Assembly of the Nb$_{3}$Sn MQXFB Quadrupole for HL-LHC

The High-Luminosity LHC (HL-LHC) project aims to upgrade the existing LHC machine to increase its integrated luminosity by a factor of ten. A key aspect of this upgrade is the inner triplet (or low-β) quadrupole magnets, the MQXF, which utilise Nb3Sn superconducting magnet technology and reach a conductor peak field of 11.3 T. The MQXFB version, manufactured at CERN, has a magnetic length of 7.2 m, making it the longest Nb3Sn accelerator magnet ever realised. A total of ten MQXFB series magnets are needed for HL-LHC (eight to be installed, and two spares). The mechanical structure is based on an aluminium shell, pre-tensioned at room temperature with a system of water-pressurised bladders and interference keys, which ensure the pre-stress of the coil-pack. The coil pre-load increases after cool-down to 1.9 K thanks to the larger thermal contraction of the aluminium shrinking cylinder compared to the rest of the magnet components. Since this structural design is used for the first time in an accelerator magnet, the accumulated experience of assembling 80% of the magnets provides valuable insights for the potential application of Nb3Sn technology in future colliders. This paper analyses the main measurements for the mechanical assembly of the MQXFB series magnets and assesses the reproducibility of the production.