Collective effects in the LHC and its injectors

Operation during 4-8 hours at a constant luminosity of five times the nominal one (with “leveling”) is required for the CERN HL-(High Luminosity)-LHC project* to be able to reach integrated luminosities of ~ 250 fb-1 per year and ~ 3 ab-1 twelve years after the upgrade. This means that the potential peak luminosity should be at least two times larger than the leveled one, i.e. a factor more than ten compared to the nominal case is contemplated. Even though the LHC had a bold beginning, reaching one third of the nominal peak luminosity at the end of the 2011 run, a factor more than thirty remains to be gained, which will be achieved only if all the collective effects are deeply understood and mastered both in the LHC and its injectors. The observations made during the 2010-2011 runs are first reviewed and compared to predictions to try and identify possible bottlenecks. The lessons learned and the possible solutions and/or mitigation measures to implement in the HL-LHC and the LHC Injectors Upgrade (LIU) projects are then discussed.