The accelerator complex at CERN is a succession of machines that accelerate particles to increasingly higher energies. Each machine boosts the energy of a beam of particles before injecting it into the next machine in the sequence. In the Large Hadron Collider (LHC) – the last element in this chain – particle beams are accelerated up to the record energy of 6.8 TeV per beam.
Linear accelerator 4 (Linac4) became the source of proton beams for the CERN accelerator complex in 2020. It accelerates negative hydrogen ions (H-, consisting of a hydrogen atom with an additional electron) to 160 MeV to prepare them to enter the Proton Synchrotron Booster (PSB). The ions are stripped of their two electrons during injection from Linac4 into the PSB, leaving only protons. These are accelerated to 2 GeV for injection into the Proton Synchrotron (PS), which pushes the beam up to 26 GeV. Protons are then sent to the Super Proton Synchrotron (SPS), where they are accelerated up to 450 GeV.
The protons are finally transferred to the two beam pipes of the LHC. The beam in one pipe circulates clockwise while the beam in the other pipe circulates anticlockwise. It takes 4 minutes and 20 seconds to fill each LHC ring, and 20 minutes for the protons to reach their maximum energy of 6.5 TeV. Beams circulate for many hours inside the LHC beam pipes under normal operating conditions. The two beams are brought into collision inside four detectors – ALICE, ATLAS, CMS and LHCb – where the total energy at the collision point is equal to 13.6 TeV.
Protons are not the only particles accelerated in the LHC. Lead ions for the LHC start from a source of vaporised lead and enter Linac3 before being collected and accelerated in the Low Energy Ion Ring (LEIR). They then follow the same route to maximum energy as the protons.
Serving a diverse experimental programme
The accelerator complex serves not only the LHC, but also a rich and diverse experimental programme. Most of the other accelerators in the chain have their own experimental halls where beams are used for experiments at lower energies.
The PSB serves the Online Isotope Mass Separator (ISOLDE) facility including HIE-ISOLDE, as well as the MEDICIS facility.
The PS serves the Antiproton Decelerator, the neutron time-of-flight (n_TOF) facility and an experimental area known as the East Area, which houses the CLOUD experiment as well as IRRAD and CHARM.
The SPS serves the North Area experiments COMPASS, NA61/SHINE, NA62, NA63, NA64, NA65 and UA9, as well as the CERN Neutrino Platform, AWAKE and HiRadMat.
The LHC serves four large experiments, ALICE, ATLAS, CMS and LHCb, as well as the smaller TOTEM, LHCf, MoEDAL-MAPP, FASER and SND@LHC experiments.
