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.
In 1911, while studying the properties of matter at very low temperature, the Dutch physicist Heike Kamerlingh Onnes and his team discovered that the electrical resistance of mercury goes to zero below 4.2 K (-269°C). This was the very first observation of the phenomenon of superconductivity. The majority of chemical elements become superconducting at sufficiently low temperature.
Superconducting heroes despite the zeroes
Below a certain “critical” temperature, materials undergo transition into the superconducting state, characterized by two basic properties: firstly, they offer no resistance to the passage of electrical current. When resistance falls to zero, a current can circulate inside the material without any dissipation of energy. Secondly, provided they are sufficiently weak, external magnetic fields will not penetrate the superconductor, but remain at its surface. This field expulsion phenomenon is known as the Meissner effect, after the physicist who first observed it in 1933.
Three names, three letters and an incomplete theory
Conventional physics does not adequately explain the superconducting state and neither does the elementary quantum theory of the solid state, which treats the behaviour of the electrons separately from that of the ions in the crystalline lattice. It was only in 1957 that three American researchers - John Bardeen, Leon Cooper and John Schrieffer - established the microscopic theory of superconductivity. According to their “BCS” theory, electrons group into pairs through interaction with vibrations of the lattice (so-called “phonons”), thus forming “Cooper pairs” which move around inside the solid without friction. The solid can be seen as a lattice of positive ions immersed in a cloud of electrons. As an electron passes through this lattice, the ions move slightly, attracted by the electron’s negative charge. This movement generates an electrically positive area which, in turn, attracts another electron. The energy of the electron interaction is quite weak and the pairs can be easily broken up by thermal energy – this is why superconductivity usually occurs at very low temperature. However, the BCS theory offers no explanation for the existence of “high-temperature” superconductors around 80 K (-193°C) and above, for which other electron coupling mechanisms must be invoked.
Type-I or Type-II, different states
The superconducting state can be destroyed by a rise in temperature or in the applied magnetic field, which then penetrates the material and suppresses the Meissner effect. From this perspective, a distinction is made between two types of superconductors. Type-I materials remain in the superconducting state only for relatively weak applied magnetic fields. Above a given threshold, the field abruptly penetrates into the material, shattering the superconducting state. Conversely, Type-II superconductors tolerate local penetration of the magnetic field, which enables them to preserve their superconducting properties in the presence of intense applied magnetic fields. This behaviour is explained by the existence of a mixed state where superconducting and non-superconducting areas coexist within the material. Type-II superconductors have made it possible to use superconductivity in high magnetic fields, leading to the development, among other things, of magnets for particle accelerators.