The Low Energy Ion Ring (LEIR) receives long pulses of lead ions from Linear accelerator 3 (Linac 3) and transforms them into the short, dense bunches suitable for injection to the Large Hadron Collider (LHC).

LEIR splits each long pulse from Linac 3 into four shorter bunches, each containing 2.2×108 lead ions. It takes about 2.5 seconds for LEIR to accelerate the bunches, in groups of two, from 4.2 MeV to 72 MeV. The ions are then at a suitable energy to be passed to the Proton Synchrotron (PS) for storage. Next, the lead ions are passed from accelerator to accelerator along the CERN complex to end up at their highest energy in the LHC. The LHC uses 592 bunches of ions per beam, so it takes around 10 minutes for LEIR to provide enough for a complete fill.

LEIR was first proposed in 1993 as a conversion of a then-existing machine, the Low Energy Antiproton Ring (LEAR). The older machine was designed to decelerate antiprotons. It finished in 1996, offering the opportunity to transfrom LEAR with an "A" into LEIR with an "I". Slowing and storing antimatter is now provided by the Antiproton Decelerator and the experiments ALPHA, AEGIS and ASACUSA.

Work on the LEIR upgrade started in 2003, and by autumn 2006, the accelerator was being used to re-commission the PS to handle ions. In 2007 LEIR did the same for the Super Proton Synchrotron. Finally, in November 2010, the Low Energy Ion Ring carried out its primary role for the first time, providing lead ions for the very first ion collisions in the LHC.