Electrons are injected into the booster synchrotron, a racetrack-shaped ring of electromagnets, and accelerated from 450 MeV to 7 billion electron volts (7 GeV) in one-half second. (By comparison, the electron beam that lights a TV screen is only 25,000 electron volts.) The electrons are now traveling at >99.999999% of the speed of light. The accelerating force is supplied by electrical fields in four radio frequency (rf) cavities. In order to maintain the orbital path of the electrons, bending and focusing magnets increase the electron field strength in synchronization with the rf field.