Synchrotron light source

This article is mostly concerned with the laboratory production and applications of synchrotron radiation. For details of physics of emission and properties, see synchrotron radiation.
Synchrotron radiation reflecting from a terbium crystal at the Daresbury Synchrotron Radiation Source, 1990

A synchrotron light source is a source of electromagnetic radiation (EM) usually produced by a storage ring,[1] for scientific and technical purposes. First observed in synchrotrons, synchrotron light is now produced by storage rings and other specialized particle accelerators, typically accelerating electrons. Once the high-energy electron beam has been generated, it is directed into auxiliary components such as bending magnets and insertion devices (undulators or wigglers) in storage rings and free electron lasers. These supply the strong magnetic fields perpendicular to the beam which are needed to convert the high-energy electron energy into photons.

The major applications of synchrotron light are in condensed matter physics, materials science, biology and medicine. A large fraction of experiments using synchrotron light involve probing the structure of matter from the sub-nanometer level of electronic structure to the micrometer and millimeter level important in medical imaging. An example of a practical industrial application is the manufacturing of microstructures by the LIGA process.

Brilliance

When comparing x-ray sources, an important measure of quality of the source is called brilliance.[2] Brilliance takes into account:

1. Number of photons produced per second
2. The angular divergence of the photons, or how fast the beam spreads out
3. The cross-sectional area of the beam
4. The photons falling within a bandwidth (BW) of 0.1% of the central wavelength or frequency

The resulting formula is:

$\text{brilliance}=\frac{\text{photons}}{\text{second} \cdot \text{mrad}^2 \cdot \text{mm}^2 \cdot 0.1\% ~ \text{BW}}$

The greater the brilliance, the more photons of a given wavelength that can be concentrated on a spot.

In most x-ray literature, the units for brilliance appear as:

Brightness, intensity, and other terminology

Different areas of science often have different ways of defining terms. Unfortunately, in the area of x-ray beams, several terms mean exactly the same thing as brilliance. Some authors use the term brightness, which was once used to mean photometric luminance, or was used (incorrectly) to mean radiometric radiance. Intensity means power density per unit of area, but for x-ray sources, usually means brilliance.

The correct meaning can be determined by looking at the units given. Brilliance is about the concentration of photons, not power. The units must take into account all four factors listed in the section above.

The remainder of this article uses the terms brilliance and intensity to mean the same thing.

Properties of sources

Especially when artificially produced, synchrotron radiation is notable for its:

• High brilliance, many orders of magnitude more than with X-rays produced in conventional X-ray tubes: 3rd generation sources typically have a brilliance larger than 1018 photons/s/mm2/mrad2/0.1%BW, where 0.1%BW denotes a bandwidth 10−3w centered around the frequency w.
• High level of polarization (linear, elliptical or circular)
• High collimation, i.e. small angular divergence of the beam
• Low emittance, i.e. the product of source cross section and solid angle of emission is small
• Wide tunability in energy/wavelength by monochromatization (sub-electronvolt up to the megaelectronvolt range)
• Pulsed light emission (pulse durations at or below one nanosecond, or a billionth of a second).

Synchrotron radiation from accelerators

Synchrotron radiation may occur in accelerators either as a nuisance, causing undesired energy loss in particle physics contexts, or as a deliberately produced radiation source for numerous laboratory applications. Electrons are accelerated to high speeds in several stages to achieve a final energy that is typically in the gigaelectronvolt range. The electrons are forced to travel in a closed path by strong magnetic fields. This is similar to a radio antenna, but with the difference that the relativistic speed changes the observed frequency due to the Doppler effect by a factor $\gamma$. Relativistic Lorentz contraction bumps the frequency by another factor of $\gamma$, thus multiplying the gigahertz frequency of the resonant cavity that accelerates the electrons into the X-ray range. Another dramatic effect of relativity is that the radiation pattern is distorted from the isotropic dipole pattern expected from non-relativistic theory into an extremely forward-pointing cone of radiation. This makes synchrotron radiation sources the most brilliant known sources of X-rays. The planar acceleration geometry makes the radiation linearly polarized when observed in the orbital plane, and circularly polarized when observed at a small angle to that plane.

The advantages of using synchrotron radiation for spectroscopy and diffraction have been realized by an ever-growing scientific community, beginning in the 1960s and 1970s. In the beginning, accelerators were built for particle physics, and synchrotron radiation was used in "parasitic mode" when bending magnet radiation had to be extracted by drilling extra holes in the beam pipes. The first storage ring commissioned as a synchrotron light source was Tantalus, at the Synchrotron Radiation Center, first operational in 1968.[3] As accelerator synchrotron radiation became more intense and its applications more promising, devices that enhanced the intensity of synchrotron radiation were built into existing rings. Third-generation synchrotron radiation sources were conceived and optimized from the outset to produce brilliant X-rays. Fourth-generation sources that will include different concepts for producing ultrabrilliant, pulsed time-structured X-rays for extremely demanding and also probably yet-to-be-conceived experiments are under consideration.

Bending electromagnets in accelerators were first used to generate this radiation, but to generate stronger radiation, other specialized devices – insertion devices – are sometimes employed. Current (third-generation) synchrotron radiation sources are typically reliant upon these insertion devices, where straight sections of the storage ring incorporate periodic magnetic structures (comprising many magnets in a pattern of alternating N and S poles – see diagram above) which force the electrons into a sinusoidal or helical path. Thus, instead of a single bend, many tens or hundreds of "wiggles" at precisely calculated positions add up or multiply the total intensity of the beam. These devices are called wigglers or undulators. The main difference between an undulator and a wiggler is the intensity of their magnetic field and the amplitude of the deviation from the straight line path of the electrons.

There are openings in the storage ring to let the radiation exit and follow a beam line into the experimenters' vacuum chamber. A great number of such beamlines can emerge from modern third-generation synchrotron radiation sources.

Storage rings

The electrons may be extracted from the accelerator proper and stored in an ultrahigh vacuum auxiliary magnetic storage ring where they may circle a large number of times. The magnets in the ring also need to repeatedly recompress the beam against Coulomb (space charge) forces tending to disrupt the electron bunches. The change of direction is a form of acceleration and thus the electrons emit radiation at GeV frequencies.

Applications of synchrotron radiation

• Synchrotron radiation of an electron beam circulating at high energy in a magnetic field leads to radiative self-polarization of electrons in the beam (Sokolov–Ternov effect).[4] This effect is used for producing highly polarised electron beams for use in various experiments.

Beamlines

Beamlines of Soleil

At a synchrotron facility, electrons are usually accelerated by a synchrotron, and then injected into a storage ring, in which they circulate, producing synchrotron radiation, but without gaining further energy. The radiation is projected at a tangent to the electron storage ring and captured by beamlines. These beamlines may originate at bending magnets, which mark the corners of the storage ring; or insertion devices, which are located in the straight sections of the storage ring. The spectrum and energy of X-rays differ between the two types. The beamline includes X-ray optical devices which control the bandwidth, photon flux, beam dimensions, focus, and collimation of the rays. The optical devices include slits, attenuators, crystal monochromators, and mirrors. The mirrors may be bent into curves or toroidal shapes to focus the beam. A high photon flux in a small area is the most common requirement of a beamline. The design of the beamline will vary with the application. At the end of the beamline is the experimental end station, where samples are placed in the line of the radiation, and detectors are positioned to measure the resulting diffraction, scattering or secondary radiation.

Experimental techniques and usage

Synchrotron light is an ideal tool for many types of research and also has industrial applications. Some of the experimental techniques in synchrotron beamlines are:

Some of the advantages of synchrotron light that allow for these practical uses are:

• High energy X-rays, short wavelength photons which can penetrate matter and interact with atoms.
• High concentration, tunability and polarization thus ensuring focusing accuracy for even the smallest of targets.

Compact synchrotron light sources

Because of the usefulness of tuneable collimated coherent X-ray radiation, efforts have been made to make smaller more economical sources of the light produced by synchrotrons. The aim is to make such sources available within a research laboratory for cost and convenience reasons; at present, researchers have to travel to a facility to perform experiments. One method of making a compact light source is to utilise the energy shift from Compton scattering near-visible laser photons from electrons stored at relatively low energies of tens of megaelectronvolts (see for example the Compact Light Source (CLS)[6]). However, a relatively low cross-section of collision can be obtained in this manner, and the repetition rate of the lasers is limited to a few hertz rather than the megahertz repetition rates naturally arising in normal storage ring emission. Another method is to use plasma acceleration to reduce the distance required to accelerate electrons from rest to the energies required for UV or X-ray emission within magnetic devices.