Heat Treatment Optimization on Nb$_{3}$Sn Strands Based on Electrical and Physical Properties

The electrical and physical properties of Nb
$_{3}$
Sn strands are strongly dependent on the heat treatment during which tin diffuses into niobium by solid-state diffusion. During diffusion, 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 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 1
$\,$
mm diameter, internal Sn Nb
$_{3}$
Sn strands produced by Kiswire Advanced Technology (KAT) for two React&Wind conductor prototypes for the Toroidal Field Coil of the EUROfusion DEMO: a
$\mathbf {66}\,$
kA/
$\mathbf {12}\,$
T and a
$\mathbf {105}\,$
kA/
$\mathbf {12}\,$
T prototype. For the optimization, four heat treatment schemes were considered and their evaluation based on I
$_{c}$
measurements at 4.2
$\,$
K, in the range of 9
$\,$
T to 15
$\,$
T, on SEM micrographic studies on grain size and shape and on hysteresis loss measurements on a vibrating sample magnetometer (VSM). Based on these results, a heat treatment schedule is proposed for the prototype DEMO conductor and the scaling law for J
$_{c}$
is updated.