Advanced examination of Nb3Sn coils and conductors for the LHC luminosity upgrade: a methodology based on computed tomography and materialographic analyses

The future of particle accelerators is strongly linked to the development of high—field magnets. The European Organization for Nuclear Research (CERN) is currently developing Nb3Sn-based magnets for the high-luminosity upgrade of the large hadron collider (HL-LHC), to fully exploit its potential and surpass the intrinsic performance limitations of Nb–Ti-based magnets. The fabrication of Nb3Sn magnets is a challenging process as it requires managing the brittle and strain sensitive conductor after the reaction heat treatment to generate the superconducting phase. Accelerator magnet coils are usually manufactured following the wind-react-and-impregnate fabrication process. This reduces the difficulty of working with brittle compounds but adds uncertainties associated with volume change during phase transition and thermal expansion/contraction differentials during the temperature ramps of the heat treatment and cooldown to cryogenic temperatures. To investigate the root causes of performance limitation or degradation observed on HL-LHC magnet prototypes, several Nb3Sn-based coils have been examined. The present paper illustrates an innovative methodology of investigations of the root causes at several fabrication stages and after cooldown and powering. The approach is based on a sequence of mesoscale observations of whole coil sections by an innovative high—energy linac x-ray computed tomography, followed by materialographic assessment of internal events, geometrical distortions and potential flaws using light microscopy. Additionally, scanning electron microscopy and focused ion beam were used to analyze damage at localized positions. This comprehensive approach provides an in-depth view of the examined coils by characterizing atypical features and imperfections in both the strands and the glass fiber/resin of the insulation system, univocally associating the limiting quenches experienced by the coils to identified physical events.