Compressive pre-strain in high-niobium-fraction Nb3Sn superconductors

Multifilamentary Nb₃Sn superconducting strands fabricated with high niobium fractions have exceptionally high critical-current densities but are sometimes marginally stable during testing. We report a technique for determining the pre-strain in such conductors, in which additional stabilizing copper is electroplated onto the conductor and the pre-strain is determined by extrapolation to the as-fabricated niobium fraction. This technique is used to measure the pre-strain in conductors with high niobium fractions of 20% to 30%. Values of the pre-strain ε_max in these conductors are reduced to the range 0.1% to 0.2%, which is significantly less than the ε_max values of 0.2% to 0.4% in traditional bronze-process Nb₃Sn conductors (where niobium fractions are typically about 10% to 15%). However, including about 20% dispersion-strengthened copper into the conductor matrix restores ε_max to the range 0.25% to 0.35%, thus providing practical levels of ε_max for magnet design in high-niobium-fraction strands.