Unleashing Light: The Cutting-Edge World of Free-Electron Laser (FEL) Sources

Free-electron Lasers are extremely versatile light sources that use an ultrarelativistic electron bunch from a linear accelerator as gain medium. They work as parametric amplifiers for an input signal that is a combination of electron bunch density and/or energy modulations, and/or a pre-existing radiation field. The output is an extremely bright pulse of FEL radiation, whose characteristics depend on the input signal and on the amplifier system itself. FELs can be designed to amplify frequencies from the THz range to the hard X-ray region. In this talk I will mainly focus on the high-frequency region of the spectrum and consider X-ray FELs (XFELs), where the standard mode of operation consists in using shot noise density fluctuations in the electron beam as input signal for the FEL amplification process and is called Self-Amplified Spontaneous Emission (SASE). SASE is a robust lasing solution, the workhorse for X-ray FELs: nevertheless, being the result of noise amplification, it inherits its chaotic signature which is reflected by limited stability and poor longitudinal coherence.