Free-electron lasers

This is a tutorial review of the basic principles and present status of free-electron lasers (FELs) with particular emphasis on high-gain FELs such as FLASH in Hamburg. With their unprecedented intensity and a pulse duration of a few 10 fs, these novel accelerator-based radiation sources in the ultraviolet and X-ray regime will open new fields in physics, chemistry, biology and material science. Assuming that the reader is unfamiliar with FELs or particle accelerators in general, this tutorial covers the basic concepts of particle accelerators and the generation of synchrotron radiation in some detail, and finally describes FELs in the low-gain and high-gain regime.