Influence of Compaction During Reaction Heat Treatment on the Interstrand Contact Resistances of Nb 3Sn Rutherford Cables for Accelerator Magnets

The high field superconducting magnets required for ongoing and planned upgrades to the Large Hadron Collider (LHC) will be wound with Nb 3 Sn Rutherford cables for which reason studies of Nb 3 Sn strand, cable, and magnet properties will continue to be needed. Of particular importance is field quality. The amplitudes of multipoles in the bore fields of dipole and quadrupole magnets, induced by ramp-rate-dependent coupling currents, are under the control of the interstrand contact resistances—crossing-strand, $R_{c}$ , adjacent strand, $R_{a}$ , or a combination of them, $R_{{\text{eff}}}$ . Although two decades ago it was agreed that for the LHC $R_{c}$ should be in the range 10–30 μ Ω, more recent measurements of LHC quadrupoles have revealed $R_{c}$ values ranging from 95 to 230 μ Ω. This paper discusses ways in which these values can be achieved. In a heavily compacted cable $R_{{\text{eff}}}$ can be tuned to some predictable value by varying the width of an included stainless steel (effectively “insulating”) core. But cables are no longer heavily compacted with the result that the crossing strands of the impregnated cable are separated by a thick epoxy layer that behaves like an insulating core. If a stainless steel core is actually present, $R_{{\text{eff}}}$ must be independent of core width. Since there is no guarantee that a fixed predetermined amount of interlayer separation could be reproduced from winding to winding it would be advisable to include a full width core.