The initial filament strain state of cable-in-conduit superconductors and the relation of this strain to large-bore, high-field magnet design

Cable-in-conduit superconductors formed by enclosing a cable of MF-Nb3Sn/Cu composites in a strong steel sheath are a useful element in magnet design and construction. However, an important feature of this type conductor, which must be understood and accounted for in magnet design, is the initial strain state of the superconducting filaments. This initial strain state results from the differential cooldown strains of the various components of the cable-in-conduit system as this system is brought from the Nb3Sn formation temperature (∼1000 K) to the operation temperature (∼4 K). The resultant strain depends on the proportions and thermomechanical properties of the constituent materials. However, empirical data suggest that the initial strain state also depends on the void fraction in the cable. In this paper, we survey the literature to provide data for a simple model to describe the mechanical link between cable and sheath. We also report on new experiments devised to further clarify this link. In addition, we discuss using the initial superconductor strain state as a parameter in magnet design.