Cryogenics is defined as the branch of physics dealing with the production of very low temperatures and their effects on the matter but more generally, cryogenics is the science and its application to physical phenomena ocurring below 120 K (-153 °C). This temperature of 120 K represents the limit where main atmospheric gases begins to liquefy (methan, oxygen, argon, nitrogen, argon, neon, hydrogen and helium).
Most of cryogenic systems use a cryogenic fluid where the latent heat of vaporization is used to maintain an object at
a constant temperature. Consequently, the choice of the cryogenic fluid depends directly of the desired temperature.
If the desired temperature is below 5 K (-263 °C), the only possible solution is the Helium because helium starts to liquefy around 5 K and all other elements are solid at this temperature. Note that helium is the only element where there is no solid phase at low temperatures when pressure is kept below 25 bar.
Moreover, helium demonstrates another state called superfluid when it is below 2.17 K. This state is designated by "He II", by opposition to the normal fluid "He I". This state is a quantum state of the matter where all the molecules behave in the same manner. It results extraordinary properties as the absence of viscosity or the perfect ability to transport the heat.
Phase diagram of helium-4
Cryogenics represents one of the biggest challenges of the
particle accelerator LHC (Large Hadron Collider). The LHC is composed of 1700
Nb-Ti superconducting magnets maintained at 1.9 K (-271.25 °C) along a ring of 27km.
To achieve this challenge, LHC makes use of superfluid helium due to its unique thermal
transport and viscosity properties.
Some LHC cryogenics key figures: