Assembly of crab cavities
Assembly of the first two crab cavities for the High-Luminosity LHC (Image: CERN)

The High-Luminosity LHC, which is expected to be operational as of 2029, will increase the LHC’s integrated luminosity by a factor of 10. To achieve this major upgrade, scientists and engineers are optimising many of the collider’s parameters. Several new key technologies, some of which are completely innovative, are being developed within the scope of the project.



More powerful focusing magnets

Increasing the luminosity means increasing the number of collisions. The aim is to produce 140 collisions each time two particle bunches meet in the centre of the ATLAS and CMS detectors, as opposed to 30 at present. To achieve this, the beam will be more intense and more concentrated than at present in the LHC. New, more powerful quadrupole magnets, generating a 12-tesla magnetic field (compared to 8 tesla for those currently in the LHC), will be installed either side of the ATLAS and CMS experiments. Twelve of these magnets, made of a superconducting intermetallic compound of niobium and tin will be installed close to each detector. The LHC’s magnets today use a niobium-titanium alloy.

Unprecedented beam optics

One particular challenge will be maintaining luminosity at a constant level throughout the lifespan of the beam. At present, it decreases as the protons collide. In the High-Luminosity LHC, the beam focusing (the concentration of the beam before impact) will be designed in such a way that the number of collisions remains levelled at a constant value for most of the physics fill. 

“Crab” cavities for tilting the beams

The crab cavities give opposite deflections, or "kicks", to the head and the tail of the particle bunches, while leaving the centre of the bunch undeflected. This results in a better overlap of the colliding bunches when the beam trajectories have a crossing angle at the interaction point. A total of sixteen crab cavities will be installed next to the ATLAS and CMS experiments.

Reinforced machine protection

As the beams will contain more particles, machine protection will need to be reinforced. This protection is based on collimators – devices that absorb particles that stray from the beam trajectory and might otherwise damage the machine. New collimators, made from a material that produces less electromagnetic interference on the beam and equipped with new instrumentation, are being developed. Around 60 of the 118 existing collimators will be replaced by new collimators and 15 to 20 new ones will be added. In addition, the active machine protection systems will require upgrading, to allow the timely detection of new, fast failure cases to trigger the safe disposal of the proton beams onto a carbon absorber block.

Crystal collimators for increased cleaning efficiency

The increased beam intensities will impose more stringent demands on the collimation system, that protects superconducting elements from quenching. For the first time in an accelerator, crystal collimators will be used as part of standard operations to mitigate collimation losses of heavy-ion beams, in the absence of the 11-tesla dipole magnets, whose installation has been deferred to after CERN’s Long Shutdown 2 (LS2). In this collimation scheme, a 4 mm-long bent crystal is used as primary collimator, instead of standard graphite-based jaws to deflect the beams halo particles onto a secondary absorber.

Innovative superconducting transmission lines

Innovative superconducting power lines will connect the power converters to the new accelerator magnets. These cables, which are around one hundred metres long, are made of a superconducting material, magnesium diboride, that works at a higher temperature than that of the magnets. They will be able to carry currents of record intensities, up to 100 000 Amperes!

A renovated injector accelerator chain for the LHC

The performance of the LHC and its successor, the High-Luminosity LHC, relies on the injector chain, the four accelerators that pre-accelerate the beams before sending them into the 27-kilometre ring. This accelerator chain has been upgraded as part of the LIU (LHC Injectors Upgrade) project. A major step in the upgrade process was achieved in 2020 when a new linear accelerator, Linac4, the first link in the chain, replaced Linac2. Improvements have also been made to the three other links in the accelerator chain: the Proton Synchrotron Booster, the Proton Synchrotron and the Super Proton Synchrotron.

Civil-engineering work

Shafts of around 80 metres deep have been dug on the ATLAS and CMS sites, as well as an underground cavern and a 300-metre-long service gallery. This gallery is to house accelerator equipment and related infrastructures that are particularly sensitive to radiation, such as power converters, which transform alternating current from the electrical network into high-intensity direct current for the magnets. The new HL-LHC underground areas are linked to the LHC tunnel by four connecting galleries. Five surface buildings are also being built on each site. 
More information about the civil engineering for the High-Luminosity LHC here.