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Multifilamentary Nb/sub 3/Sn 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 /spl epsiv//sub max/ in these conductors are reduced to the range 0.1% to 0.2%, which is significantly less than the /spl epsiv//sub max/ values of 0.2% to 0.4% in traditional bronze-process Nb/sub 3/Sn conductors (where niobium fractions are typically about 10% to 15%). However, including about 20% dispersion-strengthened copper into the conductor matrix restores /spl epsiv//sub max/ to the range 0.25% to 0.35%, thus providing practical levels of /spl epsiv//sub max/ for magnet design in high-niobium-fraction strands.

Measurements of the effects of static and cyclic (fatigue) transverse stress on the critical current of Ya-Ba-Cu-O coated tapes made by ion-beam-assisted deposition (IBAD) are reported at 76 K in self magnetic field. Ya-Ba-Cu-O films (/spl sim/1 /spl mu/m thick) on Inconel substrates (100 /spl mu/m thick) with IBAD buffer layers had critical-current densities J/sub c/ exceeding 1 MA/cm/sup 2/ and n values of about 50 (where n is an index of the sharpness of the superconductor-to-normal transition). Under static loads of 100 MPa, the degradation in J/sub c/ was less than 5% (7% at 120 MPa). When subjected to cyclic loading, there was less than 2% additional degradation in J/sub c/ after 2000 fatigue cycles. Microscopic examination of the samples indicates that this limited J/sub c/ degradation may have arisen from longitudinal cracks forming near the edges of the sample after being subjected to these high transverse pressures. This fracture mode would indicate that longitudinal side support from high-yield substrates or epoxy impregnation of magnet structures may provide additional tolerance against degradation in J/sub c/ from transverse stress.

The critical current density of Hot Isostatic Pressed (HIP'ed) and unHIP'ed Nb/sub 3/Sn Modified Jelly Roll wires has been measured as a function of magnetic field and of strain at 4.2 K. The reversibility of critical current density was investigated for both wires. The critical current and upper critical field were decreased for the HIP'ed sample. The reduced upper critical field of the HIP'ed wire was found to be less sensitive to strain than the unHIP'ed wire. The index (m) of the flux pinning scaling law is found to have increased from 0.86 to 2.14 as a result of the HIP processing.

A new probe has been developed to investigate the effect of strain and temperature on the transport critical current density of superconducting wires and tapes in high magnetic fields. The critical current of a multifilamentary niobium-tin wire has been measured as a function of magnetic field at 12 K, for both compressive and tensile strain. The ability to simultaneously vary strain, temperature and field facilitates a better understanding of how these parameters affect flux pinning mechanisms. From a technological point of view, these measurements provide essential design data for developing cryocooled high magnetic field systems.