What is an XFEL?
An X-ray free electron laser (XFEL) consists of an electron source, a linear accelerator, and undulator magnets equally spaced to produce X-ray wavelengths (typically ranging from 0.01 to 10 nm). Bunches of electrons produced by an electron gun are compressed and accelerated to almost the speed of light in a linear accelerator prior to interaction with the undulator (we could include a figure showing a photo of the undulator room at the EuXFEL and how the X-ray pulses are produced by their interaction with the undulator). Upon interaction with the undulator, the electrons wiggle via the magnets. The long length of an XFEL undulator allows the relativistic electrons to interact with their emitted radiation which causes bunching of the electrons with spacing equal to the wavelength of the emitted X-rays. As the electrons bunch proceed through, their emission becomes more coherent and allows for a stronger interaction between the two. This results in an X-ray beam with highly coherent pulses of femtosecond duration. Because the number of electrons in a bunch is on the order of a billion, this causes a massive increase in flux compared to synchrotron sources. Furthermore, the high longitudinal coherence (and thus, ultrashort duration) of the pulses pushes the peak brilliance orders of magnitude higher than that achievable at a synchrotron.
The first hard XFEL, LCLS, was built at the SLAC National Accelerator Laboratory in California and has been in operation since 2009, setting a new standard, with a peak X-ray brilliance over ten orders of magnitude higher than that of the most powerful synchrotron radiation sources. Since then, four more XFEL followed up, namely: SPring-8 Angstrom Compact free electron LAser (SACLA, Japan), the European XFEL (Eu-XFEL), the Pohang Accelerator Laboratory (PAL-XFEL, South Korea), and the Switzerland Free Electron Laser (SwissFEL, Switzerland).