The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator. It first started up on 10 September 2008, its first run was from 2010 to 2013, its second run was from 2015 to 2018 and its third run will begin in 2022.
The LHC consists of a 27-kilometre ring of superconducting magnets, used to direct and focus two high-energy particle beams. The beams travel in opposite directions in separate beam pipes at close to the speed of light, before they are made to collide inside detectors.
To accelerate particles, the accelerators are fitted with metallic chambers containing an electromagnetic field known as radiofrequency (RF) cavities. Charged particles injected into this field receive an electrical impulse that accelerates them.
Improved electrical insulation of diodes of more than 1200 magnets
The dipole magnets, which occupy 18 of the accelerator’s 27 kilometres, bend the trajectory of the particle beams. They are powered by a strong electrical current of up to 13 000 amps, which must be safely extracted if needed. For this purpose, a diode is fitted at the junction between each of the dipole magnets to create a parallel circuit to allow the current to be diverted.
To avoid short circuits, metallic debris was removed from around the diodes, using a special vacuum cleaner paired with an endoscopic camera. The electrical insulation of the diodes was also improved by developing and installing a total of 1232 special insulating caps.
22 superconducting magnets replaced
During LS2, 19 dipole magnets and 3 quadrupole magnets were replaced and the cryogenic assemblies for the High-Luminosity LHC (HL-LHC) project were installed along with additional instrumentation to study the heat loads caused by the beam.
Consolidation of the LHC external beam dumps
The LHC beam dumps consist of an 8-metre-long graphite absorber contained in a 12-mm-thick stainless-steel tube. The whole assembly, which is encased in an iron shielding structure, weighs around 7 tonnes and is filled with nitrogen gas.
One of the main modifications made was to the support system of the absorber, which is now suspended by high-resistance steel cables to allow better shock absorption. The transfer line from the LHC is also now physically disconnected from the absorber – beams will travel through the air for around ten metres – to avoid propagating vibrations to the vacuum beam tube leading from the accelerator. The upgrade also included the installation of new titanium-alloy beam “windows” to enclose the graphite part of the absorber in its nitrogen atmosphere.
The collimators are installed in two areas of the LHC (at Points 3 and 7 of the ring) and around the four big experiments (ALICE, ATLAS, CMS and LHCb). They are special devices equipped with jaws that close around the beam to clean up stray particles. The materials used for these jaws are capable of withstanding extreme pressure and temperatures as well as high levels of radiation. Some of the collimators have fixed apertures to protect the surrounding magnets.
Three types of collimators have been installed: four primary collimators, eight secondary collimators and two fixed-aperture passive absorbers.
New beam absorber
An internal beam absorber stops stray particles in order to prevent them from harming the accelerator. The new target dump injection segmented (TDIS) absorber is made up of three modules, each measuring 1.6 metres, and replaces equipment made of a single element measuring a little over four metres.
The modules comprise two jaws similar to those of a collimator, made of increasingly dense materials, namely graphite, titanium and copper, along the entire length of the absorber, which slow down and then stop the beam. One advantage of this beam absorber is its smaller size, thanks to its optimised design.
Preparing for HL-LHC: the neutral beam absorber
Two new neutral beam absorbers (TANB) were installed on both sides of the LHCb experiment to protect the accelerator equipment. Increasing the number of collisions, and therefore the number of particles in circulation, requires reinforced protection of the LHC’s equipment, as particles that diverge from the trajectory can collide with sensitive components such as superconducting magnets and interfere with their operation.
Increased cryogenic power at LHC Point 4
The LHC cooling system is made up of cryogenic islands with eight helium refrigerators in total. Each even-numbered point on the accelerator (Points 2*, 4, 6 and 8) has two refrigerators, one dating from the LEP (Large Electron–Positron Collider) era, and another, newer, refrigerator dating from the start-up of the LHC. The LEP refrigerator is composed of two cold boxes – one on the surface and the other downstream in the tunnel, which cool the helium from room temperature to 20 K (-253.15 °C) and from 20 K to 4.5 K respectively – and a unit located in a cavern generating superfluid helium at 1.9 K.
The Point 4 refrigerators are crucial for the HL-LHC because, as well as cooling Sectors 3–4 and 4–5, they must also cool the sections where the radiofrequency cavities are installed, which require a considerable amount of cooling.
To achieve this important extra 2 kW, the four turbines and heat exchangers in each of the cold boxes at Point 4 have been replaced with higher-performing equivalents. This task was relatively straightforward to carry out for the cold box at the surface, which is easily accessible to workers, but more arduous for the cold box in the tunnel.