Interstrand Coupling Properties of LARP High Gradient Quadrupole Cables in Response to Variations in Cable Design and Heat Treatment Condition

Calorimetric measurement of coupling loss versus frequency has been measured on two sets of cored and uncored LARP high gradient quadrupole Nb3Sn Rutherford cables. Studied are the responses of the resulting interstrand contact resistances (ICR) to variation of stainless-steel (SS) core width and position and to variation of reaction-heat-treatment (RHT) condition. One pair of cables (an early HQ-series type) with and without core had received RHT under 20 MPa uniaxial face-on pressure. Another set of cables (recent QXF type) furnished with SS cores of various widths had received RHT under ambient pressure. The results were displayed as cable-cross-sectional micrographs and plots of ICR versus percent core coverage (W). The HQ cables were tightly compacted and produced results consistent with a previously expected continuous ICR versus W variation. On the other hand the QXF cables were uncompacted such that their upper and lower layers were separated by what is referred to as a full-width “pseudocore”; as a result their ICRs were independent of the widths of the SS cores. Compaction versus noncompaction is discussed and future research directions suggested.Funding was provided by the U.S. Department of Energy, Office of High Energy Physics, under Grants No. DE-SC0010312 and DE-SC0011721 (OSU) and DE-AC02-05CH11231 (LBNL).To obtain usefully high Reffs for compacted cored cables coverage values of 80-90% are required. But as summarized in [10] for any given W in that range a considerable scatter in Reff can be expected depending on the type, width, and placement of the core. For a fixed set of such parameters the reproducibility of Reff has not been explored or quantified; a core may be useful in moderating any such variations in practice. A full-width “epoxy pseudocore” is present in all the noncompacted QXF cables. For these, Reff depends predominantly on Ra. Future studies might investigate how the Ra of another group of the same cables, ostensibly prepared in the same way, would respond to what might turn out to be a different set of side constraints. Similarly, it would be useful to explore the level of local compaction in cables in different parts of an accelerator magnet prepared under a given constraint. Finally, explorations of ICR vs “level of constraint” are needed