AC loss in cored, stabrite-coated, superconducting cables in response to external compaction and variation of core thickness and width

Abstract

AC loss due to coupling currents in a Rutherford cable can be modified by changing the interstrand contact resistance (ICR) by: adjusting the level of native oxidation of the strand, coating it, or by inserting a ribbon-like core into the cable itself. With regard to cored cables further effective-ICR adjustments can be achieved by changing: (i) the degree of compaction during manufacture, (ii) the thickness of the core at fixed overall thickness, and (iii) the width of the core. We report on the results of magnetic and calorimetric AC loss measurements on stainless-steel-cored stabrite cables which had been: (i) externally compacted by rolling to thicknesses of 0%, 6%, 9%, and 11% below the standard thickness, (ii) internally compacted by being furnished with cores of thicknesses 1 mil ( 25 μm ) and 2 mil ( 50 μm ), and (ii) furnished with cores whose widths, w core, were about 20%, 50%, 75% and 100% of the maximum available width. The measurements were made in applied AC fields that were directed perpendicular (face-on, FO) and parallel (edge-on, EO), respectively, to the broad cables faces. It was noted that: (i) the introduction of the core dramatically reduced the FO loss, (ii) core-induced compaction moderately increased the side-by-side ICR at higher curing temperatures and hence increased the EO loss, (iii) increases in either external or internal compaction still further increased the losses, particularly those in the FO direction (which of course had already been strongly core-suppressed). The loss results were interpreted in terms of the FO-measured effective ICRs, R ⊥,eff, which were found to decrease from 198 to 37 μΩ during the compaction of a 170 °C-heat treated (“cured”, HT) cored cable and from 38 to 10 μΩ in the case of a 200 °C HT one. With increase in the w core of a 170 °C HT cable R ⊥,eff increased from 2.5 to 123 μΩ , passing through 15 μΩ at a w core of 57% w core,max. The results show that there is an opportunity for achieving the LHC-target R ⊥,eff of 15±5 μΩ either by compacting the standard cored cable ( T f=200 °C) by 4–7% or by reducing the width of the core ( T f =170 ° C ) to within 57±7% of w max. We prefer the latter route since it exposes more of the strands to crossover-type current sharing.