Synchrotron X-ray sources
At a synchrotron facility, bunches of electrons, several GeV in energy, move in a large, carefully steered, closed electron beam loop containing bending elements and linear segments, collectively called the storage ring. In each section, magnetic devices are inserted - bending magnets in the curved sections, insertion devices called wigglers and undulators in the straight sections - to bend, wiggle or undulate the path of the electrons while they pass around the ring. Due to the acceleration experienced in the bending magnets or insertion devices, the electrons emit a narrow fan of intense white (polychromatic) radiation ranging from soft UV to hard X-rays over a very tightly defined angle tangential to the ring. The radiation is 'tunable' by cutting out fine bands (few eV or 10-5 Å wide) of wavelengths appropriate for particular experiments with monochromator crystals that selectively pass the wavelength of choice. The intensity of X-rays generated by modern 3rd generation synchrotron sources is so high that radiation damage to crystals has become a major concern, and this has given rise to the near-exclusive use of cryo-crystallographic techniques, in which crystals are kept at near-liquid nitrogen temperatures to minimize radiation damage. Synchrotron radiation has additional features that make it attractive for advanced applications. Because it is pulsed, it can be exploited for examining time-dependent phenomena, and because it is highly polarized, it can be used to examine polarization-dependent and angle-dependent effects.