Artificial Pinning in Nb$_3$Sn Wires

Two different processes are known to create “artificial” pinning centers in superconductors: 1) high energy irradiation; and 2) nanoinclusions. Own results on the critical current density $J_c$ of $Nb_3Sn$ wires after irradiation by protons are compared to the effects obtained by neutrons (on the same wires) and by nanoinclusions from the literature. Characteristic differences and similarities are discussed. After high energy irradiation, the observed enhancement of $J_c$ is attributed to the formation of defect clusters. The contributions due to grain boundary pinning on one hand and of point pinning on the other can be separated, using a two-component model. This is in contrast to wires with nanoinclusions, where point pinning effects occur simultaneously to those due to enhanced grain boundary pinning, as a consequence of a reduction of the A15 grain size. In both cases, the enhancement $ΔJ_c$ in $Nb_3Sn$ wires decreases gradually with increasing applied field and vanishes at $B_{c2}$. In binary $Nb_3Sn$ wires with oxide nanoinclusions, an increase of $J_c$ by $100 A/mm^2$ with respect to the present RRP wires is expected at fields between 17 and 18 T.