Spontaneous Super-radiative Emission of Ultra-short Terahertz Wave Pulses from Short Photo-injector Electron Bunches

Developing of photo-injectors makes it possible to form bunches of ps and sub-ps, of ultra-relativistic electrons with comparatively high charges (total charge 0. 1-1nC). Since such bunches are sufficiently compact, their spontaneous radiation can be organized in the terahertz (THz) and sub-THz frequency ranges. This is a process of spontaneous radiation followed by amplification of a single wave cycle, if the wave group velocity is close to the bunch velocity. The bunch compactness is provided by the self-compression of the bunch under the action of its radiation fields, despite the Coulomb repulsion of electrons inside the bunch. The radiation in the so-called negative mass regime is considered. In this case, Coulomb interaction can contribute to additional stabilization. As a result, formation of an ultra-short (several cycles) wave packet occurs, when the bunch moves through several undulator periods with high (approximately several tens of percent in the optimized systems) efficiency of extraction of the electron energy and high intensity ($\sim$100MV/m) of the peak wave field.