Superradiant and stimulated-superradiant emission of bunched electron beams

TL;DR

This paper reviews the fundamental processes of superradiant and stimulated-superradiant emission from bunched electron beams, extending models to nonlinear regimes and discussing implications for free-electron lasers across various frequency regimes.

Contribution

It introduces a comprehensive model of coherent emission processes, including nonlinear effects like TESSA, and connects these to practical FEL strategies for power enhancement and efficiency.

Findings

Superradiance scales with the square of the number of electrons.

Stimulated-superradiance can be enhanced with seed radiation.

The model applies across optical to X-ray regimes.

Abstract

We outline the fundamental coherent radiation emission processes from a bunched charged particles beam. In contrast to spontaneous emission of radiation from a random electron beam that is proportional to the number of particles, a pre-bunched 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 (ST-SR) 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 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, we review previous theories and experiments 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.