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 will sit in a 1.9 K bath of superfluid helium at atmospheric pressure. This bath will be 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 is essential to reuse the four existing LEPII 12 kW, 4.5 K cryoplants. Their cooling power will be increased by 50% and 1.9 K stages will be added. Again for economic reasons, innovative technology is being sought for the helium compressors. Multi-stage, cold centrifugal compressors are needed and valuable input will be provided by present work at the Continuous Electron Beam Accelerator Facility CEBAF in the US, on a system at about on third LHC scale.
In all, LHC cryogenics will need 40,000 leak-tight pipe junctions, 12 million litres of liquid nitrogen will be vaporised during the initial cooldown of 31,000 tons of material and the total inventory of liquid helium will be 700,000 litres.