Effects of neutron irradiation on pinning force scaling in state-of-the-art Nb₃Sn wires

We present an extensive irradiation study involving five state-of-the-art Nb₃Sn wires which were subjected to sequential neutron irradiation up to a fast neutron fluence of 1.6 × 10²² m⁻² (E > 0.1 MeV). The volume pinning force of short wire samples was assessed in the temperature range from 4.2 to 15 K in applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated state and after each irradiation step. Pinning force scaling computations revealed that the exponents in the pinning force function differ significantly from those expected for pure grain boundary pinning, and that fast neutron irradiation causes a substantial change in the functional dependence of the volume pinning force. A model is presented, which describes the pinning force function of irradiated wires using a two-component ansatz involving a point-pinning contribution stemming from radiation induced pinning centers. The dependence of this point-pinning contribution on fast neutron fluence appears to be a universal function for all examined wire types. (paper)