Ac loss and interstrand contact resistance in bare and coated NbTi/Cu Rutherford cables with cores

Ac loss due to coupling currents in a Rutherford cable can be controlled by increasing the interstrand contact resistance through adjusting the level of native oxidation of the strand, coating it, or by inserting a ribbon-like core into the cable itself. In an investigation of coupling loss, magnetic and calorimetric measurements were performed on: (i) a `reference pair' of bare and stabrite-coated uncored Rutherford cables; (ii) a series of stabrite-coated cables with cores of titanium, stainless steel, and kapton ribbon; and (iii) a series of bare-Cu cables with fixed overall thickness but with cores of successively increasing thickness. Measurements were made both with and without the release of uniaxial pressure between `curing' and measurement and, in the former case, after reapplication of in-cryostat (`cold') pressure. The total ac loss was measured as a function of the ramp rate of a magnetic field applied in either the face-on (FO, perpendicular to the cable's broad face) or edge-on (EO) orientations. From the coupling-current loss components, standard formulae enabled the interstrand contact resistances R_⊥ (crossover) and R_∥ (side-by-side) to be determined. These were combined, for the purpose of discussion, into an effective FO-measured contact resistance, R_⊥,eff(R_⊥,R_∥). It was noted that under the pressure-release measurement condition: (i) although the inclusion of a core (of any of the three materials) generally brings about a strong suppression of the FO loss, its presence at fixed cable outer dimensions causes an increase in the EO loss presumably as a result of increased side-by-side contact; (ii) increases in the core thickness, again at fixed cable size, resulted in still further increases in the side-by-side contact, and concurrent small reductions in the existing large R_⊥,eff(R_⊥,R_∥). Although previously reported results had confirmed that the insertion of a core into a stabrite cable removed the cold-pressure sensitivity of its R_⊥,eff - a highly desirable outcome - it was noted that the cored-enhanced R_⊥,effs were then much larger than the 20 or so µΩ called for by dipole magnet designers. With a view to correcting this problem a series of cables with reduced-width cores was proposed for future fabrication and measurement.