Supercooling the LHC
CERN's Large Hadron Collider (LHC) is the new research instrument of the world's elementary particle physics community. This discovery-making machine will explore the structure of matter and basic forces of nature on a scale never attained before. Research will focus in particular on the Higgs boson, the hypothetical particle which plays a key role in explaining the origins of mass but has not yet been observed. Beams of protons circulating in opposite directions along the 27-km-circumference LHC, at 99.999999% of the speed of light, are brought into collision at the heart of 4 large detectors installed in deep underground caverns, where the resulting spray of particles is analysed and recorded. The LHC is to produce up to 600 million such particle collisions per second. High-field superconducting magnets are required to guide and focus the beams. This implies high electrical currents, which can only be accommodated by superconducting windings made of niobium-titanium alloy offering no resistance to electricity, and therefore no dissipation. To maintain the magnet windings in the superconducting state under high currents and high fields, they must be cooled down to -271°C, just 1.9°C above absolute zero by sub-cooled helium. At this temperature, the helium becomes superfluid, i.e. it exhibits high thermal conductivity and minimal friction and viscosity. The magnets are thus better cooled, so that they can operate at higher field and are better stabilized against thermal disturbances. In view of the sheer scale of the project and of its technical novelty, specific technologies were developed over the years by CERN, national laboratories and the specialized industry, thus permitting construction to start a decade ago. While most of the components and technical systems were procured satisfactorily from industry in the 20 CERN Member States, as well as in Canada, India, Japan, Russia and the USA, some difficulties plagued the procurement of the compound cryogenic line circling the ring, causing delay in the project installation. After vigorous corrective actions taken by CERN and the contractor Air Liquide, this unprecedented installation was completed in October 2006 with a better performance than initially expected. The LHC constitutes the largest helium cryogenic system in the world, with some 100 tons of liquid helium cooling 36 000 tons of equipment at a temperature "colder than outer space" as says Philippe Lebrun, who is in charge of the main technical systems of the project and President of the IIR's Commission A1.