NEWS & VIEWS

(incoherent) undulator radiation with an electron beam. As a beam of electrons passes through an undulator, the interaction of the beam with the radiation it emits causes it to be modulated into small groups (micro-bunches) separated by a distance equal to the wavelength of the radiation. In turn, these micro-bunches emit further radiation at a wavelength equivalent to this distance, causing them to contribute coherently to the growing radiation field. This process requires the generation of electron beams of extremely high current and small emittance and energy spread