The Effect of Ground Motion on the LHC and HL-LHC Beam Orbit

The High Luminosity Large Hadron Collider (HL--LHC) will require unprecedented orbit stability at the low-beta experiments, ATLAS and CMS. Because the effect of seismic noise might become a relevant source of luminosity loss, several studies have been conducted to characterize the actual ground motion in the LHC tunnel. In preparation for the official groundbreaking of the civil engineering work for the high luminosity upgrade, which started on 15 June 2018 in parallel to LHC beam operation, seismic sensors were installed to permanently monitor the ground stability close to these experiments. Compactor work and shaft excavation were expected to induce vibrations in the accelerator magnets that would cause orbit disturbance, beam loss, and potentially premature beam dumps. This paper summarises the observations made on the LHC beams and uses this data to benchmark estimates of the impact of low-frequency vibrations on the closed orbit at the main interaction points and collimators. The impact of ground motion on HL--LHC operation is then estimated using the expected operational scenario and the estimated mechanical stability of the new HL--LHC triplet magnets that is based on recent measurements of mechanical magnet transfer functions.