LEP uses a circular ring of magnets to guide the particles round a thin vacuum pipe. They pass through regions in which they are accelerated by successive boosts, until each bunch is travelling around the 27 km ring 10,000 times every second. Four bunches of electrons travel one way around the ring and four bunches of positrons travel the other way around, each bunch contains a hundred thousand million particles (1011). It is relatively easy to "hold" all the particles within the same ring of magnets, although they are travelling in opposite directions, since the two types of particle have opposite electric charge.
Once accelerated to the maximum speed, the paths of the particles are crossed at four evenly spaced points on the ring, allowing the electrons and positrons to collide. The bunches of particles are focussed down to roughly the thickness of a human hair, but still only one in 40,000 collisions of electron and positron bunches results in a head to head collision of two particles. Because of this the LEP experiments last several hours although bunches collide every 22 millionths of a second.
