Many of the ELETTRA beamlines receive their X-rays from two special types of devices called wigglers and undulators.
Considering the following questions: why allow the electrons to illuminate each beamline only once for every turn around the ring? Why not force them into a local zig-zag path, so that a beamline located at the end of such a path can collect light several times per turn instead of once ?
This is precisely how a wiggler works: it consists of a periodic series of magnets, placed in a ring section where the electron path would otherwise be straight; because of its action, the electrons are forced to wiggle around the straight path. The result is a very high flux of X-rays along the wiggler beamline. An undulator is similar to a wiggler except for one point: it forces the electrons into a much weaker zig-zag, so that during the entire zig-zag motion synchrotron light continues to illuminate the undulator beamline. The result is a longer pulse of light rather than a series of short bursts. Without short pulses, there is no wide band of wavelengths, thus the undulator emission is not spread in a wide band but concentrated, producing high levels of flux and brightness. The technical specification of ELETTRA undulators and wigglers make it possible to perform record levels of flux and brightness.
The undulators reveal another link between synchrotron light and relativity. Consider the following question: why is the emitted undulator wavelength not equal to the magnet array period (a few centimeters) -- but 20-30 million times smaller, as required for X-rays? The explanation may be given in terms of relativity: imagine the undulator seen by an electron that travels towards it, along its axis. The electron moves at light-like speed, thus it sees the undulator squeezed along its length, due to the famous relativistic effect called Lorentz contraction.
Therefore, the emitted wavelength is reduced with respect to the undulator period -- in the case of ELETTRA, by a factor of 3,000-4,000. Furthermore, the emitted wavelength is affected by the same relativistic Doppler effect that produces the torchlight phenomenon. In the present case, the Doppler effect causes a further wavelength reduction by 6,000-8,000. Thus, relativity squeezes twice the wavelength, and transforms the rather large undulator period into supershort X-ray wavelengths.