Synchrotron Radiation

When electrons (or any charged particles) move at speed close to c in a circular path, they emit electromagnetic radiation in a narrow beam in the direction that they were travelling. This radiation is strongly polarized in the plane of motion and has a broad spectrum; it is called synchrotron radiation. ("Synchrotron" is the name given to any circular accelerator that uses microwave electric fields for acceleration and magnets for steering.)
If the particle motion is non-relativistic (the speed is much less than the speed of light c), the emitted radiation is much weaker and has the frequency of the circling motion: it is called cyclotron radiation. The radiation switches gradually from cyclotron-type to synchrotron-type as the speed increases.

The following is adapted from the Stanford Linear Accelerator Center (SLAC) webpage http://www.slac.stanford.edu/gen/edu/about-6.html

For high-energy physicists the synchrotron radiation is a nuisance because the electrons lose energy and must be reaccelerated to keep them travelling around the ring. Otherwise the match between their momentum, the magnetic field and the radius of the ring is quickly lost. As a particles approaches close to the speed of light the amount of power lost to synchrotron radiation for a fixed radius ring grows rapidly, as the fourth power of the "gamma factor"

E/mc² = (1-(v/c)²)^-1/2, where E is the (relativistic) energy, m is the (rest) mass, and v is the speed of the particle. This means that one needs to make bigger radius rings to store higher energy particles and sets a practical limit to the energy that can be reached in a synchrotron. (Note that the limit is quite different for protons than for electrons, since the proton's mass is 1836 times the electron's).
However, at the energies of the SPEAR storage ring at SLAC, the synchrotron radiation turns out to give a beautiful beam of x-rays which another group of physicists have exploited as a tool to study many aspects of the structure of matter at the atomic and molecular scale, from surface properties of semiconductor materials to the structure of protein molecules. This is the work of the division of SLAC known as the Stanford Synchrotron Radiation Laboratory (SSRL).
Originally this work began as a "parasite" project, using whatever synchrotron radiation was produced during high-energy physics use of the storage ring. Over time so much interesting research developed that SSRL has now completely taken over the SPEAR storage ring and built a new low-energy (3 GeV) accelerator so that SPEAR can be filled with electrons even when the main SLAC accelerator is busy doing other research. Additional magnets known as "wigglers" have been added in straight sections of the SPEAR ring to produce even more intense x-ray beams due to radiation as electrons "wiggle" through the alternating sections of magnetic field from these magnets.