results of a heat treatment optimization performed on a 1mm diameter, internal Sn $\mathrm{Nb_3 Sn}$ strands produced by KAT (Korea) for a $66\,$kA / $12\,$T prototype React&Wind conductor for the Toroidal Field Coil of EUROfusion DEMO. The heat treatments and the $j_c$ measurements on ITER barrels were performed at SPC (Villigen, Switzerland) at $4.2\,$K, in the range of $9\,$T to $15\,$T, the SEM micrographic studies on grain size and shape were conducted at CERN and the hysteresis loss was measured on a vibrating sample magnetometer (VSM) at ENEA (Frascati, Italy). Eventually, a heat treatment schedule is proposed for the prototype DEMO conductor and the scaling law for $j_c$ is updated.
