Source of @C01130@ in which the @A00106@ 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 @W06659@, \(\lambda _{\text{L}}\), is approximately given by \[\frac{\lambda _{\omega }}{4\ E^{2}}\] with \(\lambda _{\omega }\) being the wiggler period and \(E\) the electron's energy in \(\mathrm{MeV}\).