During operation, each beam of the LHC contains about 2500 particle bunches, each in turn containing roughly 100 billion protons. The energy stored in one of these beams, 320 megajoules (MJ), is considerable, the equivalent of a TGV train travelling at 150 km/h. After the current long shutdown (LS2), in Run 3 (2021-2024), it will reach 555 MJ.
But when these beams need to be stopped, such high energy levels are a real challenge. “To stop a beam, we need to direct it to a beam dump, a device that absorbs particle beams,” explains Marco Calviani, leader of the Targets, Collimators and Dumps section in the EN-STI group. “At the LHC, there is an external absorber for each of the two beams. They are located in two purpose-built underground caverns at Point 6 of the accelerator.”
During Run 2, the internal peak temperature of the absorbers reached up to 1000 °C in just 100 microseconds after each beam dump. After LS2, when the LHC beams will be even more intense, the temperature could rise to 1500 °C. To cope with this, the LHC’s 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.
After ten years of loyal service, however, the LHC absorbers are showing signs of wear and tear. “We have detected nitrogen leaks caused by movement of the steel tube: each time a beam impacts it, the tube receives a large fraction of the energy released by the particle shower, which results in a rapid thermal expansion and vibrations,” explains Marco Calviani. “Comparing computer simulation models with instrumentation data gathered during Run 2 has given us a better understanding of the behaviour of the absorber during impact and the origin of the vibrations.”
An upgrade of the LHC beam dumps was therefore added to the menu for LS2. One of the main modifications being made is to the support system of the absorber, which will now be suspended by high-resistance steel cables to allow better shock absorption. The transfer line from the LHC will also be physically disconnected from the absorber – beams will travel through the air for around ten metres – to avoid the propagation of vibrations to the vacuum beam tube leading from the accelerator. The upgrade work also includes the installation of new titanium-alloy beam “windows” to enclose the graphite part of the absorber in its nitrogen atmosphere.
But there’s a problem: after ten years of use, the LHC’s main absorbers have reached a level of radioactivity that prevents teams from working in close proximity to them for long periods. “As we cannot work on the used absorbers, we have decided to upgrade the two spare absorbers, which will now become the main absorbers,” explains Marco Calviani.
The upgrade work began at the start of February and should be finished by August, just in time for the start of the cool down of the accelerator. Instrumentation attached to the upgraded absorbers will collect data during the next run with a view to guiding the design of the absorbers for the HL-LHC, since these will need to absorb beams with an energy of 710 MJ. “This work would not be possible without the strong commitment of all involved groups and departments within the ATS Sector and HSE,” concludes Marco Calviani.