The Large Hadron Collider ( LHC ) has been Shut Down For Major Upgrades

The Large Hadron Collider has been Shut Down For Major Upgrades

The Large Hadron Collider (LHC) is getting a big boost to its performance. Unfortunately, for fans of ground-breaking physics, the whole thing has to be shut down for two years while the work is done. But once it’s back up and running, its enhanced capabilities will make it even more powerful.

The essence of the Large Hadron Collider is to accelerate particles and then direct them to collide with each other in chambers. Cameras and detectors are trained on these collisions, and the results are monitored in minute detail. It’s all about discovering new particles and new reactions between particles, and watching how particles decay.

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This shutdown is called Long Shutdown 2 (LS2.) The first shutdown was LS1, and it took place between 2013 and 2015. During LS1 the power of the collider was improved, and so were its detection capabilities. The same will happen during LS2, when engineers will reinforce and upgrade the whole accelerator complex and the detectors. The work is in preparation for the next LHC run, which will start in 2021. It’s also to prepare for the a project called the High-Luminosity LHC (HL-LHC) project, which starts in 2025.

The High-Luminosity LHC, which is expected to be operational after 2025, will increase the LHC’s luminosity by a factor of 10. To achieve this major upgrade, scientists and engineers are optimising all of the collider’s parameters. Several technologies, some of which are completely innovative, are being developed.

More powerful focusing magnets

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Production of superconducting cable for the High-Luminosity LHC. (Image: Julien Ordan/CERN)

Increasing the luminosity means increasing the number of collisions. The aim is to produce 140 collisions each time the 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.

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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 and disappear. 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 constant.

“Crab” cavities for tilting the beams

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Assembly of the first crab cavity. (Image: Julien Ordan/CERN)

This innovative superconducting equipment will give the particle bunches a transverse momentum before they meet, enlarging the overlap area of the two bunches and thus increasing the probability of collisions. A total of sixteen crab cavities will be installed on either side of each of the ATLAS and CMS experiments.

Reinforced machine protection

LHC collimator. (Image: Claudia Marcelloni/CERN)

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.

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More compact and powerful bending magnets

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Test of a dipole coil prototype for the High-Luminosity LHC. (Robert Hradil, Monika Majer/

The LHC ring is already full of equipment. To allow the insertion of additional collimators, two 15-metre-long dipole magnets will be replaced with two pairs of shorter magnets (each measuring 5.5 metres) and two collimators. These new dipole magnets will be more powerful since they must bend the trajectory of the protons over 11 metres instead of 15. They too are based on the superconducting intermetallic niobium-tin compound and will generate an 11-tesla magnetic field, compared with 8.3 tesla for the current dipole magnets.

Innovative superconducting transmission lines

Innovative superconducting power lines will connect the power converters to the accelerator. 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 amps!

A renovated accelerator chain

Linac 4
The Linac4 accelerator. (Image: Maximilien Brice/CERN)

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 is being upgraded as part of the LIU (LHC Injectors Upgrade) project. A major step in the upgrade process will come in 2020 when a new linear accelerator, Linac4, the first link in the chain, will replace the current Linac2. Improvements are also planned for the three other links in the accelerator chain: the Proton Synchrotron Booster, the Proton Synchrotron and the Super Proton Synchrotron.

Civil-engineering work

A shaft of around 80 metres will be dug on ATLAS and CMS site, as well as an underground cavern and a 300-metre-long service tunnel. This tunnel will house equipment that is particularly sensitive to radiation, such as power converters, which transform alternating current from the electrical network into high-intensity direct current for the magnets. It will be linked to the LHC tunnel by four connecting tunnels. Five surface buildings will be built on each site.
More information about the civil engineering for the High-Luminosity LHC here.

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