Synchrotrons combine ideas from many of the preceding machines. The particles are accelerated by radio frequency cavities in a circular path of fixed radius using magnetic fields that increase in strength as the particles get faster.

The bending magnets, called dipoles, are placed along the beam path rather than over the whole area of the orbit, so the design can be scaled up to very large sizes – CERN’s LEP (Large Electron Positrons) was in a tunnel 27 km long and ultimately produced 105 GeV electron beams.

Particles are injected into a synchrotron from a pre-accelerator. If they are already moving ultra-relativistically then their speed around the ring will be nearly constant as they are ‘accelerated’ and so the radio frequency can be kept constant. As electrons reach 0.98c at only 2 MeV, this is almost always the case with electron machines but not for proton machines. The rest energy of the proton (~ 1 GeV) is much higher than that of the electron, and so as the protons become more energetic, the magnetic field strength and the radio frequency have to be increased.

As with all cyclic machines, the particles in a synchrotron are accelerating when they move in their circular path and so they emit electromagnetic radiation, called synchrotron radiation. This is emitted tangentially to the orbit and has so many uses in different areas of medicine and science that synchrotrons have been built simply to generate synchrotron radiation.

Because of their large mass, synchrotron radiation is not really an issue for protons, but for electron machines it is a major concern. The radiation obviously takes energy away from the particles and the power lost goes as

where r is the radius of the orbit and g is the Lorentz factor,

The accelerator designers want to make g as large as possible, so the size of the machine has to increase dramatically. LEP had an orbit radius of 3100 m but, when running at its highest energies of 100 GeV per beam, was losing 23 MW as synchrotron radiation and curved sections of the tunnel were out of bounds to all personnel.

During 2000-2006 the LEP is being removed from its tunnel and replaced by the LHC (Large Hadron Collider) which will produce 7 TeV proton beams. Although the beam energy will be higher by a factor of nearly 100, the lower g due to the larger proton mass means that synchrotron losses will only be 5 kW.

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?