Accelerating charged particles will produce electromagnetic radiation, including radio waves, infrared radiation (heat), visible light, ultraviolet radiation, and X-rays. For example, heating a material like a metal will cause the atoms to vibrate with more energy, which accelerates the electrons and causes them to emit infrared light. Heating the metal more will make it glow “red hot” and then “white hot” as higher energy light is produced. In air, a hot metal will also react with oxygen (or burn), but a hot metal in an inert gas (an incandescent light bulb) will emit visible light, but won’t produce many X-rays. Even a metal in a vacuum tube will start emitting electrons before emitting very many X-rays. Such electrons from a metal filament can be accelerated by an applied voltage and then abruptly decelerated by striking a metal plate. Such a “tube source” can produce ultraviolet light and X-rays – this is how X-rays are produced for most laboratory and medical applications
Electrons taking a curved path in a magnetic field are also accelerated and emit light. This light is called synchrotron radiation. For high-energy electrons traveling in strong magnetic fields, synchrotron radiation will span the full electromagnetic spectrum, from infrared up to X-rays. While low-energy electrons emit light with dipole radiation perpendicular to the direction of acceleration, electrons traveling at relativistic speeds will emit radiation along the tangent of their curved path (which is also perpendicular to the direction of acceleration). This creates a beam of radiation at the tangents of each bending magnet. Conveniently, a series of bending magnets can make a closed loop or a ring. Relativistic electrons can travel in a large vacuum tube around such a ring (many thousands of times per second) and emit light from each of the bending magnets. For particle accelerators, synchrotron radiation gives an annoying but unavoidable loss of energy. For scientists interested in using X-rays and other radiation to study the properties of matter, these are excellent light sources, and dedicated synchrotrons have been build especially for this purpose and used for many decades.
Modern synchrotrons are large rings (with circumferences from 100 to 1000 m) composed of dozens of bending magnets and with electrons traveling in an evacuated chamber in the ring at energies in the GeV range (a few thousand times the rest mass of the electron, so with speeds more than 99% of the speed of light). These deliver highly collimated beams of radiation into the X-ray regime to many ports or beamlines that can use the X-rays for scientific measurements. Because the electrons lose some energy when emitting light, some energy must be given to them in order to maintain them in a stable energy and orbit for more than a few round trips. This is done using radio-frequency cavity that produces an oscillating electromagnetic field, resulting in a storage ring that maintains an electron beam at fixed energy for many hours at a time. Although the electron beam is in an extremely high vacuum, some electrons will scatter from the residual gas or from each other, so that the current in the ring will eventually decay. A separate ring will accelerate electrons up to the desired energy so that they can be injected at the proper energy when needed. Depending on the machine, this may happen one or twice per day or every few minutes.
Wigglers and Undulators
If one bending magnet produces a bright beam of X-rays, why not use more? An array of alternating magnets will accelerate the electrons and produce light at each bend, enhancing the intensity of the light, but leaving the net electron path unchanged. For magnet arrays with a relatively high spatial period (that is, the magnets are relatively far from each other), the radiation from each magnet is independent and the intensity from N magnets is N times that for one bending magnet. This is called wiggler. Wigglers with N from 3 to 50 are used in some storage rings, to produce higher intensities X-rays than is available from bending magnets. Because the bending magnets main purpose is to steer the beam around the storage ring, the magnet fields will be set by the needs of the storage ring, whereas a wiggler can have higher field magnets, and so produce higher energy X-rays.
If the spatial period between magnets is reduced (typically to the range of centimeters), then light of certain wavelengths produced from one bend will be in phase with that from subsequent bends, and add constructively. For these particular wavelengths, the intensity will goes as N2, while intensities at other wavelengths will be suppressed. In this regime, the magnetic array is called an undulator. The constructive interference over many periods (typically between 10 and 100) requires extremely uniform path-lengths for the electrons, so that the light from the undulator at their peak intensities is much more highly collimated than that from a wiggler or bending magnet.
The N2 intensity increase in intensity for undulators only happens at particular wavelengths or energies, but is a harmonic nature. That is, if light with energy E is produced, then energies of 3E, 5E, etc will also be produced. Even harmonics are suppressed as these come from successive magnets: light from the electrons deflected by one magnet will be slightly displaced from those of the next bend, and so be slightly off-axis. On the other hand, electrons from one bend will overlap in space with those of the third and other odd-numbered magnets. This tight spatial and angular tolerance makes undulators beams particularly bright, a term which combines all of the quantities of intensity, spatial extent, angular extent, and energy purity (see the figure comparing brightness of undulator, bending magnets and other light sources). Undulator X-ray sources are highly collimated, which makes it possible to focus their beams to very small spot sizes, on the order of 1 x 1 micron.
The wavelength/energy produced by an undulator depends on the magnetic field, and so can be changed by changes that field. Most undulators used in current synchrotrons are made from very strong permanent magnets, and the fields are changed by physically moving the distance between the magnets. Some undulators (and some wigglers) use super-conducting electron-magnets, so that the peak wavelength or energy can be changed by changing the current flowing through the device. For all undulator beamlines, the wavelength or energy can be selected for the needs of the experiment.
The synchrotrons run by the US D.O.E that are used for SEES beamlines are optimized for these magnetic arrays or insertion devices (both wigglers or undulators), by having a straight section in the storage ring, between the bending magnets that define the closed loop for the circulating electron beam. SEES beamlines use all bending magnets, wigglers, and undulators, as appropriate for the needs of the experimental station.
Timing Modes and Pulsed X-rays
The radio-frequency cavity in the storage ring maintains the electron energy with an oscillating field. This accelerates some electrons but slows others, creating bunches of electrons at discrete spacings in the storage ring. This make synchrotron beams inherently pulsed sources. That pulsing can vary but is often in the MHz range. This can be exploited for timing experiments such as pump-probe measurements. The timing structure can also be used for nuclear and synchrotron Mössbauer spectroscopies.