The 4.5K refrigerators for the LHC are all present and correct!

Acceptance procedures for the last of the LHC 4.5 K refrigerators have been completed at Point 6. All that now remains to be done, to have the LHC refrigeration system ready by the end of 2005, is to upgrade the refrigerators recovered from LEP and install the 1.8 K cooling system.

By now you will be well aware that the LHC's superconducting magnets cannot operate unless they are cooled to 1.8 K (-271°C) (see Bulletin 13-14/2002). Eight 4.5 K refrigerators, one for each sector of the LHC, are required to achieve this. In December last year the last of these refrigerators, specially designed for the LHC, was accepted at Point 6. "It is a big step forwards for the LHC's refrigeration system, as it takes its cooling capacity at 4.5 K to 140 kW, that is to say almost 40,000 litres of liquid helium per hour, a capacity never previously attained. The progress achieved since the days of LEP is impressive, particularly if one bears in mind the extent of the infrastructure required, " (see graph) underlines Serge Claudet, head of the LHC Cryogenic Refrigeration Team.

The process began in 1998, when contracts were signed with Air Liquide in France and Linde in Switzerland. The design and industrial engineering phases that followed made it possible for the first deliveries to take place from the middle of 2000, while LEP was still in operation. After a period of intensive tests, the first refrigerator built by Air Liquide was accepted at Point 1.8 in March 2002. It has since been used to supply the test benches for the superconducting magnets. In December 2002, a second refrigerator was accepted at Point 4, followed by the two refrigerators manufactured by Linde in August and December 2003, at Points 8 and 6.
Each of these refrigerators consists of a compressor unit and a cold box. The compressor unit supplies compressed helium gas at 20 bar and at room temperature. The cold box houses the heat exchangers and the expansion turbines that provide the cooling necessary to liquefy the helium to 4.5 K.
This sounds straightforward enough, but in reality for each compressor unit "it means 100 tonnes of equipment sitting on 120 tonnes of concrete with an installed power of 4.2 MW, representing an electricity cost of CHF 6000 per day of operation at full power," explains Serge Claudet. As for the cold boxes, you have to picture each of them as weighing 100 tonnes and occupying a volume of more than 100 m3 (see photographs). Automatic operation of the refrigerators requires 850 signals to be processed per refrigerator!

Udo Wagner and Henning Gruehagen, the two CERN project engineers responsible for liaising with Air Liquide and Linde respectively, commented that this marks the end of "an important stage" and represents an "interesting and epic achievement" for themselves and all their colleagues.
However, the story of the LHC refrigeration system does not end with the installation of the final 4.5 K refrigerator. From now on, the LHC Cryogenic Refrigeration Team will focus on upgrading the four LEP refrigerators, already in place at Points 2, 4, 6 and 8. Simultaneously, they will finish installing the cryogenic infrastructure, the vertical transfer lines, the pipe work, the storage of fluids, etc., as well as the 1.8 K cooling system to supplement the 4.5 K system. All these installations will gradually be brought into service in order to test the other cryogenic system assemblies. This should enable final adjustments to be made and provide the experience necessary to face up to the next challenge, that of cooling the first sector of the machine in 2005.