First CERN Cold Masses for the HL-LHC Interaction Regions

In the framework of the HL-LHC project, CERN has the responsibility to develop, qualify and assemble three different types of cold masses, namely Q2 (final focus quadrupole in the triplet), CP (corrector package) and D2 (separation dipole). In this paper, we describe the various designs, which incorporate several novel features. For example, for the cold masses containing an MQXFB quadrupole – based on a bladder-and-key structure with aluminium cylinders – we proposed and qualified a longitudinal welding process that takes into account the developed length of the loaded magnet (measured with a laser tracker), the geometry of the stainless steel half shells of the liquid helium vessel, and the welding shrinkage, so to match the requirements of mechanical uncoupling in the transversal plane, while introducing a sound fixed point in the longitudinal direction. In the case of the D2 cold mass, we introduced an orbital welded joint between two half units, with on one side the main recombination dipole, and on the other the dipole correctors, which comes with specific challenges for the relative alignment. Then, in the CP design we integrated a large number (nine) of high order correctors, together with a nested dipole corrector. We report as well on the special test cold mass developed to allow fast track cold testing of the MQXFB magnet in a standalone horizontal configuration, which was not in the initial baseline of the project. Besides detailing the various designs, the paper provides feedback from the manufacturing experience of the first units.