VUV and X-ray free-electron lasers: The technology and its scientific promise

Currently available, fully coherent (laser) light sources emit radiation only in a limited range of wavelengths in the infrared, visible and near-ultraviolet range, excluding their use in all the measurements needing tunable photons of energy higher than a few eV. Consequentially, the strong scientific need for tunable, coherent radiation sources from the vacuum ultraviolet (VUV) to the X-ray in the femtosecond and picosecond time domain has spurred international research efforts to develop a new generation of research facilities. Laser-driven light sources that use non-linear processes to create very high harmonics and the interaction between an ultra-short laser pulse and an electron bunch in a storage ring are both able to produce radiation pulses in the femtosecond time domain and in the soft EUV and X-ray region, but with a relatively low useful photon flux on the sample. In contrast Free-Electron Lasers (FELs) can produce light pulses with peak brilliance as much as ten orders of magnitude higher than the pulses generated in present third-generation synchrotron light sources and with photon energies ranging from the VUV to the hard X-rays, i.e. from about 10eV (120nm) to 10keV (0.12nm). This manuscript reviews the scientific fields that can be opened or extended by the new techniques along with the progress of FEL physics and the supporting accelerator physics and technology.