A limiting factor of direct voltage and cascade accelerators is that they expose the particle to the entire voltage at once and so are limited by the problem of electrical breakdown. A radio frequency linear accelerator uses a smaller but changing electric field over and over again to increase the energy of the particle. The particles pass through tubes called cavities which are alternately charged by the alternating supply.

Initially, each tube is longer than the last so that, as the particle gets faster, it traverses each one in the same time to keep in step with the changing field. In high energy machines, and therefore especially in electron accelerators, as particle approaches the speed of light and its speed remains nearly constant, the tubes are evenly spaced out.

Ignoring relativity, the energy gained by the particle is equal to NqV where N is the number of cavities and V is maximum voltage of the RF supply. For example, an early Berkley machine had 30 cavities with a peak voltage supply of 43 kV to accelerate singly-ionised mercury atoms to 1.3 MeV.

To reach very high energies, a large number of cavities are needed and so the machine becomes very long. The world’s largest such machine is SLAC, the Stanford Linear Collider, a 20 GeV electron accelerator two miles long. The CLIC accelerator planned by CERN could be nearly 40 km long.

A synchrocyclotron is simply a cyclotron with the accelerating supply frequency decreasing as the particles become relativistic and begin to lag behind. Although in principle they can be scaled up to any energy they are not built any more as the synchrotron is a more versatile machine at high energies.

Introduction        Direct voltage and cascade machines           Cyclotrons

Betatrons                    Linear accelerators and the synchrocyclotron                Synchrotrons       

    Fixed target verses collider machines        Lepton verses Hadron machines        The Future?