After several years of complex design, manufacture and planning, the CMS collaboration, in close cooperation with experts from CERN's Vacuum, Surfaces and Coating group (TE department), have in recent months been installing the new heart of the detector: the beam pipe. This fragile, 36-m-long component, in which the LHC beams collide at the Interaction Point, will be one of the last elements of the experiment to be installed before closing the detector.
The design of the new beam pipe has to comply with the numerous demands of physics, vacuum and integration requirements. From either side of the Interaction Point, the cylindrical section of the central chamber, with a diameter of 43.4 mm, has been extended from 1.6 m to 3.1 m to be compatible with the Phase 2 Tracker sub-detector that will be installed during LS3.
An important change with respect to the previous layout of the beam pipe consists in a new vacuum pumping group, moved away from the detector at 16 m from the Interaction Point, to facilitate maintenance.
Another key motivation behind the beam pipe layout change is the reduction of the radiation dose received by personnel during interventions. The new aluminium alloy used for the beam pipe reduces the induced radioactivity by a factor of 5 compared to the stainless steel used for the old beam pipe. This alloy has been chosen as the main material of the experimental vacuum chambers for Run 3 and the HL-LHC era.
After a series of acceptance tests, the vacuum chamber segments were equipped with a set of temperature sensors and then wrapped with heating foils, the so-called "bake-out jackets", that will be used to heat up the beam pipe from ambient temperature to 230 °C after the installation. The bake-out will activate the non-evaporable getter (NEG) material already coating the inner surface of the vacuum chambers, which will act as a distributed vacuum pump, constantly absorbing any residual gas. This will clean up stray particles and help to achieve the ultra-high vacuum that is essential inside the beamline of any particle collider to prevent collisions between the circulating beam particles and residual gas molecules. Such collisions would scatter the beam, creating a noisy background for the detector and degrading the beam life.
Following a detailed installation sequence of the vacuum pipe segments, in parallel, at both ends of the experiment, the mechanical installation of the chambers and all their operational and temporary supports was completed at the end of April.
With the mechanical installation completed, a global leak test will be performed on the chambers, with the aim of reaching an ultimate pressure of 10−11 millibars. Then, the detector endcaps will be positioned in the bake-out configuration for a duration of 168 hours. A final step of ultra-pure neon injection will complete the activity, readying the new beam pipe for Run 3.
