Source of coherent radiation in which the active medium is an electron beam moving at speeds close to the speed of light in the spatially periodic magnetic field produced by an array of magnets (the wiggler). The emitted wavelength, ${\lambda }_{\text{L}}$, is approximately given by $$\frac{{\lambda }_{\omega }}{4E^2}$$ with ${\lambda }_{\omega }$ being the wiggler period and $E$ the electron's energy in $\mathrm{MeV}$.