FREE ELECTRON LASER

A free electron laser comprising: an electron source, a linear accelerator, an undulator, electron beam optics and a deceleration unit. 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 the electrons 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. An electron beam separator separates the decelerated electron beam from the accelerated electron beam. The electron beam separator comprises a magnet system operable to generate a separator magnetic field, the separator magnetic field having a field direction that is perpendicular to the first direction and a field strength that increases in a second direction away from a point of entry of the decelerated electron beam into the separator magnetic field, the second direction being perpendicular to the field direction and at an acute angle to the first direction.