# 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.

## References

- W. Ackermann et al., Nat. Photonics
**1**, 336–342 (2007)CrossRefADSGoogle Scholar - M. Altarelli et al. (eds.), XFEL, The European X-ray free-electron laser. Technical design report, DESY 2006-097 (2006). Fulltext available at http://arxiv.org/
- J. Amann et al., Nat. Photonics
**6**, 693 (2012)CrossRefADSGoogle Scholar - A. Bohm,
*Quantum Mechanics*(Springer, New York, 1979)CrossRefzbMATHGoogle Scholar - R. Bonifacio, C. Pellegrini, L. Narducci, Opt. Commun.
**50**, 373 (1984)CrossRefADSGoogle Scholar - M. Cornacchia et al., J. Synchrotron Radiat.
**11**, 227 (2004)CrossRefGoogle Scholar - F. Curbis et al., Simulation studies for an X-ray FEL based on an extension of the MAX IV linac, in
*Proceedings of the 2013 FEL Conference*, New York, WEPSO07 (2013)Google Scholar - F.-J. Decker et al., Improving and maintaining FEL beam stability of the LCLS, in
*Proceedings of the 2014 IPAC Conference*, Dresden, THPRO035 (2014)Google Scholar - W. Decking and V. Blank., Design and construction of hard X-ray self-seeding setups for the European XFEL (XFELSEED), coordinated Russian-German project in the framework of the ‘Ioffe-R
*ö*ntgen’ cooperation (2014)Google Scholar - Y. Ding, Z. Huang, R. Ruth, Phys. Rev. ST Accel. Beams
**13**, 060703 (2010)CrossRefADSGoogle Scholar - Y. Ding et al., Phys. Rev. Lett.
**109**, 254802 (2012)CrossRefADSGoogle Scholar - P. Emma et al., Phys. Rev. Lett.
**92**, 074801-1 (2004a)CrossRefADSGoogle Scholar - P. Emma, M. Borland, Z. Huang, Femtosecond and Subfemtosecond X-ray pulses from a self-amplified spontaneous-emission-based free-electron laser, in
*Proceedings of the 2004 FEL Conference*, Trieste, TUBIS01 (2004b)Google Scholar - P. Emma et al., Nat. Photonics
**4**, 641 (2010)CrossRefADSGoogle Scholar - C. Emma et al., Phys. Rev. ST AB
**17**, 110701 (2014)ADSGoogle Scholar - U. Fano, Phys. Rev.
**124**, 6 (1961)CrossRefGoogle Scholar - U. Fano, A.R.P. Rau,
*Atomic Collisions and Spectra*(Academic, Orlando, 1986)Google Scholar - W. Fawley et al., NIM A
**483**, 537 (2002)CrossRefADSGoogle Scholar - W.M. Fawley et al., Toward TW-level LCLS radiation pulses, in
*Proceedings of the FEL 2011 Conference*, Shanghai, TUOA4 (2011)Google Scholar - J. Feldhaus et al., Opt. Commun.
**140**, 341 (1997)CrossRefADSGoogle Scholar - Y. Feng, J. Hastings, P. Heimann, M. Rowen, J. Krzywinski, J. Wu, X-ray optics for soft X-ray self-seeding the LCLS-II, in
*Proceedings of the 2010 FEL Conference*, Malmo, TUBP10 (2010)Google Scholar - Y. Feng et al., System design for self-seeding the LCLS at soft X-ray energies, in
*Proceedings of the 2012 FEL Conference*, Nara, TUOBI01 (2012)Google Scholar - G. Geloni, V. Kocharyan, E. Saldin, Ultrafast X-ray pulse measurement method, DESY 10-008 (2008). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Scheme for generation of fully coherent, TW power level hard x-ray pulses from baseline undulators at the European XFEL, DESY 10-108 (2010a). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Scheme for generation of highly monochromatic X-rays from a baseline XFEL undulator, DESY 10-033 (2010b). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Cost-effective way to enhance the capabilities of the LCLS baseline, DESY 10-133 (2010c). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Scheme for femtosecond-resolution pump-probe experiments at XFELs with two-color ten GW-level X-ray pulses, DESY-10-004 (2010d). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Self-seeded operation of the LCLS hard X-ray FEL in the long-bunch mode, DESY 10-239 (2010e). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, J. Modern Opt.
**58**, 1391 (2011a)CrossRefADSGoogle Scholar - G. Geloni, V. Kocharyan, E. Saldin, Production of transform-limited X-ray pulses through self-seeding at the European X-ray FEL, DESY 11-165 (2011b). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Opt. Commun.
**284**, 3348 (2011c)CrossRefADSGoogle Scholar - G. Geloni, V. Kocharyan, E. Saldin, Self-seeding scheme with gas monochromator for narrow-bandwidth soft X-ray FELs, DESY 11-049 (2011d). Fulltext available at http://arxiv.org/
- G. Geloni, V. Kocharyan, E. Saldin, Gas-filled cell as a narrow bandwidth bandpass filter in the VUV wavelength range, DESY 11-055 (2011d). Fulltext available at http://arxiv.org/
- G. Geloni et al., Wake monochromator in asymmetric and symmetric Bragg and Laue geometry for self-seeding the European XFEL, DESY 13-013 (2013). Fulltext available at http://arxiv.org/
- O. Grimm, K. Klose, S. Schreiber, Double-pulse generation with the FLASH injector laser for pump-probe experiments, in
*Proceedings of EPAC 2006 Conference*, Edinburgh, THPCH150 (2006)Google Scholar - T. Hara et al., Nat. Commun. (2013). doi:10.1038/ncomms3919Google Scholar
- J. Harries et al., J. Phys. B: At. Mol. Opt. Phys.
**46**, 164021 (2013)CrossRefADSGoogle Scholar - T. Inagaki et al., Hard X-ray self-seeding setup and results at SACLA, in
*Proceedings of the 2014 FEL Conference*, Basel, TUC01 (2014)Google Scholar - T. Ishikawa et al., Nat. Photonics
**6**, 540 (2012)MathSciNetCrossRefADSGoogle Scholar - Y. Jiao et al., Phys. Rev. ST Accel. Beams
**15**, 050704 (2012)CrossRefADSGoogle Scholar - H.-S. Kang, Current status of PAL-XFEL project, in
*Proceedings of the 2014 FEL Conference*, Basel, WEODB103 (2014)Google Scholar - I.S. Ko, J.-H. Han, Current status of PAL-XFEL, in
*Proceedings of the 27th Linear Accelerator Conference*, Geneva, MOIOB04 (2014)Google Scholar - A. Kondratenko, E. Saldin, Part. Accel.
**10**, 207 (1980)Google Scholar - H.A. Kramers, La diffusion de la lumière par les atomes, in Atti del Congresso Internazionale dei Fisici, 2, 545 (1927)Google Scholar
- N.M. Kroll, P. Morton, M.N. Rosenbluth, IEEE J. Quantum Electron.
**QE-17**, 1436 (1981)CrossRefADSGoogle Scholar - R. de L. Kronig, J. Opt. Soc. Am.
**12**, 547–557 (1926)Google Scholar - A. Lin, J.M. Dawson, Phys. Rev. Lett.
**42**, 2172 (1986)Google Scholar - R.R. Lindberg, Y.V. Shvydko, Phys. Rev. ST Accel. Beams
**15**, 050706 (2012)CrossRefADSGoogle Scholar - V. Lucarini et al.,
*Kramers-Kronig Relations in Optical Materials Research*, (Springer, Berlin/Heidelberg 2004)Google Scholar - A. Lutman et al., J. Phys. A: Math. Theor.
**42**, 045202 (2009)MathSciNetCrossRefADSGoogle Scholar - A.A. Lutman et al., Phys. Rev. Lett.
**110**, 134801 (2013)CrossRefADSGoogle Scholar - A. Lutman et al., Phys. Rev. Lett.
**113**, 254801 (2014)CrossRefADSGoogle Scholar - A. Marinelli et al., Nat. Commun. (2015). doi:10.1038/ncomms7369Google Scholar
- H. Morgan, D. Hederer, Phys. Rev. A
**29**, 4 (1984)CrossRefGoogle Scholar - T.J. Orzechovski et al., Phys. Rev. Lett.
**57**, 2172 (1986)CrossRefADSGoogle Scholar - E. Prat, S. Reiche, Self-seeding design for SwissFEL, in
*Proceedings of the 2013 FEL Conference*, New York, WEPSO5 (2013)Google Scholar - D. Ratner et al., Phys. Rev. Lett.
**114**, 054801 (2015)CrossRefADSGoogle Scholar - T. Raubenheimer, The LCLS-II, a new FEL facility at SLAC, in
*Proceedings of the 2014 FEL Conference*, Basel, WEB001 (2014)Google Scholar - E. Saldin, E. Schneidmiller, M. Yurkov,
*The Physics of Free-Electron Lasers*(Springer, Berlin, 1999)Google Scholar - E. Saldin, E. Schneidmiller, Yu. Shvyd’ko, M. Yurkov, NIM A
**475**, 357 (2001)Google Scholar - S. Serkez et al., Phys. Rev. ST AB
**18**, 030708 (2015)ADSGoogle Scholar - D. Shu et al., J. Phys.: Conf. Ser.
**425**, 052004 (2013)ADSGoogle Scholar - Y. Shvydko, R. Lindberg, Phys. Rev. ST Accel. Beams
**15**, 100702 (2012)CrossRefADSGoogle Scholar - P. Sprangle, C.M. Tang, W.M. Manheimer, Phys. Rev. Lett.
**43**1932 (1979)CrossRefADSGoogle Scholar - R. Ganter et al. (eds.), SwissFEL conceptual design report, PSI Bericht Nr. 10-04 (2012)Google Scholar
- J. Toll, Phys. Rev.
**104**, 6 (1956)MathSciNetCrossRefGoogle Scholar - R. Treusch, W. Brefeld, J. Feldhaus, U. Hahn, The seeding project for the FEL in TTF phase II. Annual report (2001)Google Scholar
- X. Wang et al., Phys. Rev. Lett.
**103**, 154801 (2009)CrossRefADSGoogle Scholar - J. Welch, Seeding at LCLS FEL, Joint DESY and University of Hamburg Accelerator Physics Seminar, 5 Feb 2013, DESY Hamburg (2013), http://www-mpy.desy.de/AccPhySemDESY/y2013/JWelch_SeedingAtLCLS.pdf
- J. Wu, Recent experimental and theoretical results on tapering for high intensity extraction, in
*Workshop on Advanced X-Ray FEL Development*, Hamburg (2014), http://www.xfel.eu/events/workshops/2014/workshop_on_advanced_x_ray_fel_development/ - J. Wu et al., Staged self-seeding scheme for narrow bandwidth, ultra-short X-ray harmonic generation free electron laser at LCLS, in
*Proceedings of the 2010 FEL Conference*, Malmo, TUPB08 (2010)Google Scholar - J. Wu et al., Simulation of the hard X-ray self-seeding FEL at LCLS, in
*Proceedings of the 2011 FEL Conference*, Shanghai, MOPB09 (2011)Google Scholar - I. Zagorodnov, Beam dynamics simulations for XFEL (2011), http://www.desy.de/fel-beam/s2e/
- D. Zhu et al., Appl. Phys. Lett.
**101**, 034103 (2012)CrossRefADSGoogle Scholar