Stress induced normal--Superconducting transition in multifilamentary Nb3Sn conductors

In multifilamentary Nb3Sn conductors produced by a solid state diffusion process the critical current is reduced by the compression which acts on the Nb3Sn due to the stronger thermal contraction of the bronze. Externally applied tensile stress reduces the compression and enhances the critical current Ic. Icruns through a maximum as a function of the strain of the conductor. Icwas measured as a function of the strain up to a flux density of 16 T. Current enhancement factors of about 2 have been observed for technical conductors near 16 T. The results can be described by the theory of E.J. Kramer using a strain dependence of the upper critical flux density Bc2. In flux densities near Bc2of the stress-free state the case is possible that the conductor is not superconducting in the initial state but becomes superconducting beyond a certain strain and returns reversibly into the normal state for higher strain.