Two-beam based two-stage EEHG-FEL for coherent hard X-ray generation

The ability to generate stable and coherent short wavelength radiation is a great promise of the externally seeded free-electron laser. However, the frequency up-conversion efficiency is limited to a small number in a single stage setup. In this paper, we propose a new scheme to produce coherent hard X-ray FEL with two-beam based two-stage echo-enabled harmonic generation configuration. Electron beams with quite different properties are separately used in each stage and a monochromator is adopted to purify the radiation from first stage for seeding the second stage. Three-dimensional start-to-end simulations have been performed with realistic parameters to show the feasibility of the proposed scheme.