LS1 Report: A cold, cold summer
The cooling of the LHC is advancing quickly, with the second sector having now reached 200 K (about -73°C). By the end of the summer, four of the sectors will have been cooled. To achieve this, trucks carrying around 20 tonnes of nitrogen each are clocking up the miles to bring the cryogenic liquid to CERN. When the whole process is complete, almost four times the mass of the Eiffel Tower will have been cooled, using more than 10,000 tonnes of nitrogen and 140 tonnes of helium.
Liquid nitrogen, arriving to CERN on trucks, is injected into exchangers that pre-cool the helium flow used to cool the magnets.
Cooling a sector (about 3 kilometres long) of the LHC is a fairly complex operation involving several stages. This summer, for the first time, the first two sectors will be cooled to 20 K (and not directly to the nominal temperature of 1.9 K) and will be maintained at this temperature for two weeks. “This plateau is necessary to allow the teams to carry out checks on the joints that have been repaired during LS1,” explains Gérard Ferlin of the TE Department, who is in charge of cryogenics operations at the LHC.
Cooling to 20 K is achieved in two stages. The first consists of lowering the temperature to 80 K by cooling helium using nitrogen. “Every day for the first 10 days, we receive five to six truckloads of nitrogen from the Lyon area,” explains Gérard. “This represents about 110 tonnes of nitrogen per day for a total of around 1200 tonnes per sector. The trucks arrive at CERN from 6 a.m. to 10 p.m. The liquid nitrogen is then injected into exchangers that pre-cool the helium flow used to cool the magnets.”
After this first stage, to lower the temperature of the helium from 80 K down to the 20 K plateau (and later the final 1.9 K), the operators use the refrigerators present in each sector of the LHC, which cool the helium using turbo-expanders.
At the end of the first LHC run, the helium inventory totalled 140 tonnes. CERN has only a limited capacity for storing helium, so the precious molecules were temporarily sent back to the suppliers. Now they are coming back to CERN. “Now that our refrigerators are operational again, we can store liquid helium in our tanks until it is time to reinject it into the accelerator,” confirms Laurent Tavian, head of the Cryogenics Group in the TE Department. “Planning the restart has been a real challenge: in August and September, we will have five to six sectors being filled at once, which would have been impossible without the new tanks installed in 2009.”
Despite the tight schedule, the operators are always keeping a close eye on safety: “We have very strict procedures to ensure that people working in the tunnel and on the installations are completely safe," explains Laurent. “We have a checklist of all the systems related to safety. Cooling is only given a green light when the configuration of all the systems has been validated by the various people in charge.”
The final figures relating to the cooling of the machine are impressive: the 27 kilometres of the LHC consist of more than 36,000 tonnes of equipment, about four times the mass of the Eiffel Tower. To cool this enormous mass, we will have used more than 10,000 tonnes of nitrogen and 140 tonnes of helium. According to the current schedule, all eight sectors of the LHC will be cooled to 1.9 K by October 2014.
| Meanwhile, elsewhere in the LHC... Teams are carrying out their final tests prior to cool-down in Sectors 1-2, 2-3 and 5-6. Leak tests are being performed on the vacuum sub-sectors of Sectors 3-4 and 4-5, and pressure tests are underway in Sector 7-8. Following last month's cool-down of Sector 6-7, the first CSCM (Copper Stabilizer Continuity Measurement) tests are being conducted. |
