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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.

  • Compact storage rings are small devices (circumference ~30 m or less) utilizing bending magnet radiation for mostly industrial applications.
  • 1st generation synchrotron radiation light sources are operated partially for synchrotron radiation and partially for other programs (e.g., high energy physics). Typically these light sources were parasitic on the other program.
  • 2nd generation synchrotron radiation light sources are dedicated for synchrotron radiation but are not designed for low emittance or with many straight sections for insertion devices.
  • 3rd generation synchrotron radiation light sources are dedicated for synchrotron radiation and designed for low emittance and with many straight sections.

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:

  • Energy is the energy of the electrons in the storage ring. The highest energy light sources have energies of 6-8 GeV; medium energy machines are usually in the range 1.5-3 GeV; machines primarily designed for ultraviolet light production are around 0.5 to 0.8 GeV.
  • Current is the current of the circulating electron beam.
  • Circumference is the circumference of the storage ring.
  • Number of useable straight sections is the number of straight sections that can accommodate insertion devices and experimental beam lines.
  • Number of useable bending magnet ports is the number of bending magnet ports that can accommodate experimental beam lines.
  • Top up mode is the process to maintain the electron current at a preset level by frequent injection of additional electrons.

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.

  • Wiggler magnets produce intense, energetic radiation over a wide range of energies with a brightness 100 times that of bending magnets.
  • Undulators produce radiation of selected energies (harmonics) at high brilliance with a brightness 1,000 times that of bending magnets.

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.

  • Flux is a measure of the intensity integrated over all vertical opening angles (above and below the plane of the electron orbit) of the photon beam and is the appropriate measure for experiments that use the entire unfocused X-ray beam.
  • Brightness is the number of photons emitted per second, per square millimeter of source size, per square milliradian of opening angle, within a given spectral bandwidth (usually 0.1%). Brightness is a measure of the concentration of the radiation and increases as the size and divergence of the electron beam decrease. Undulators produce the brightest beams of synchrotron radiation.

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.

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Last Modified: May 2, 2014
Please forward all questions about this site to: Gary Schroeder