Critical current changes in Nb3Sn irradiated with fast neutrons at 6 K

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Abstract
Multifilament copper‐clad wires of Nb3Sn have been irradiated at 6 K to a dose of 1.8×1018 n/cm2 (E≳0.1 MeV). The critical current (Jc) was measured as a function of dose and applied magnetic field to 33 kOe. After 1.2×1018 n/cm2, an increase of 33% in Jc occurred at 33 kOe from an initial value of 1.5×106 A/cm2 at 4.5 K. The increase was smaller at lower applied fields, and Jc actually decreased for HJc eventually saturated and then decreased as the dose increased, with the saturation dose increasing with increasing applied field. The critical temperature (Tc) changed by Jc recovered after a 295 K anneal. The Jc changes are explained as an increase in flux pinning from radiation‐induced defect cascades that leads to an increase in Jc greater than the decrease in Jc which results from a decrease in Tc of 1 K and from the increase in Hc2 produced by the increase in normal resistivity. A model is proposed that allows the prediction of the dose dependence of various terms in the pinning‐force density and explains the field and dose dependence of the Jc changes. Measurements of the superconducting–to–normal‐state thermal transition indicate that the decrease in thermal conductivity of the Cu cladding is important when considering the degradation of the thermal transition current that occurs at high doses.