Cryogenics is the branch of physics that deals with the production and effects of very low temperatures. The Large Hadron Collider (LHC) is the largest cryogenic system in the world and one of the coldest places on Earth. All of the magnets on the LHC are electromagnets – magnets in which the magnetic field is produced by the flow of electric current. The LHC's main magnets operate at a temperature of 1.9 K (-271.3°C), colder than the 2.7 K (-270.5°C) of outer space.

The LHC's cryogenic system requires 40,000 leak-tight pipe seals, 40 MW of electricity – 10 times more than is needed to power a locomotive – and 120 tonnes of helium to keep the magnets at 1.9 K.

Extreme cold for exceptional performances

Magnets produce a magnetic field of 8.33 tesla to keep particle beams on course around the LHC's 27-kilometre ring. A current of 11,850 amps in the magnet coils is needed to reach magnetic fields of this amplitude. The use of superconducting materials – those that conduct electricity with no resistance – has proven to be the best way of avoiding overheating in the coils and of keeping them as small as possible.

Superconductivity could not happen without the use of cryogenic systems. The coils' niobium-titanium (NbTi) wires must be kept at low temperatures to reach a superconducting state. The LHC's superconducting magnets are therefore maintained at 1.9 K (-271.3°C) by a closed liquid-helium circuit.

Cryogenic techniques essentially serve to cool the superconducting magnets. In particle detectors they are also used to keep heavy gases such as argon or krypton in a liquid state, for detecting particles in calorimeters, for example.

Three steps to cooling

The layout of the LHC magnet cooling system is based on five "cryogenic islands" which distribute the cooling fluid and convey kilowatts of cooling power over several kilometres.

The entire cooling process takes weeks to complete. It consists of three different stages. During the first stage, helium is cooled to 80 K and then to 4.5 K. It is injected into the cold masses of the magnets in a second stage, before being cooled to a temperature of 1.9 K in the third and final stage.

During the first stage, some 10,000 tonnes of liquid nitrogen are used in heat exchangers in the refrigerating equipment to bring the temperature of the helium down to 80 K.

The helium is then cooled to 4.5 K (-268.7°C) using turbines. Once the magnets have been filled, the 1.8 K refrigeration units bring the temperature down yet further to 1.9 K (-271.3°C).

In total, the cryogenics system cools some 36,000 tonnes of magnet cold masses.

Tonnes of helium for the big chill

Helium was a natural choice of coolant as its properties allow components to be kept cool over long distances. At atmospheric pressure gaseous helium becomes liquid at around 4.2 K (-269.0°C). However, if cooled below 2.17 K (-271.0°C), it passes from the fluid to the superfluid state. Superfluid helium has remarkable properties, including very high thermal conductivity; it is an efficient heat conductor. These qualities make helium an excellent refrigerant for cooling and stabilising the LHC's large-scale superconducting systems.

Helium circulates in a closed circuit while the machine is in operation.