Cool days ahead
The outlook is sunny for CERN now that all the sectors are cold. The mood is growing feverish in anticipation of the LHC start-up!
One of the LHC cryoplants.
At the time this issue goes to press the LHC has been almost entirely cooled to 1.9K (-271ºC). Cooling of the 1700 main magnets commenced in January 2007, with Sector 7-8. Two months later, the temperature had dropped below 2K, making this 3.3-kilometre section the world’s largest superconducting installation cooled with superfluid helium. Cooling of the other sectors continued, with Sector 4-5 following in July 2007.
In all, more than 10'000 tonnes of liquid nitrogen were needed to cool down the eight sectors, followed by 130 tonnes of helium to fill them. "This amount of helium is approximately one percent of global annual production," says Laurent Tavian, head of the Cryogenics Group. "CERN gets its helium under two delivery contracts, most of it coming from two sources: one in Algeria and the other in Russia. For LHC filling, the rates of delivery to CERN had to be increased by a factor of five, to a peak of 40 tonnes per month!"
During cool-down, numerous operations were conducted. "For example, the final phase of cooling to 1.9K required the deployment of complex pumping systems combining several stages of cryogenic centrifugal compressors," explains Tavian. In the first few sectors these operations, of considerable technical difficulty, took more time to complete, as the responsible teams had to hone their skills. Thus, while cooling down and testing of Sectors 7-8 and 4-5 took place over periods of roughly eight months, for the last sectors both operations were completed in only three months.
Now that the entire accelerator is cold, the process of commissioning is continuing with electrical tests (see BULLETIN No. 30-31/2008) to verify the functioning of the magnets. Following these final tests, the machine will be ready for start-up. The first beam should be circulating in the accelerator in the first week of September.
The cooling process
The cooling down of the sectors takes place in three stages. In the first stage, 1250 tonnes of liquid nitrogen is used above-ground for the purpose of pre-cooling the helium, which is then piped to the sectors underground. The temperature of the helium drops to 80K, while the nitrogen evaporates into the atmosphere, creating a vapour plume. In the second stage the dipoles are cooled to 4.5K. Finally, the third stage entails the use of a pumping system to lower the pressure of the liquid helium so as to reach the requisite 1.9K (approximately -271°C). At that temperature, the helium is superfluid and loses its viscosity, increasing its capacity for heat transfer and wetting of the superconductors.
LHC Cooldown Status: