CRYOGENICS
The cryogenic technology chosen for the LHC was developed by the Commissariat a l'Energie atomique in Grenoble, and pioneered industrially for the Tore Supra fusion tokamak at Cadarache, France. It uses superfluid helium, which has unusually efficient heat transfer properties, allowing kilowatts of refrigeration to be transported over more than a kilometre with a temperature drop of less than 0.1 K.
LHC superconducting magnets sit in a 1.9 K bath of superfluid helium at atmospheric pressure. This bath is cooled by low pressure liquid helium flowing in heat exchanger tubes threaded along the string of magnets. The reliability and efficacy of this sophisticated cryoloop are key factors in achieving the required magnet performance.
The LHC cryogenic system is very large as well as very cold. Refrigeration power equivalent to over 140 kW at 4.5 K is distributed around the 27 km ring. To save costs, it was essential to reuse the four existing LEPII 12 kW, 4.5 K cryoplants. Their cooling power was increased by 50% and 1.9 K stages added. Again for economic reasons, innovative technology was developed for the helium compressors. Multi-stage, cold centrifugal compressors are needed and valuable input was provided by the work at the Continuous Electron Beam Accelerator Facility CEBAF in the US, on a system at about on third LHC scale.
In all, the LHC cryogenics system has 40,000 leak-tight pipe junctions. 12 million litres of liquid nitrogen was vaporised during the initial cooldown of 31,000 tons of material and the total inventory of liquid helium is about 150 tonnes.
A more detailed explaination is available here