# Self-Seeded Free-Electron Lasers

## Abstract

Self-seeding is an active filtering method for Free-Electron Lasers (FELs) enabling the production of nearly Fourier-limited pulses in the X-ray frequency range where external seeding is not available. Schematically, it is composed by three parts: a Self-Amplified Spontaneous Emission (SASE) FEL working in the linear regime, a monochromator, and an FEL amplifier. Active filtering is achieved by letting the FEL pulse produced in the SASE FEL through the monochromator, while the electron beam is sent through a bypass, and its microbunching is destroyed due to dispersion. The filtered SASE pulse, serving as a seed, is recombined with the electron beam at the entrance of the FEL amplifier part. It is then amplified up to saturation and possibly beyond via post-saturation tapering. This allows for the production of high-brightness, nearly single-mode FEL pulses. The technique has been or will be implemented in a number of X-ray FEL (XFEL) facilities under operation or in the construction phase. In this chapter, we review the principle of self-seeding, its practical realizations, and related techniques.

## Keywords

Self-seeding, Longitudinal coherence, Single-mode FEL, Tapering## Notes

### Acknowledgements

I am grateful to Evgeni Saldin (DESY) for the careful reading of the manuscript and his continuous advice. I thank Serguei Molodtsov (European XFEL) for his interest in this work and his support.

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