The accelerator complex at CERN is a succession of machines that accelerate particles to increasingly high energies. Each machine boosts the energy of a beam of particles before injecting it into the next machine in the sequence, of which the Large Hadron Collider (LHC) is the last.
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 a kinetic energy of 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 25 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, where the particle beams are accelerated up to the record energy of 6.5 TeV per beam and made to collide in the four interaction points of the LHC, where the four main detectors (ALICE, ATLAS, CMS and LHCb) record the high-energy collisions.
During the upcoming Run 3 of the LHC, a new energy record is planned, with acceleration up to 6.8 TeV per beam.
Built in 1994, Linac3 is the starting point for ions, mainly lead nuclei, which are collided by the LHC during dedicated ion runs and also used in fixed-target experiments.
New equipment has been installed to significantly increase beam intensity and quality. A new high-capacity oven, used to evaporate lead, was installed, with a new source crucible especially developed to prevent lead oxide blockage. The plasma chamber’s old stainless-steel internal coating has been replaced with a 20-micrometre-thick aluminium coating, which is better suited for the production of a higher intensity beam. A new remotely controlled extraction system, drawing the lead ions from the plasma chamber, now helps to fine-tune the extracted beam and may also increase beam intensity. The heating and ventilation systems of the entire facility were also completely replaced.
Linac4, the newest machine in CERN’s accelerator chain, was connected to the rest of the accelerator complex in August 2020, replacing the retired Linac2. This almost 90-metre-long linear accelerator lies 12 metres underground at the very beginning of the proton beam acceleration chain. It accelerates negative hydrogen ions (H-, consisting of a hydrogen atom with an additional electron) to a kinetic energy of 160 MeV, significantly higher than the 50-MeV operation of Linac2, providing proton beams for the entire accelerator complex, from the PS booster to the LHC.
Proton Synchrotron Booster (PSB)
The Proton Synchrotron Booster receives beams of negative hydrogen ions from Linac4 at 160 MeV. During their injection into the PSB, the ions are stripped of their two electrons to leave only protons, which are accelerated to 2 GeV for injection into the Proton Synchrotron (PS).
The PSB underwent a complete revamp during LS2. The most notable changes are the total replacement of its acceleration system, with the installation of one universal broadband radio frequency (RF) system, covering the whole relevant frequency range, instead of the three separate-function RF systems previously in use; a new cooling system, with cooling towers in two renovated buildings; a new power supply system, supplying the magnets with electrical intensities of 5500 amps, instead of 4000 amps previously; and a brand new injection region with fast bumper magnets, stripping foils and new beam instrumentation to allow the H- charge exchange injection from Linac4.
To cope with the increase in energy and the use of negative hydrogen ions at injection, the injection transfer lines from Linac4 to the PSB and the extraction transfer lines from the PSB towards the PS have been completely replaced, with the addition of new magnets and new beam dumps and the modification of beam instrumentation. A whole host of new sensors, beam position monitors, beam loss monitors and wire scanners have been installed to monitor and measure the denser particle beams that will circulate in this machine.
Proton Synchrotron (PS)
The Proton Synchrotron (PS), with a circumference of 628 metres, was first operated in 1959 and has 100 room-temperature main magnet units to bend and focus the beam. In the past few years, tests have identified weak points in the magnet system: 50 magnets needed refurbishment. Seven of them were repaired during the last long shutdown (LS1) and the remaining 43 were refurbished in LS2.
To cope with the almost 50% increase in injection energy from 1.4 to 2 GeV, the 20-metre-long injection line into the PS was replaced and a new septum magnet, four new bumper magnets and a new fast injection-kicker magnet were installed, as were two novel internal beam dumps. A new broadband RF system, which had already been prototyped and tested with beam before LS2, as well as new feedback amplifiers for the main accelerating RF system, were installed to ensure beam stability for more intense and brighter beams.
Two novel BGI (beam gas ionisation) beam profile monitors were installed to enable the machine operators to get a very precise insight into the quality of the beams, as well as to prepare beams for the future High-Luminosity LHC.
Super Proton Synchrotron (SPS)
The Super Proton Synchrotron (SPS) measures nearly 7 kilometres in circumference and operates up to 450 GeV.
An increase in radiofrequency power is required to prepare for beams in the High-Luminosity LHC, which will be twice as intense. A new technology, developed by CERN with industry, has been applied for the first time to renew the SPS acceleration system: its solid-state amplifiers are the core components of two new power plants that feed two of the radiofrequency (RF) cavities (i.e. accelerating cavities) with a power of 1.6 megawatts each. The remaining cavities, the layout of which has been completely changed, continue to be fed by the four existing 1-megawatt RF power plants.
To enhance the stability of the more intense beams in the SPS, about 100 connections between different beam chambers have been adequately shielded to limit the undesired electromagnetic interaction of the beam with the surrounding environment. The adjacent focusing magnets have been coated with amorphous carbon, a technology developed at CERN to prevent the formation of electron clouds in the presence of beam.
Another major overhaul was the replacement of the SPS beam dump with a new one capable of absorbing high-intensity particle beams with a wide range of energies – from 14 to 450 GeV.
A new fire-detection system for the SPS tunnel now uses an aspirating smoke-detection mechanism capable of sucking in air from up to 700 metres away, and the tunnel’s access system has been fully revamped.