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What is a free-electron laser (FEL)?

The free-electron laser (FEL) is a synchrotron radiation source that generates coherent radiation of very high brilliance. Coherent radiation is defined as radiation whose components (wave packets) oscillate in fixed relationships to one another. Because of the coherence of the radiation, the FEL is called a laser. In principle, free-electron lasers cover large parts of the spectral range, but they are optimized for a certain range. For example, the Particle Physics Lab FEL in Dubna operates in the millimetre range, the FLASH (free-electron laser in Hamburg) at DESY in the UV range (6 to 30 nm) and the European XFEL in Schenefeld and Hamburg cover the X-ray range down to 0.05 nm. Such free-electron lasers are often referred to as X-ray lasers.

The first part of a free-electron laser consists of a particle accelerator in which electrons are accelerated to almost the speed of light. In the second part, the electrons are brought onto a slalom course in special magnetic field arrangements (so-called undulators) and emit radiation in the process. FELs have this structure in common with conventional modern synchrotron radiation sources. The trick in an FEL is to allow the electrons to interact on their way through the magnetic field of the undulator with radiation that has exactly the same wavelength as the radiation emitted by the electrons. The result is an FEL that shines many times more intensely.

The microbunching effect ensures that the electron bunch is microstructured by the interaction with the generated laser radiation. The electron bunch is structured into thin slices that are aligned perpendicular to the direction of flight. These slices have a distance exactly aligned to the radiation, which is equal to the wavelength, so that all electrons in the packet can radiate coherently at the same time.

The wavelength of an FEL can be tuned by varying the energy of the electrons. Modern FELs deliver coherent high-intensity radiation up to the X-ray range.

Sources: Wikipedia / Welt der Physik