Asymmetric behaviour of Jc(ε) in Nb3Sn wires and correlation with the stress induced elastic tetragonal distortion

The effect of uniaxial strain on the critical current of 0.8 m long Nb3Sn wires up to 21 T is studied by the modified Walters spring (WASP). For Nb3Sn wires, prepared by both the bronze route and the internal Sn diffusion process, the critical current density as a function of the uniaxial strain epsilon is found to exhibit an asymmetric behaviour on both sides of the strain epsilon m, where J(c) reaches its maximum. Revisiting earlier x-ray and neutron diffraction measurements on bronze route processed wires between 10 and 600 K, it is shown that the asymmetric behaviour of J(c) (epsilon) on both sides of the strain value epsilon(m) is connected to individual variations of the stress-induced tetragonal lattice parameters a and c. The present measurements of J(c) versus strain for Nb3Sn wires show stronger strain dependence for wires prepared by the internal Sn diffusion method with respect to those obtained by the bronze route. The reasons for this difference are attributed to the individual details of the filament configuration in both types of wire, for example the different Sn distributions inside the filaments and the very different filament sizes, 4 and 80 mu m, respectively.