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 Nb
₃
Sn 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 Nb
₃
Sn. 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 Nb
₃
Sn 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 Nb
₃
Sn cable can be achieved by inserting a suitably resistive core rather than an insulating (SS) one.