The LHC vacuum system: Commissioning up to nominal luminosity

The Large Hadron Collider (LHC), currently under operation at CERN, is colliding intense proton beams at the highest energy frontier up to ∼14 TeV in the centre of mass. This superconducting storage ring is at the origin of the discovery in 2012 of the so-called 'Higgs' Boson explaining the origin of the mass of weak bosons W+,W− and Z also discovered at CERN in 1983. The arc vacuum system, which operates at cryogenic temperatures, consists of a 1.9 K cold bore which houses a 5-20 K beam screen. Beam collisions are performed inside a Non-Evaporable-Getter coated vacuum system located in long straight sections held at room temperature. These vacuum systems were designed to be stable under ion bombardment, to cope with intense VUV synchrotron radiation flux and to mitigate beam induced multipacting effects. In this paper, the LHC beam vacuum system design is recalled. Its operation, challenges and achieved performances during the commissioning phase will be discussed in detail.