VUV and X-ray free-electron lasers

Recent advances in linear accelerators, new developments in laser driven low-emittance electron guns, and the feasibility of ultra-precise long undulators open up the exciting possibility of building single pass Free-Electron Lasers (FELs) based on self-amplified spontaneous emission (SASE). These FELs promise to provide extremely intense, polarised ultra-short pulse radiation for the Vacuum-Ultraviolet (VUV) and X-ray regimes. In addition their high peak and average brilliance, the tunability of the photon energy and the lateral coherence of the radiation will make the FELs to unique sources. World wide several projects for VUV and X-ray FELS have been initiated. At the Deutsches Elektronen Sychroton DESY (Hamburg) at the VUV–FEL (Phase I) for the first time laser emission was observed in the VUV regime. The principle of operation and the components of FELs based on SASE will be discussed in context with the VUV and X-FEL projects. Schemes to overcome the statistical fluctuations and to increase the longitudinal coherence will be presented and examples for basic and applied research with the FELs will be given.