Currently the biggest electron accelerator is LEP (Large Electron Positron Collider) at CERN. Its circumference is 27 kilometres and the whole accelerator is situated more than 100 meters below ground.

LEP is a collider where the two particle beams consist of electrons and positrons (anti electrons) respectively. The beams of particles are accelerated in opposite directions in orbits next to each other inside the beam tube. There after the beams are made to collide with each other.

To achieve the high energy needed, the particles are accelerated turn after turn in the circular accelerator. Special acceleration fields increase the energies of the particles for each turn. Different types of magnets force the particles to move in well-defined orbits.

Four steps of acceleration increase the energies of the particles with up to 400 MeV per turn, to a top energy of about 100 GeV. Four bunches of electrons and positrons can circulate inside the ring for hours, with a speed very close to the speed of light.

Inside LEP there is 3 368 magnets and 272 superconducting acceleration steps. All these parts need to be frozen down to a temperature of minus 269 degrees Celsius.