A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams.Large accelerators are best known for their use in particle physics as colliders (e.g. the LHC at CERN, RHIC at Brookhaven National Laboratory, and Tevatron at Fermilab), FACET at SLAC National Accelerator. Other kinds of particle accelerators are used in a large variety of applications, including particle therapy for oncological purposes, and as synchrotron light sources for the study of condensed matter physics. There are currently more than 30,000 accelerators in operation around the world.There are two basic classes of accelerators: electrostatic and oscillating field accelerators. Electrostatic accelerators use static electric fields to accelerate particles. A small-scale example of this class is the cathode ray tube in an ordinary old television set. Other examples are the Cockcroft–Walton generator and the Van de Graaff generator. The achievable kinetic energy for particles in these devices is limited by electrical breakdown. Oscillating field accelerators, on the other hand, use radio frequency electromagnetic fields to accelerate particles, and circumvent the breakdown problem. This class, which was first developed in the 1920s, is the basis for all modern accelerator concepts and large-scale facilities.Rolf Widerøe, Gustav Ising, Leó Szilárd, Donald Kerst, and Ernest Lawrence are considered pioneers of this field, conceiving and building the first operational linear particle accelerator, the betatron, and the cyclotron.Because colliders can give evidence of the structure of the subatomic world, accelerators were commonly referred to as atom smashers in the 20th century. Despite the fact that most accelerators (but not ion facilities) actually propel subatomic particles, the term persists in popular usage when referring to particle accelerators in general.