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What Does a Light Source Look Like Today?

The largest light source facilities
The largest light source facilities are campuses onto themselves with administrative, office, and laboratory buildings in addition to the light source itself. (Courtesy: European Synchrotron Radiation Facility)

Today’s synchrotron-based light source is a large, roughly circular machine (usually about the size of a football field to much larger) that accelerates electrons to almost the speed of light. The accelerator components include an electron gun, one or more injector accelerators (usually a linear accelerator and a synchrotron but sometimes just a large linear accelerator) to increase the energy of the electrons, and a storage ring where the electrons circulate for many hours. In the storage ring, magnets force the electrons into circular paths. As the electron path bends, light is emitted tangentially to the curved path and streams down pipes called beamlines to the instruments where scientists conduct their experiments

Synchrotron light is always emitted in the forward direction in a narrow cone, like water droplets from a spinning wet tire. As the electrons sweep around the curve, they generate a horizontal fan of light. The higher the kinetic energy (i.e. the speed) of the electrons, the narrower the emission cone becomes. The spectrum of the emitted radiation also shifts to shorter wavelengths as the energy increases, so that sources designed to produce x rays tend to have higher energy and to be larger and more expensive than those designed for ultraviolet. In sum, the storage ring is a stable, continuous source of light over a wide wavelength range. Together with additional features, such as controllable polarization (both linear and circular), laser-like collimation, and pulsed time structure, these characteristics make synchrotron light sources the x-ray source of choice for a wide range of research.

Components of a synchrotron light  source
Components of a synchrotron light source typically include (1) an electron gun, (2) a linear accelerator, (3) a booster synchrotron, (4) a storage ring, (5) beamlines, and (6) experiment stations. (Courtesy: Australian Synchrotron, Illustrator: Michael Payne)

In the newer light sources, the storage ring is specifically designed to include special magnetic structures known as insertion devices (undulators and wigglers). Insertion devices generate specially shaped magnetic fields that drive electrons into an oscillating trajectory for linearly polarized light or sometimes a spiral trajectory for circularly polarized light. Each bend acts like a source radiating along the axis of the insertion device, hence the light is very intense and in some cases takes on near-laser-like brightness.

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