From the CERN website, posted 17 December 2014:

Last week the cryogenics team at CERN finished filling the arc sections of the Large Hadron Collider (LHC) with liquid helium. The helium, which is injected into magnetsthat steer particle beams around the 27-kilometre accelerator, cools the machine to below 4 degrees kelvin (-269.15°C).

The process of filling the LHC is an important milestone on the road to restarting theaccelerator at higher energy, though it will still take many weeks to cool the entire accelerator to its nominal operating temperature of 1.9 K (-271.3°C).

The electromagnets that steer particle beams around the LHC must be kept cold enough to operate in a superconducting state – the temperature at which electricity can pass through a material without losing energy to resistance. The niobium-titanium wires that form the coils of the LHC’s superconducting magnets are therefore maintained at 1.9 K by a closed liquid-helium circuit. This is colder than the average temperature – 2.7 K – in outer space.

Some 1232 dipole magnets will produce a magnetic field of 8.33 tesla to keep particle beams on course around the LHC's 27-kilometre ring. A current of 11,850 amps in the magnet coils is needed to reach magnetic fields of this amplitude. The use of superconducting materials has proved to be the best – and most cost-effective – way to avoid overheating the coils.

Find out more:

• Cryogenics at the LHC
• About the higher energy restart