The cooling system at the Large Hadron Collider (LHC for short) is probably the second most exciting aspect of the project. The first, of course, is the precision-guided particle beams that physicists expect to help recreate the conditions at the start of the universe. In comparison to such particle physics pyrotechnics, the cooling system may not seem too flashy, but it is extremely important. Without its state of the art cooling systems, the LHC could not possibly do its job. Each particle beam collision will contain about 7 teraelectron volts; though the largest collision so far recorded is only about 3.5 TeV, the LHC has already created the largest energy man-made collisions of particle beams.
In order to operate safely and allow scientists to capture the elusive Higgs-Boson particle, the cooling system needs to keep the LHC at tremendously low temperatures. Cooling systems keep the system at a chilly 1.8 degrees Kelvin – that’s -271 degrees Centigrade for those keeping track at home. 1.8 degrees K is very close to absolute zero, the temperature at which all molecular motion stops, in order to induce superconductivity in the electromagnets guiding the particle beams. These temperatures are so low that they necessitated the installation of never before used cooling equipment, which was manufactured by Linde AG.
The only coolant on Earth capable of such low temperatures is liquid helium, an ultra-cold fluid with some unique properties. The cooling system, therefore, relies heavily on liquid helium. It consists of eight separate cooling systems, each of which is spaced apart along the 17 mile ring of the LHC. To each cooling system is attached a cold box, which houses a compression system. The cooling systems are directly responsible for maintaining the magnets at superconducting temperatures, which is no small feat. It takes several weeks to cool the magnets down to 1.8 K, which means lengthy delays when any given part fails or needs to be switched out.
Image of LHC Dipole, taken from Flickr.
The cooling systems were designed and manufactured by Linde Kryotechnik AG, a Swiss company. For their work in the design of the cooling systems, CERN presented Linde with a Gold Hadron award in 2003. Representatives from Linde and two other corporations are responsible for the operation of the cooling systems.
These cooling systems make use of two different super-cooled liquids: liquid helium and liquid nitrogen. A total of 130 tons of liquid helium and 10,000 tons of liquid nitrogen are used in the system, which must be completely leak-free to operate. A leak in any one of the 40,000 welds in the system can cause serious damage and a months-long shutdown.
In order to guide the tiny particle beams, extremely powerful magnets must be used. Each magnet has a field of 8.36 Tesla (typical electromagnets can’t exceed 2 Tesla). The magnets are 14 meters long and have an inner diameter of 56 millimeters.
In order to achieve the extreme field strength required of them, the magnets make use of advanced technologies: they are made of a niobium-titanium alloy and encased in copper. This is not an altogether new technology, having been invented in the 1960s, but it is being operated at much colder temperatures than in the past. These super-cold temperatures and high field strength limit the materials that can be used. While operating, the magnetic cables need to be able to carry 15,000 amps at 1.5 Kelvin while resisting many hundreds of tons of force in each meter of the cable.
To maintain the 1.9 K operating temperature, the LHC draws on a 150 kW refrigeration capacity at 4.5 K (20 kW at 1.9 K). This refrigeration capacity is obtained by use of heat exchangers around the magnets. The heat exchangers use liquid helium, which is one thousand times more heat conducting than copper, as a working fluid. The heat exchanger contains a mix of liquid and vapor helium; the vapor is taken to a header, where it is compressed down to atmospheric pressure.
Another view of the LHC with magnet. Image from Flickr.
There have, unfortunately, been setbacks at the LHC that have set back research several months; these accidents have often been caused by failures in the cooling system. Even the slightest failure in cooling can result in catastrophe, as the magnets alone contain about 10 gigajoules of energy. A helium leak caused by faulty insulation on the magnets led to a year-long shutdown after just a week of operation. Other issues have arisen, typically also involving the cooling system in some aspect.