Synchrotron radiation (SR) is emitted when charged particles moving with relativistic speeds are forced to follow curved trajectories in magnetic fields. The first visual observation of SR was in 1948 from the General Electric synchrotron in the USA during investigations into the design and construction of accelerators suitable for the production of very high energy electrons. Over the next 50 years, an explosive growth in the building of accelerators optimised for SR production has turned this interesting but also limiting radiative energy loss into a valuable research tool.

In general, three kinds of magnets are used to make the necessary magnetic fields: bending magnets, wigglers and undulators. In bending magnets, a simple dipole structure is used to constrain the electrons in a curved path. The radiation emitted is extremely intense and extends over a broad wavelength range from the infrared through the visible and ultraviolet, and into the soft and hard x-ray regions of the electromagnetic spectrum. A typical output curve for a bending magnet source has a smooth spectral distribution with a broad maximum near the so-called critical wavelength. This critical wavelength depends on the square of the energy of the electrons and the bending radius in the dipole. In practical units of angstroms, it can be calculated as: lambda(c) = 18.64 / (B * E2), where B (the dipole field) is in Tesla and E (the ring energy) is in GeV. The critical wavelength (or energy) has the property that one-half of the power is radiated above this wavelength and one-half below. 

This high intensity and broad spectral range when combined with other properties such as a high degree of polarization and collimation makes SR a powerful tool for basic and applied research in physics, chemistry, biology and medicine, as well as finding applications in such technologies as x-ray lithography, materials characterisation and micromechanics.

High-field wiggler magnets are often used as sources in order to increase the flux at shorter wavelengths. We can best think of a wiggler as a sequence of bending magnets of alternating polarities which gives a 2N enhancement in the flux, where N is the number of poles. The properties of wiggler SR are thus very similar to that of dipole radiation with a reduction in the critical wavelength as a consequence of the higher field. For superconducting wiggler magnets a value of 6 Tesla, as opposed to around 1.2 Tesla for conventional dipoles, would be typical. 

Undulators, consisting of periodic magnetic arrays, cause small electron deflections comparable in magnitude to the natural emission angle of the SR. The radiation emitted at the various poles interferes coherently resulting in the emission of a pencil-shaped beam peaked in narrow energy bands at the harmonics of the fundamental energy. For N poles, the beam's opening angle is decreased by N1/2 and thus the intensity per solid angle increases as N2. The two figures below illustrate the two forms of output curves.

Last Modified 13 June 2009