High efficiency tapered free-electron lasers with a prebunched electron beam from echo-enabled harmonic generation

Abstract

Undulator tapering based on a prebunched electron beam and a strong seed laser has been demonstrated as a promising method to greatly enhance free-electron laser (FEL) efficiency and suppress sideband instability. Different from the former works that introduce an ideal bunching or a seed laser or a self-seeding mechanism, in this paper, we propose to enhance FEL efficiency by optimizing the tapering strategy with a prebunched electron beam from echo-enabled harmonic generation (EEHG). The simulation results demonstrate that the efficiency enhancement can be effectively maintained in a quite long undulator, in which the whole FEL efficiency is improved by more than 2 orders and a clear center spectrum inherited from EEHG can be preserved. Benefitting from a stable initial phase space with an appropriate tapering strategy, the nature of stabilities can be well preserved at both time and frequency domain. Through jumping the resonant phase, an effective sideband suppression can be realized and the stability of output power is improved. In this scheme, the inhomogeneous shot noise and microbunching instability also have an important impact on efficiency enhancement and output stability. It shows a different instability feature from the first stage to the following tapered undulator in comparison to the self-seeding tapered scheme.