RRP Nb3Sn Subelement Shear Dependence on Hexagonal Subelement Stack Orientation and the Strand's Position Within a Rutherford Cable

Abstract

Superconducting strands built into Rutherford cables undergo high deformation at the cable edges. This deformation negatively impacts subelement integrity and subsequently the Residual Resistance Ratio (RRR) of the stabilization Cu in Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands after heat treatment due to compromised diffusion barriers [1]. Rutherford cables for the high luminosity upgrade of the LHC at CERN (HL-LHC) use Restack Rod Process (RRP) Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires with a 108/127 subelement stack. Numerous transverse cross-sectional samples from the cables made under the HL-LHC Accelerator Upgrade Project (“HL-LHC AUP”) [2] have been mounted, polished, and analyzed with light microscopy. The number of sheared subelements within a strand was seen to depend not only on a strand's position in the overall cable (i.e. center vs. edge), but also on its specific position within the cable edge. Moreover, the rotational angle of the subelement stack within a strand, relative to the cable's centerline plane, appears to play a role. There is a need to understand these dependencies, as they might impact the applicability of any criterion for a maximum number of sheared subelements (typically < 15% of subelements) applied to series production quality assurance, often imposed to ensure acceptable RRR degradation. By gathering data across many HL-LHC AUP cables and utilizing a more statistical approach, we aim to establish more accurate success criteria for future R&D cables using RRP wire when the available cable runs (and thus samples) are limited.