Large Hadron Collider
The European Laboratory for Particle Physics (CERN), hosts a unique arrangement of interlocking particle accelerators. Each of the lower-energy machines delivers particles that are either used directly in experimental studies, or are accelerated further by a higher-energy machine. The highest-energy accelerator at CERN is the Large Hadron Collider (LHC). This is a synchrotron that accelerates beams of protons or ions in both directions around a near-circular tunnel, with a circumference of 26,659 metres (16.6 miles) – a little over half the length of the channel tunnel (31.4 miles). The LHC tunnel, which is at a depth below ground of between 45 metres (towards lake Geneva) and 170 metres (at the foot of the Jura mountains), was excavated in the 1980s, and originally housed a different machine, the Large Electron- Positron (LEP) collider.
Particles reach the LHC after having their energy boosted by a linear accelerator and a series of three synchrotrons. Protons arrive at the LHC with an energy of 0.45 TeV and, under 2011 operating conditions, are accelerated to 3.5 TeV. Beams of protons travelling in opposite directions are made to collide in four caverns, each of which hosts one of the main LHC experiments: ALICE, ATLAS, CMS, LHCb. The collision energy is 7.0 TeV, the sum of the energies of the two protons involved. Following upgrades in 2013, the collision energy will be doubled to 14.0 TeV.
Keeping protons with energies of up to 7.0 TeV circulating at the LHC requires powerful magnetic fi elds. The main contribution to these comes from 1232 electromagnets, each 14.3 metres in length. The magnets are operated in liquid helium, at a temperature of –271.3°C – colder than outer space (–270.5°C). At this temperature, the niobium-titanium alloy of the magnet coils acts as a superconductor, allowing the large electric currents needed to produce magnetic fi elds of the required strength. Overheating magnets caused damage shortly after protons were fi rst circulated in the LHC, in September 2008. Remedial work, to repair the damage and to prevent a repeat of the problem, were carried out during an extended shutdown.
The LHC has been the world’s highest-energy particle accelerator since November 2009. It took the world record for collision frequency in April 2011, and improves on this on a regular basis.