Status and challenges of the Nb-Ti and Nb3Sn interaction region magnets for High-Luminosity LHC

• Ezio Todesco - ezio.todesco@cern.ch (CERN)

Room: ZOOM About one hundred magnets of six different types shall be installed in the High Luminosity LHC (HL-LHC) in the years 2026-2028 at CERN. These accelerator magnets are characterized by (i) a large aperture (105 to 150 mm) inducing large accumulation of stress due to electromagnetic forces, (ii) tight requirements on field quality due to the large values of the particle amplitudes in this region, and (iii) small series (6 to 20 magnets of the same type) giving a limited possibility of feedback and tuning during production. The magnet design, construction and test are based on CERN collaborations with institutes and industrial partners in USA, Spain, Italy, Japan and China. Three types of correctors are based on Nb-Ti technology and conductor peak field in the 2 to 4 T range: for all of them the protoype phase has been sucessfully completed. The production is well advanced for the superferric correctors, and is starting for the canted cos theta correctors and for the nested correctors. The separation and recombination Nb-Ti dipoles D1 and D2, with a 4.5-6 T bore field range, are both in the prototype phase after the completion of the short model program. The most challenging magnet, the Nb3Sn quadrupole with conductor peak field above 11 T is in the prototype phase at CERN and halfway through the production phase in the USA. In this paper we will first give an update on the values of the magnet parameters with respect to the initial baseline presented in ASC-2015. Then, for each type of magnet, we will give an overview of the main achievements obtained so far, pointing out which project requirements have been fulfilled, and we will outline the technical points still needing validation from the prototype program. We will conclude giving an outlook of the most critical issues that can be expected along the production.

Advanced Examination of Nb3Sn Coils and Conductors for the LHC luminosity upgrade: Computed Tomography and Materialographic Analyses

• Stefano Sgobba - stefano.sgobba@cern.ch (CERN)

Room: ZOOM The future of particle accelerators is inevitably and strongly linked to the development of high – field magnets that enable higher energies and higher luminosity to be attained. The European Organization for Nuclear Research (CERN) is currently developing several Nb3Sn-based magnets for the High-Luminosity upgrade of the Large Hadron Collider (HL-LHC), in order to fully exploit its potential and surpass the intrinsic performance limitations of NbTi based magnets. The fabrication of Nb3Sn magnets is a challenging process as it requires to manage the brittleness and strain sensitivity of the conductors once they have undergone the reaction heat treatment to generate the superconducting Nb3Sn phase (A15). Accelerator magnet coils are usually manufactured following the wind-react-and-impregnate fabrication approach. This reduces the difficulty of working with brittle compounds, but adds uncertainties associated to volume change during phase transition for the formation of Nb3Sn and thermal expansion / contraction differentials of all the magnets’ components. In order to investigate the root causes of performance limitation or degradation observed on present magnets, several HL-LHC dipole and quadrupole magnet 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. Internal shear and bending loads on unsupported superconducting wires, which can cause their dislocation as well as cracks in the aggregates of Nb3Sn filaments, are suspected to be the main cause of limitation or degradation. The approach is based on a sequence of mesoscale observations of whole coil sections through non-destructive testing by an innovative high energy linac X-ray Computed Tomography (CT) technique, followed by materialographic assessment of internal events, geometrical distortions, and potential flaws using Light Optical Microscopy (LOM). Additionally, Scanning Electron Microscopy (SEM) and Focussed Ion Beam (FIB) were used to analyze strands or sub elements’ damage at particular localized positions as well as failure modes. This comprehensive approach provided an in-depth view of the examined coils by identifying and characterizing atypical features and imperfections in both the superconducting phase of the strands and the glass fibre/resin system, and univocally associate the quenches experienced by the coils to identified physical events, under the form of broken superconductive filaments or damaged strands.