A refrigerator inside CERN’s Large Hadron Collider (LHC) that cools helium from room temperature to 4.5 K (-268.65°C) has undergone major improvements in preparation for a new project.

The LHC is the world’s largest and most powerful particle accelerator designed to produce some of nature’s tiniest and most exostic subatomic particles by smashing protons together.

Housed underground, deep beneath the international border separating France from Switzerland, the LHC accelerates protons in a ring of superconducting magnets almost to the speed of light before they collide and explode, giving rise to particles possibly never seen before.

It’s also one of the coldest places on Earth. The 1.9 K (-271.3°C) operating temperature of its main magnets is even lower than the 2.7 K (-270.5°C) of outer space.

To get the LHC to this temperature, 120 tonnes of liquid helium flow around a closed circuit in the veins of the accelerator.

The LHC cooling system is made up of cryogenic islands with eight helium refrigerators in total.

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Source: CERN

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.

“These refrigerators date back to 1994, but they have undergone a number of upgrades since then, in particular in preparation for the LHC in 2006,” says Emmanuel Monneret, an engineer from the TE-CRG group working on the refrigeration project.

“On that occasion, their cooling power was increased from 12 to 16 kW at 4.5 K.”

During LS2, further upgrades have been carried out on the LEP refrigerator at Point 4, increasing its cooling power to 18 kW at 4.5 K, in preparation for a new project, the HL-LHC (High-Luminosity LHC).

This project aims to crank up the performance of the LHC in order to increase the potential for discoveries after 2027.

The objective is to increase luminosity by a factor of 10 beyond the LHC’s design value.

Luminosity is an important indicator of the performance of an accelerator: it is proportional to the number of collisions that occur in a given amount of time.

The higher the luminosity, the more data the experiments can gather to allow them to observe rare processes.

“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,” Monneret continued.

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.

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Source: CERN

The cold box located on the surface is approximately 6 metres long and 3 metres in diameter. All the elements are vacuum insulated to limit thermal radiation.

CERN said 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.

“We had not anticipated that it would be impossible to get inside the tunnel cold box, which is much more compact than the one on the surface,” Monneret explained.

“Working in close collaboration with the manufacturer, we eventually found a solution to allow us to replace the turbines and exchangers from the outside.”

The HL-LHC, which should be operational from the end of 2027, will allow physicists to study of known mechanisms in greater detail, such as the Higgs boson, and observe rare new phenomena that might reveal themselves.

For example, the HL-LHC will produce at least 15 million Higgs bosons per year, compared to around three million from the LHC in 2017.