While the LHC takes a technical pause, scientists, engineers and technicians are busy starting machine maintenance and upgrades

As with everything in this world, scientific instruments have a limited life-span, and from time to time they need a revamp. But compared to other objects, a technical pause for the Large Hadron Collider (LHC) involves thousands of international scientists, engineers and technicians, state-of-the-art technology, and… a huge key. Back in 2015, after the LHC’s first long shutdown (LS1), the LS1 coordination team handed this key to the CERN Control Centre (CCC) operators. Not to open a door, but as a simple gesture to symbolise a shift in responsibilities. The operators kept the key for almost three years, as they checked the performance of the machine 24/7. Today, after a successful machine run, the operators mark the start of the second long shutdown (LS2), and pass the key to the LS2 coordinators to keep for the next two years.

The first task of LS2 is to bring the machine back to room temperature. The LHC uses superconductors that work at the astonishing temperature of −271 °C. The warming process requires almost four months, as more than 100 tonnes of liquid helium need to be slowly removed. Then, major upgrades and improvements will start.

Teams will be working to a tight schedule to improve the machine for both the short-term and long-term future, including preparations towards the High-Luminosity LHC (HL-LHC) project foreseen for after 2025. Reaching the HL-LHC goals means delivering a more intense beam of particles to the LHC, and the team working on the injectors plan a series of modifications in the next two years. One includes replacing the now retired linear accelerator Linac 2 with the new Linac 4. While Linac 2 accelerated protons, the new addition will accelerate hydrogen ions (H−), made of one proton and two electrons, along an almost 90-metre-long machine, placed 12 metres underground.

The next accelerator in the chain, the Proton Synchrotron Booster (PSB), will strip off the electrons of H-, leaving only protons. The negatively charged hydrogen ions coming from Linac 4 get attracted to the newly-obtained protons, and the result is a more intense, concentrated beam that will continue its journey towards the LHC. In order to cope with these new requirements, the PSB will be equipped with completely new injection and acceleration systems, and the Super Proton Synchrotron (SPS), the last injector before the LHC, will have a new radio-frequency system.

Replacement of LHC dipole magnets during LS1 (Image: Anna Pantelia/CERN)

Of the many renovations taking place inside the LHC, teams will replace more than 20 magnets. They will also install new lifts to travel 100 metres underground to the LHC tunnel, innovative power converters and unprecedented superconducting technologies. Teams will open up the interconnections between the LHC dipole magnets to consolidate the diodes, which are used to bypass the current from one magnet to the next in case of a rise in temperature. This is essential for the machine to reach a beam energy of 7 TeV, another objective of the HL-LHC upgrade.

While engineers and technicians perform maintenance and consolidation underground, above ground, physicists sift through the wealth of data gathered so far. We will share the LS2’s key moments for both the accelerators and the experiments over the coming months.