A free-electron laser (FEL) is a type of laser that produces very intense, coherent radiation in the X-ray, ultraviolet, and infrared regions of the electromagnetic spectrum. FELs work by passing a beam of high-energy electrons through a periodic magnetic field, causing the electrons to emit radiation through the process of stimulated emission. This results in a highly tunable and very powerful laser beam that can be used for a wide range of scientific research applications, including materials science, biology, chemistry, and physics. FELs have the advantage of being able to produce light with very short wavelengths and high peak powers, making them invaluable tools for studying the structure and dynamics of matter at the atomic and molecular level.