Free electron laser

Abstract

A free electron laser comprising: an electron source (21), a linear accelerator (22), an undulator (26), electron beam optics and a deceleration unit (28'). The electron source is operable to produce a bunched electron beam. The linear accelerator arranged to impart energy to electrons in the bunched electron beam produced by the electron source. The undulator is operable to produce a periodic magnetic field and is arranged so as to guide the bunched electron beam along a periodic path about a central axis of the undulator such that they interact with radiation in the undulator, stimulating emission of coherent radiation. The electron beam optics is arranged to direct the bunched electron beam back into the linear accelerator after it leaves the undulator so as to extract energy from electrons in the bunched electron beam. The deceleration unit is arranged to extract energy from electrons in the bunched electron beam after it has left the undulator. The deceleration unit comprises one or more resonant cavities (33), and an energy dissipation mechanism. The bunched electron beam is directed through the one or more resonant cavities so as to excite one or more resonant standing wave modes therein.