During LS2, the LHC’s two external beam dumps were removed from the tunnel and replaced with spare ones. After ten years of operation, they were showing signs of degradation, notably nitrogen leaks. Before being installed, the spare dumps were modified and upgraded to prevent the same problems from occurring during Run 3 (see this Bulletin article published in 2020).

To find out more about the cause of the nitrogen leaks, an endoscopy was carried out in July 2020. It revealed unexpected cracks in the beam dump’s two extruded graphite discs (see box). An action plan was drawn up by the SY-STI (Sources, Targets and Interactions) group as more information was needed with Run 3 on the horizon, especially with a view to designing new spare beam dumps for the LHC and, beyond that, beam dumps for the HL-LHC. To access the dump’s three main components – high-density, low-density and extruded graphite (see below) – there was only one possible solution: perform an “autopsy” on one of the dumps. Given its radioactivity levels, this was easier said than done.

“To reach the heart of the beam dump, we needed to be able to open it…,” says project leader Ana-Paula Bernardes. “But its duplex-stainless-steel-alloy housing was extremely difficult to cut. The first attempt, in January 2021, under a framework contract, was unsuccessful: it was impossible to cut through it manually without exceeding the radiation dose limits. We considered outsourcing the job to a specialised external company with the right equipment for the task, but the costs and time frames were incompatible with the project.”

Thanks to the expertise and versatility of CERN’s teams, a solution was eventually found in house: the SY-STI and BE-CEM (Controls, Electronics and Mechatronics) groups worked together to develop two techniques that would allow the dump’s housing to be cut remotely. The first involved an automated circular saw mounted on a rail, and the second a robot arm equipped with a cutter.

Several trial runs were carried out on a mock-up in order to “choreograph” the operation and thus limit, as far as possible, the time spent in close proximity to the dump. The circular saw was ultimately used to make five cuts, in a radiation airlock installation created specially for the job: two radial cuts to separate the low-density graphite block and three longitudinal cuts to remove the stainless-steel-alloy housing.

“As the endoscopy had already shown, both extruded graphite discs were cracked. Against all expectations, the low-density graphite was generally in good condition, as were the high-density-graphite blocks,” says Ana-Paula Bernardes.

“It was important to check the condition of the various components of the dump and establish how resistant they were, for various reasons,” explains Marco Calviani, leader of the Targets, Collimators and Dumps (STI-TCD) section in the SY department. “First of all, we needed to be sure that the dumps currently installed in the LHC – which are built from the same components as the autopsied dump – would withstand the energy levels of Run 3; next, we wanted to know what strategy to adopt for the two new spare dumps, which we need to design and manufacture by 2023, and especially for the dumps for the future HL-LHC.”

The results of the autopsy validated the use of low-density and high-density graphite for Run 3, but ruled out the use of extruded graphite for the design of the spare dumps. Other studies are under way at the HiRadMat facility (see the corresponding article entitled “What will the future LHC beam dumps be made of?”) to confirm these results and to test new materials, notably for the HL-LHC beam dumps. What about the dumps that are already in place? “The modifications made before their installation should greatly improve their resistance for Run 3, even though the energy to be dissipated is set to increase from 320 MJ to 540 MJ,” says Marco Calviani. “Don’t forget that the previous dumps withstood the onslaught for ten years!”

What are the current LHC beam dumps made of?

The LHC’s external beam dumps comprise a graphite dump measuring 8.5 metres in length and 722 mm in diameter, contained in a 12-mm-thick 318LN stainless-steel-alloy tube. In total, each dump weighs 6.2 tonnes.

Each beam dump is made of several graphite blocks of varying density: high-density isostatic graphite; a stack of 1700 2-mm-thick low-density expanded graphite discs; and two extruded graphite discs (the black bands), which hold the low-density graphite stack together.