Magnetic, calorimetric, and transport studies of coupling and interstrand contact resistance in Nb3Sn rutherford cables with bimetallic cores of stainless steel bonded to copper

Magnetic and calorimetric techniques were used to measure coupling loss in small-scale (typically 20-strand) Rutherford cables inclosing bimetallic stainless steel-and-copper cores. The effects of two types of core were studied: (i) a double core consisting of two strips of bimetallic Cu/SS with the stainless steel (SS) surfaces facing each other; (ii) a single composite core with Cu sandwiched between two strips of SS -- a kind of "thick-core reference". A "no-core reference" cable was also included in the measurement set. In the double-core cable the facing SS strips (with their included surface oxide layer) were intended to inhibit crossover interstrand contact at the same time that side-by-side contact would be enhanced by the diffusion bonding of the outer strip surfaces to the strand during the cable's prolonged heat treatment under a uniaxial pressure of 20 MPa. Coupling loss in this double-core cable was small but measurable and very much less than that of the uncored cable. Coupling loss in the composite core cable was immeasurably small. The losses were expressed quantitatively in terms of interstrand contact resistances (ICR) deduced from the magnetic- and calorimetric coupling-loss results. ICR data were also obtained by direct measurement using the four-terminal transport method.