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-lik e core into the cable itself. In an investigation of coupling loss, magneti c and calorimetric measurements were performed on: (i) a 'reference pair' o f bare and stabrite-coated uncored Rutherford cables; (ii) a series of stab rite-coated cables with cores of titanium, stainless steel, and kapton ribb on; and (iii) a series of bare-Cu cables with fixed overall thickness but w ith cores of successively increasing thickness. Measurements were made both with and without the release of uniaxial pressure between 'curing' and mea surement 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-curren t loss components, standard formulae enabled the interstrand contact resist ances R-perpendicular to (crossover) and R-parallel to (side-by-side) to be determined. These were combined, for the purpose of discussion, into an ef fective FO-measured contact resistance, R-perpendicular to ,R-eff (R-perpen dicular to, R-parallel to). It was noted that under the pressure-release me asurement condition: (i) although the inclusion of a core (of any of the th ree materials) generally brings about a strong suppression of the FO loss, its presence at fixed cable outer dimensions causes an increase in the EO l oss presumably as a result of increased side-by-side contact; (ii) increase s in the core thickness, again at fixed cable size, resulted in still furth er increases in the side-by-side contact, and concurrent small reductions i n the existing large R-perpendicular to ,R-eff(R-perpendicular to, R-parall el to). Although previously reported results had confirmed that the inserti on of a core into a stabrite cable removed the cold-pressure sensitivity of its R-perpendicular to ,R-eff-a highly desirable outcome-it was noted that the cored-enhanced R-perpendicular to ,R-eff s were then much larger than the 20 or so mu Omega called for by dipole magnet designers. With a view to correcting this problem a series of cables with reduced-width cores was pr oposed for future fabrication and measurement.