Figure 1.

Schematic layouts for SASE and self-seeding

Typical spectra corresponding to SASE and self-seeding are shown, respectively. The central part shows two types of electron distribution and the corresponding radiation emission. Incoherent radiation is emitted owing to the random phases of electrons at the entrance of undulator (left). If the electron beam exhibits micro-bunching at the radiation wavelength scale, a powerful coherent radiation pulse is generated (right). With increasing bunching, the fraction of coherent emission increases, leading to an exponential growth of radiation power along the undulators.