Electro-mechanical limits of RRP Nb3Sn wires under transverse stress: numerical and experimental analysis

• Tommaso Bagni - tommaso.bagni@physics.uu.se (Uppsala University)

Room: ZOOM Future particle colliders in search for new physics rely on the development of Nb3Sn wires capable to withstand the large stresses generated by Lorentz forces during magnets operation. These stresses can cause a permanent reduction of the transport properties generated by residual deformation of the Nb3Sn crystal lattice as well as the formation of cracks in the brittle Nb3Sn filaments. Studies for the development of the High Luminosity LHC (HL-LHC) showed that nominal transverse compressive stresses above 150 MPa may be sufficient to generate cracks in the wires. Therefore, it has become essential to develop new methods for the characterization of the Nb3Sn wires capable to provide strategies to improve the mechanical limits of the wires. In this seminar, we will focus on Restacked-Rod-Process (RRP®) Nb3Sn wires designed for the 11 T dipoles of HL-LHC. The final goal of the study is to develop a Finite Element Model (FEM) to reproduce the behavior of Nb3Sn wires under transverse stress based on real geometry and benchmarked on single-wire experiments. The transverse electro-mechanical tests were performed at the University of Geneva and provided an accurate evaluation of the mechanical wire limits and irreversible regime, essential for the investigation of the root cause of the degradation. The wire geometry for the FEM and the evaluation of the formation of the crack during the transverse tests were obtained by exploiting X-Ray tomography, performed at the European Synchrotron Radiation Facility (Grenoble, FR), combined with artificial intelligence methods for image analysis. The FEM preliminary results confirmed that the residual stresses on the Nb3Sn, caused by plastic deformation of the copper matrix, are the main reason for the critical current degradation in the considered RRP wire. Furthermore, the model already provides strategies for the mechanical improvement of such wire.