Changes in the upper critical fields and critical currents of Nb3Sn and V3Ga owing to neutron radiation damage

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Abstract
Ambient-temperature reactor irradiations of Nb3Sn and V3Ga filamentary superconductors have produced increases in the critical current Ic over a low-fluence regime <∼2×1018 n/cm2 (E≳1.0 MeV) followed by severe reductions of Ic at higher fluences as the critical temperature Tc→0. The low-fluence increases in Ic are attributed to increases in the upper critical field Hc2 with increasing resistivity. In V3Ga, however, because of paramagnetic effects, it appears that the changes in the Ginzburg-Landau upper critical field H*c2(GL) govern the behavior of Ic, and not the measured Hc2 as is the case in Nb3Sn. In order to compare the measured Hc2 of V3Ga with estimates based upon H*c2(GL), spin-orbit scattering must be included in the Maki analysis of Hc2. The spin-orbit scattering does not increase with increasing normal-state resistivity, but rather appears to be decreasing. From the irradiation behavior of Hc2 and H*c2(GL) in V3Ga it is argued that in treatments such as flux-pinning analysis where a reduced field h≡H/Hc2 is used to established scaling laws, it is necessary to use a reduced field h*=H/H*c2(GL) because of the paramagnetic limiting effects.