Magnetic characterization of Nb3Sn strands under applied strain conditions

We performed magnetic characterizations, using a vibrating sample magnetometer, of an internal tin-type Nb3Sn technological strand. In order to compare the properties of this sample under different strain conditions, the same strand was tested under compression, obtained by swaging and compacting into thin stainless steel tubes before the heat treatment, as well as after a complete relaxation of the filament strain components, by chemical etching of the Cu and bronze matrix components. In particular, we analysed the Kramer extrapolated upper critical field Bc2K, and the zero-field critical temperature Tc(0) and its distribution, for all the samples. In order to clarify the effect of the strain due to both the stainless steel reinforcement and the copper matrix thermal pre-compression, we compared our experimental results with the predictions from well established scaling laws based on uniaxial models. Good agreement has been found with measured critical temperature Tc and extrapolated upper critical field Bc2K values, with proper choice of fit parameters. On the other hand, we observed a broadening of the superconducting Tc transition with increasing strain. This has to be ascribed to the effect of radial strain components, which cannot be neglected in the present work.