When a charged particle, such as an electron, is accelerated, it emits electromagnetic radiation. Electrons energized into a range where their velocities are close to that of light give off radiation in the form of X-rays; this type of electromagnetic radiation is known as synchrotron radiation. A storage ring is a facility designed to energize electrons or positrons to high enough velocities that they can be accelerated in this way, and the resulting X-rays can then be used for research purposes. The X-ray fluxes (number of X-ray photons available to an experiment per unit time) and the brilliance (number of X-ray photons per unit time per unit area per unit divergence) of these storage-ring sources are higher than those achievable with laboratory X-ray sources. Storage rings, as the name implies, are set up so that the electrons or positrons travel in a closed loop. Usually the loop contains both straight sections and curved sections; the electrons travel in the straight sections without changing their primary direction, but they are bent (with magnetic fields) around the curved sections. X-ray radiation is given off in the bent sections. If the electrons are forced to move up and down or sideways through the straight sections (also with magnetic fields) then X-rays can be given off out of those areas too.
When a slow electron is accelerated, electromagnetic radiation is emitted in all directions in roughly equal proportions. The theory of relativity can be used to show that as the electron velocities approach that of light, the beams of X-rays that are produced out of a storage ring become increasingly flattened so that the beam comes out (a) in the plane in which the electrons are traveling, and (b) in a fairly narrow linear path, tangential to the direction in which the electrons are traveling at the time. We can therefore capture the emitted X-rays as they travel along that path and do experiments with them. These paths are called "beamlines".
There are a few dozen storage rings in the world that produce high fluxes and high brilliance. They are characterized as first-generation, second-generation, and third-generation sources. First-generation sources produce X-rays as a byproduct of an effort to accelerate electrons, so X-rays are only available under circumstances that do not interfere with experiments involving the electron beams themselves. Second-generation sources are dedicated sources: the purpose is the production of X-rays. Third-generation sources are also dedicated, and they are distinguished from second-generation sources by the fact that they have a large number of insertion devices, each of which consists of an array of electromagnets and permanent magnets placed within ("inserted") in the straight sections of the ring to produce a wiggling motion of the electrons. This wiggling motion produces extremely brilliant X-ray beams.
There are three third-generation sources: the European Synchrotron Radiation Facility in Grenoble, France; the Advanced Photon Source in Argonne, Illinois; and SPRing-8 in Japan. They were built in the order just given. ESRF is essentially complete and has been producing useful data for several years. The core of the APS was completed in 1995, and about half of the beamlines have been built out. The core of SPRing-8 was completed in 1997, and some of its beamlines have been built out.
The beamlines at the APS are organized into sectors, each of which consists of one insertion device beamline and one bending-magnet beamline. Most sectors have now been assigned to research groups called Collaborative Access Teams, each of which is responsible for funding, building, and then doing experiments on the beamlines within its sector. IIT is in charge of three sectors: sector 10, operated by Materials Research Collaborative Access Team (MR-CAT), sector 17, operated by the Industrial Macromolecular Crystallography Association Collaborative Access Team (IMCA-CAT), and sector 18, operated by Biophysics Collaborative Access Team (BioCAT). Other sectors are operated by other universities, by government labs, by industrial teams, or by combinations of the above.
The science that is done at the APS all involves the use of X-rays, but it covers a wide range of applications. IMCA-CAT and two other CATs at the APS specialize in macromolecular crystallography. MR-CAT and several other CATs offer research involving various techniques relevant to materials science, including powder diffraction, X-ray absorption spectroscopy, and single-crystal diffraction from small molecules. BioCAT does all types of X-ray research on biomolecules other than macromolecular crystallography; already underway are programs in biological X-ray absorption spectroscopy, fiber diffraction, and small-angle scattering. Other groups at the APS are doing X-ray imaging, environmental elemental analysis, archeological research, and other efforts.
As you tour the APS today, be alert to the various techniques that the individual CATs are employing and the scientific results toward which these efforts are directed.