Variation of the Critical Properties of Alloyed Nb-Sn Wires After Proton Irradiation at 65 MeV and 24 GeV

A recent proton irradiation study on Ta and Ti alloyed industrial multifilamentary Nb_3Sn wires has now been extended to fluences up to 1.38 × 10^21 p/m^2, the Bragg peak region being located outside of the wire. In the present work, magnetization measurements were performed up to 14 K and 10 T, to follow and determine with more precision the development of the upper critical field B_c2 with fluence. It was found that the critical temperature, T_c, decreases linearly with increasing fluences, about 3% up to the highest fluence 1.38 × 10^21 p/m^2. The transition width does not change after irradiation, thus reflecting a homogeneous damage in analogy to neutron irradiation. Both the critical current density, J_c, and the upper critical field, B_c2, were found to increase in the considered fluence range. It was obtained that the larger enhancement of J_c, (about 45% for Ta alloyed wires and 100% for Ti alloyed wires at 10 T) is not correlated to that of B_c2 (about 5% for Ti alloyed and 10% for Ta alloyed up to the highest fluence). The enhancement of J_c in Ta alloyed wires is very similar for both PIT and RRP processing, thus assigning a major importance to the nature of the additive. The present results after irradiation were analysed applying the two-mechanism model on the volume pinning force, taking into account both grain boundary pinning and point pinning. A comparison between the present results and those achieved after neutron irradiation on the same Nb_3Sn wires shows that protons cause considerably higher damage than neutrons: the same effect on J_c and T_c is already observed at fluences being one order of magnitude smaller.