Glossary of Common Terms
Used with Storage-Ring-Based Synchrotron Radiation Light Sources
Synchrotron radiation is the name given to light radiated by an electric charge following a curved trajectory, for example, a charged particle under the influence of a magnetic field. Synchrotron radiation is a natural phenomenon that has existed since the Big Bang. It is in the starlight that we see at night, generated by charged particles of matter spiraling through the cosmos. However, a manmade, controllable source of such radiation was not found until the middle of the twentieth century when accelerators for charged particles first appeared. High-energy electron accelerators emerged as viable synchrotron radiation sources because, as electrons approach the speed of light, the synchrotron radiation increasingly is emitted in a narrow, forward-directed cone. Thus, the radiation is concentrated in a small solid angle and can be readily used by researchers.
Key components of a synchrotron radiation light source (at right) are (1) an electron gun, (2) a linear accelerator, (3) a booster synchrotron, (4) a storage ring, (5) beamlines, and (6) experiment stations.
Generation: Synchrotron light sources are commonly referred to as belonging to one of four generations of machines.
Storage rings maintain the electrons in a fixed orbit at a particular speed. The storage ring contains components such as magnets, insertion devices, and radio frequency cavities, which respectively keep the electrons in their orbit, intensify the light beam produced by the electrons, and supply energy to the electrons to increase their energy. Some parameters of a storage ring are:
Insertion devices are periodic arrays of magnets designed to produce a series of deflections of the electron beam in the place of its straight-line orbit in the storage ring. Such devices are inserted into the straight sections of the storage ring with one array of magnets above and one array below the electron beam path. As the charged particles pass through the alternating field, their deflections produce extremely intense synchrotron radiation. They are the key devices for the generation of synchrotron light in third generation storage rings. Instead of one magnet deflecting the electron beam and generating a single fan of light, an entire array of magnets deflect the beam. Each deflection adds to the intensity of the light.
Flux and brightness (or brilliance) are measures of the intensity of the radiation based on a measure of the number of photons per second in a narrow energy bandwidth (usually 0.1%) per unit solid angle.
Polarization is a measure of the alignment of the electric field vector of the light. Normally linearly polarized, special undulators can alter the polarization to produce variable ellipticity and helicity, which will enable a wide variety of polarization-dependent studies.
Coherence is a measure of the alignment of the phases of the electric field vectors of the light, i.e., a measure of the degree to which the waves are in phase across a light beam at any instant (transverse or spatial coherence) and the degree to which they remain so as the light propagates (longitudinal or temporal coherence). The transverse coherence of a synchrotron radiation beam is proportional to the brightness and hence is the highest for undulator beams. The high spatial and temporal coherence of light from undulators facilitates both tight focusing for microscopy and advanced imaging technologies such as holography.
Emittance is a measure of the size of the electron beam in position momentum phase space and is a constant of the storage ring. Designers of storage rings endeavor to make the emittance as small as possible as this increases the brightness of the photon beams that are generated by various components of the ring.
Last Modified: May 2, 2014