A Combined Testing and Modelling Methodology for the Mechanics of High-Field Superconducting Magnets

The use of superconducting composite cables based on Nb3Sn, an intermetallic compound of Niobium and Tin, is one of the favorite routes to reach magnetic fields higher than 10 T in state-of-the-art accelerator magnets. The brittle and nonlinear nature of the epoxy-impregnated Nb3Sn Rutherford cable makes challenging to predict its mechanical limits and, consequently, the overall performance of the magnet. In the case of collared magnet structures, peak stresses in coils are usually reached during the collaring procedure performed at room temperature. Hence, it is essential to extensively study stress distribution within the superconducting coils and at their interfaces with other components during this phase. In this context, a combined experimental/numerical methodology was developed at CERN to investigate the effects of the assembling process and geometrical imperfections on the mechanical response of the collared coil of the 11 T HL-LHC dipole. The results of the experimental tests were benchmarked against 3-D non-linear finite element models, using material constitutive laws that mimicked the non-linear responses of the Nb3Sn cable, and embedding geometrical imperfections as measured on the tested components. The proposed methodology, used in the early stages of superconducting magnets development, may help to identify limitations in the mechanical design, and understand the impact of geometrical imperfections and tolerances on stress distribution in the magnet structures.