LHC: A cool 1.8 K is achieved for the first time

An LHC cryogenic unit installed in its final underground location has for the first time achieved 1.8 Kelvin (-271°C), the temperature at which the accelerator is to operate in 2007.

Some of the team responsible for supervising the installation and commissioning of the cryogenic unit at Point 8. From left to right: Guillaume Vincent from the Air Liquide-Linde-Serco consortium in charge of operating CERN's cryogenic facilities, Gérard Ferlin, CERN project engineer, and Adrien Forgeas from CEA Grenoble.

The LHC's cryogenic system has taken a major new step towards the very cold temperatures needed for the experiments. On 7 April, the cryogenic unit at Point 8 operated at 1.8 K (-271°C), the temperature that will be needed throughout the LHC ring in 2007. The accelerator will be cooled to this temperature close to absolute zero to allow the magnets of which it is made to reach the superconducting state and produce the very high magnetic fields needed to bend the trajectory of the protons accelerated at close to the speed of light.

As the niobium-titanium alloy at the heart of the LHC magnets becomes superconducting at a slightly higher temperature, the temperature of the liquid helium, 4.5 K, could be considered sufficient. "But at 1.8Â K we can get more out of the superconducting material and the helium's cooling capacity", says Serge Claudet, who is in charge of the team responsible for the cryogenics production for the LHC. At this temperature helium becomes superfluid, which allows it to flow with virtually no viscosity and gives it a greater heat transfer capacity.

The amazing properties obtained at this temperature justify the challenges that have to be overcome to achieve it. The LHC cooling system is much more complex than just a series of freezers. Many tonnes of equipment and machines - compressor stations, cold boxes with expansion turbines and heat exchangers, interconnecting lines - have to be installed. Altogether, the assembly constitutes a unique system.

The cooling is done in two stages. Eight refrigerators - one for each octant of the LHC - cool the system to 4.5Â K. Four new refrigerators have been in place since the end of 2003 (see Bulletin 07/2004), while four old refrigerators recovered from LEP are currently being adapted for the LHC. Each of these enormous refrigerators is backed up by a sophisticated pumping system that allows the final 3Â degrees of cooling to be obtained to reach 1.8 K. The closer to absolute zero, the harder it gets. According to the laws of thermodynamics, the boiling temperature of a fluid varies according to pressure. In order to cool a fluid, the saturation pressure has to be reduced by pumping. For example, water will boil at 100°C in a kettle located at sea level but requires only 84°C at the top of Mont Blanc and only 10°C at a pressure of around 100 times lower than atmospheric pressure. The 1.8 K cooling system is based on this principle. To achieve a pressure of 15 millibars, the system uses both hydrodynamic centrifugal compressors operating at low temperature and positive-displacement compressors operating at ambient temperature.

It was one of these complex units, built by the Swiss-Japanese consortium IHI-Linde that, as part of the cooling chain, achieved 1.8 K for the first time at the beginning of April. The pre-series units had been validated in 2002 and 2003 but only at the surface, using specially adapted testing methods. The small 6-man team from CERN and CEA Grenoble which supervised the work can be proud of its achievements. "The installation and commissioning of these machines imposes strict technical requirements", says project engineer, Gérard Ferlin, "and what's more, we will have to do it again another seven times".

The four units built by IHI-Linde have already been installed and will be acceptance-tested later this year. The other four units, made by the French company Air Liquide, are currently being installed and will be tested in 2006, marking the start of the big chill for the 27 kilometres of the LHC tunnel.

Diagram showing the different phases of the helium. First the helium is cooled to 4.5 K. Then the pressure is reduced to 15 millibars and the temperature drops to 1.8 K, at which point the helium becomes superfluid.