X-ray Free-electron Lasers

Abstract

We review the present status and properties of X‐ray free‐electron lasers in operation or under construction in the nanometer and sub‐nanometer wavelength range, and the novel possibilities they offer for the study of atomic and molecular processes. We also discuss recent developments in relativistic electron beam physics that give us the possibility of designing a new generation of X‐ray free‐electron lasers that: a. are more compact; b. reduce the radiation pulse duration to one femtosecond or below; c. extend the photon energy to the 50 keV region. These results are obtained by reducing the electron bunch charge while at same time maximizing the beam brightness and reducing the bunch length to a value near or smaller than the free‐electron laser cooperation length. In the last case the radiation pulse is fully coherent in the longitudinal and transverse space. The increase in beam brightness can also be used to reduce the beam energy needed for a given radiation wavelength, when, at the same time, the undulator period is reduced. The simultaneous decrease in beam energy and undulator period leads to a more compact free‐electron laser, while the high beam brightness reduces the gain length and increases the coherent radiation intensity.