Observation of coherent effect in undulator radiation

The coherent effect in spontaneous emission has been observed in a low-energy, low-gain, waveguide-mode free-electron laser. The intensity of the radiation was more than ${10}^3$ times stronger than that of the incoherent radiation. This enhancement is due to coherent radiation emitted by the electron beam, which is randomly modulated by micropulses whose typical pulse width (∼35 ps) is comparable to the radiation wavelength. The measured power tends towards a quadratic dependence on the electron-beam current.