What is a free electron laser?

A free electron laser (FEL) is a linear accelerator that is designed to produce very fast bursts of light with extremely high peak brightness. FEL's make some of the most brilliant light sources ever made by humankind.

In a FEL the optical amplification takes place in an undulator which is fed with high energy electrons from an electron accelerator. FEL's have been constructed to produce radiation with wavelenghts ranging from the terahertz region until the x-ray regime.

Free-electron lasers can achieve very high peak powers, and the tunability of FELs makes them highly desirable in many disciplines, including physics, chemistry, biology and materials science.

To create an FEL, a beam of electrons is accelerated to almost the speed of light. The beam passes through an undulator, a side to side magnetic field produced by a periodic arrangement of magnets with alternating poles across the beam path. The radiation is produced in a process called self-amplified spontaneous emission (SASE). 

This amplification process is induced by the interaction of the X-ray radiation with the electrons: Because the radiation is faster than the electrons speeding along their slalom path, the radiation overtakes the electrons flying ahead and interacts with them along the way, accelerating some of them and slowing others down. As a result, the electrons gradually organize themselves into a multitude of thin disks. The key property of this process is the fact that all of the electrons in a given disk emit their light “in sync.” This produces extremely short and intense X-ray flashes with the properties of laser light. This is the SASE principle of self-amplified spontaneous emission. Since the structure of thin disks takes some time to fully form in the undulators, free-electron lasers require very long undulators. At the European XFEL, they will be more than 100 metres long.

Sources: European XFEL, Wikipedia, SLAC Stanford Linear Accelerator Laboratory