Enhancement of the critical current of multifilamentary Nb3Sn conductors by tensile stress

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Abstract
The critical current Ic and the strain ε within multifilamentary Nb3Sn conductors subjected to a uniaxial tensile stress were measured. Ic increases with increasing ε and passes through a maximum Icm at a strain εm. The ratio Icm/Ic is a function of ε−εm only and independent of conductor type. As the ratio γ of the cross‐sectional area of the conductor taken up by Nb3Sn to that of bronze decreases, εm increases. For γ=0.06, the smallest value used, εm=0.9% and Icm/Ic0=1.4 at 4.4 T (Ic0: initial value of Ic). This effect occurs because the coefficient of thermal expansion is greater for bronze than for Nb3Sn. The resulting pressure to which the Nb3Sn is subjected at 4.2 K reduces Ic. Initially, an externally applied axial stress reduces the pressure upon the Nb3Sn and produces an increase in Ic. Further increase of the stress exerts a resultant tensile stress upon the Nb3Sn and brings about a reduction in Ic. The technical applicability of the conductor is improved by the internal compressive stress since less degradation in Ic occurs at high tensile loads when compared to the case of a stress‐free conductor.