When electrons (or any charged particles) are accelerated to
keep them in a circular path they will radiate electromagnetic radiation in a
narrow beam in the direction that they were travelling. This radiation is called
synchrotron
radiation. ("Synchrotron" is the name given to any circular
accelerator that uses microwave electric fields for acceleration and magnets for
steering.)
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 the particles
approach close to the speed of light the amount of energy lost to synchrotron
radiation for a fixed radius ring grows rapidly. This means that one needs to
make bigger radius rings to store higher energy particles and provides a
practical limit to the energy that can be reached in a synchrotron.
However, 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.
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
specialised synchrotron storage rings were built. Additional magnets known as
"wigglers" were added in straight sections of the 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.
