The fundamental coherent radiation emission processes from a bunched charged particles beam are outlined. In contrast to spontaneous emission of radiation from a random electron beam that is proportional to the number of particles, a prebunched electron beam can emit spontaneously coherent radiation proportional to the number of particles—squared, through the process of (spontaneous) superradiance (SP-SR) (in the sense of Dicke’s). The coherent SP-SR emission of a bunched electron beam can be even further enhanced by a process of stimulated superradiance in the presence of a seed-injected radiation field. In this review, these coherent radiation emission processes for both single bunch and periodically bunched beams are considered in a model of radiation mode expansion. The general model of coherent spontaneous emission is extended to the nonlinear regime, particularly for undulator (wiggler) interaction: tapering-enhanced stimulated-superradiant amplification (TESSA). Processes of SP-SR and TESSA take place in tapered wiggler seed-injected free-electron lasers (FELs). In such FELs, operating in the x-ray regime, these processes are convoluted with other effects. However these fundamental emission concepts are useful guidelines for the strategy of wiggler tapering efficiency and power enhancement. Based on this model, previous theories and experiments are reviewed on coherent radiation sources based on SP-SR (coherent undulator radiation, synchrotron radiation, Smith-Purcell radiation, etc.), in the THz regime and on-going works on tapered wiggler efficiency-enhancement concepts in all optical frequency regimes up to x rays.

• Received 22 April 2018
• Accepted 20 February 2019

DOI:https://doi.org/10.1103/RevModPhys.91.035003