Effects of Core Type, Placement, and Width on the Estimated Interstrand Coupling Properties of QXF-Type Nb3Sn Rutherford Cables

The coupling magnetization of a Rutherford cable is inversely proportional to an effective interstrand contact resistance R_eff, a function of the crossing-strand resistance R_c, and the adjacent strand resistance Ra. In cored cables, R_eff continuously varies with W, the core width expressed as percent interstrand cover. For a series of un-heat-treated stabrite-coated NbTi LHC-inner cables with stainless-steel (SS, insulating) cores, R_eff (W) decreased smoothly as W decreased from 100%, whereas for a set of research-wound SS-cored Nb₃Sn cables, R_eff plummeted abruptly and remained low over most of the range. The difference is due to the controlling influence of R_c - 2.5 μΩ for the stabrite/NbTi and 0.26 μΩ for Nb₃Sn. The experimental behavior was replicated in the R_eff (W )´s calculated by the program CUDI, which (using the basic parameters of the QXF cable) went on to show in terms of decreasing W that: 1) in QXF-type Nb₃Sn cables (R_c = 0.26 μΩ), Reff dropped even more suddenly when the SS core, instead of being centered, was offset to one edge of the cable; 2) R_eff decreased more gradually in cables with higher R_c´s; and 3) a suitable R_eff for a Nb₃Sn cable can be achieved by inserting a suitably resistive core rather than an insulating (SS) one.