Analysis of the effect of Nb 3Sn strand bending on CICC superconductor performance

Abstract

Assessment of measured values of the mechanical stiffness of Nb 3Sn cable-in-conduit conductors (CICC) suggests that longitudinal strand bending due to local magnetic loads, transverse to the conductor axis, is sufficiently high to affect the superconductor performance when the magnetic field is over about 10 T. A current diffusion model is applied to simulated current transfer between filaments inside a Nb 3Sn strand in a cable during a T cs (ramped temperature at constant current) measurement. It can be shown that the effect of cyclic bending strains along such strands, with a characteristic wavelength of about 5 mm, can be to produce a drop in the effective ` n' value (the power factor in the voltage–current relation) of the strand as it starts to enter the current sharing region, for electric fields up to 100 μV/m, from 30 to under 10. The effect is dependent on the internal resistivity of the strands as well as the bending strain distribution, but estimates suggest that for transverse resistivity ranges typical of Nb 3Sn strands, both the ` n' value can be reduced and/or the eventual voltage runaway point can decrease. Strands with a high mean stress level (as is expected with a steel jacket) are predicted to be more sensitive than those operating near zero mean intrinsic strain (as with an incoloy or titanium jacket). Although clearly not the only factor, this effect could explain recent experimental results on large coils as well as short conductor samples, where unexpectedly low cable ` n' values have been found. By providing a link between strain and ` n', it also suggests a mechanism by which current (and hence magnetic load) cycling can affect the superconducting performance, as observed in some recent tests.