'An accelerator usually consists of a vacuum chamber surrounded by a long sequence of vacuum pumps, magnets, radio-frequency cavities, high voltage instruments and electronic circuits. Each of these pieces has its specific function. The vacuum chamber is a metal pipe where air is permanently pumped out to make sure the residual pressure is as low as possible. Inside the pipe, particles are accelerated by electric fields. Powerful amplifiers provide intense radio waves that are fed into resonating structures, the Radio-Frequency (RF) cavities. Each time the particles traverse an RF cavity, some of the energy of the radio wave is transferred to them and they are accelerated. To make a more effective use of a limited number of RF cavities, accelerator designers can force the particle beam to go through them many times, by curving its trajectory into a closed loop. That is why most accelerators will look roughly circular. Curving the beam's path is usually achieved by the magnetic field of dipole magnets. This is because the magnetic force exerted on charged particles is always perpendicular to their velocity - perfect for curving the trajectory! The higher the energy of a particle, the stronger the field that is needed to bend it. This means that, as the maximum magnetic field is limited (to some 2 Tesla for conventional magnets, some 10 Tesla for superconducting ones), the more powerful a machine is, the larger it needs to be.'