J/sub c/ changes after neutron irradiation of Nb3Sn at 8 K

In order to understand radiation effects in fusion magnet materials under operating conditions, the critical current (J/sub c/) and critical temperature (T/sub c/) have been measured up to 3.2 T as a function of dose after fast neutron irradiation at 60K in Nb3Sn and after thermal neutron irradiation at 80K in 0.1 a/o 235U--Nb3Sn. Experimentally determining an equivalent fast-neutron dose for the latter fission-fragment damage allows comparison of the irradiations. Increases in high J/sub c/ material (1.5 x 106 A/cm2 at 4.5 K and 3.2 T) were observed after 1 x 1018 n/cm2 (E greater than 0.1 MeV). An extension of the dose up to 2.5 x 1019 n/cm2 resulted in decreases in J/sub c/ by a factor of 20 and decreases in T/sub c/ from 18 to 110K. The results are explained by a model that considers enhanced flux pinning (F/sub p/) by the radiation-induced defect cascades at low doses. At higher doses the decreases in T/sub c/ dominate and lower F/sub p/. The field dependence of the F/sub p/ changes can be explained by considering changes in H/sub c2/. The model predicted the observed J/sub c/ changes in the 235U--Nb3Sn. The J/sub c/ changes differ from those found after neutron irradiations at 4000K due to the different flux pinning characteristics of the different defect structures