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Refrigeration's role in the world's biggest experiment

A CRYOGENIC refrigeration system capable of generating temperatures as low as -271 degrees Celsius is part of the world's biggest science experiment- the Large Hadron Collider.
First reported in ACR News magazine in December 2007, the central part of the Large Hadron Collider is the world’s largest fridge and is colder than deep outer space.

The LHC, a massive particle accelerator, was built at the CERN physics laboratory near Geneva and is contained in an underground tunnel (17 miles in circumferance). A liquid helium refrigeration system is used to cool the LHC's circular tunnel, which crosses the border between Switzerland and France at four points.

On September 10, scientists at the LHC sent two beams of protons travelling at almost the speed of light in opposite directions along the tunnel to create a head-on collision between the two beams.

The LHC tunnel contains two pipes enclosed within 'superconducting' magnets, with each pipe containing a proton beam. The tunnel must be cooled down to -271°C in order for the LHC’s superconductive magnets to remain superconductive and guide the two proton beams around the LHC ring route as they travel at almost the speed of light.

The two beams are directed around the ring by magnets to four points where collisions between the two beams can take place.

The superconductive magnets sit in a 1.9K bath of superfluid helium at atmospheric pressure. It is cooled by low pressure liquid helium flowing in heat exchanger tubes threaded along the string of magnets. Refrigeration power equal to more than 140kW at 4.5kW is distributed around the underground ring.

Linde supplied each of the LHC's eight 4.5kw refrigeration units which incorporate a pumping system to divide the temperature by 2.5 in order to achieve the 1.9K.

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.

Approximately 96 tonnes of liquid helium is needed to keep the magnets at their operating temperature of -271°C (1.9 K), making the LHC the largest cryogenic facility in the world at liquid helium temperature.

Scientists behind the LHC hope to learn how matter behaved a fraction of a second after the Big Bang. They hope the LHC experiments will answer questions such as what gives matter its mass, what the invisible 96% of the universe is made of, why nature prefers matter to antimatter and how matter evolved from the first instants of the universe’s existence.

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