X-ray free-electron lasers (XFELs)1,2 are widely operated on the basis of self-amplified spontaneous emission (SASE)3,4, where spontaneous radiation from the electron beam is amplified along the magnetic field in undulators. Despite their high intensities, SASE-XFELs have a broad spectrum due to the stochastic starting-up process5. To narrow the bandwidth, self-seeding has been proposed6,7 and recently demonstrated8,9, where the seed pulse produced by monochromatizing the SASE-XFELs from the first section of undulators using a thin crystal in transmission geometry is amplified in the remaining undulators. Here, we present an efficient self-seeding scheme using the Bragg reflection to produce a seed pulse. We applied this scheme to SPring-8 Angstrom Compact free-electron LAser (SACLA)10, and produced nearly Fourier-transform-limited XFEL pulses that correspond to an increase in spectral brightness by a factor of six compared with SASE-XFELs. This achievement will not only enhance the throughput of present XFEL experiments but also should open new opportunities for X-ray science.

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The authors thank Y. Sano, T. Hirano, Y. Morioka, Y. Kohmura and K. Yamauchi for their support in the characterization of the silicon channel-cut crystal monochromator. The authors also thank D. Zhu, A. Robert and Y. Feng for sharing their experiences on transmission self-seeding at the Linac Coherent Light Source (LCLS). The authors are grateful for the valuable advice from K. Tamasaku. The authors acknowledge T. Hasegawa, S. Tanaka, Y. Tajiri and other members of the SACLA team for help with the accelerator operation and data analysis. This research was partially performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2018A8040).

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Author notes

  1. These authors contributed equally: Ichiro Inoue, Taito Osaka.


  1. RIKEN SPring-8 Center, Sayo, Japan

    • Ichiro Inoue
    • , Taito Osaka
    • , Toru Hara
    • , Takashi Tanaka
    • , Takahiro Inagaki
    • , Toru Fukui
    • , Shunji Goto
    • , Yuichi Inubushi
    • , Hiroaki Kimura
    • , Ryota Kinjo
    • , Haruhiko Ohashi
    • , Kazuaki Togawa
    • , Kensuke Tono
    • , Mitsuhiro Yamaga
    • , Hitoshi Tanaka
    • , Tetsuya Ishikawa
    •  & Makina Yabashi
  2. Japan Synchrotron Radiation Research Institute, Sayo, Japan

    • Shunji Goto
    • , Yuichi Inubushi
    • , Hiroaki Kimura
    • , Haruhiko Ohashi
    • , Kensuke Tono
    • , Mitsuhiro Yamaga
    •  & Makina Yabashi


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M. Yabashi, T.T. and H.T. conceived the idea of self-seeding using a Si channel-cut crystal monochromator. T.O., I.I. and M. Yabashi designed and characterized the Si(111) channel-cut crystal monochromator. I.I. and T.O. designed the commissioning plan with advice from all co-authors. All of the co-authors jointly performed the commissioning of the reflection self-seeding. I.I. and T.O. analysed the experimental data. T.T. and R.K. performed simulation studies. I.I., T.O., T.T. and M. Yabashi co-wrote the manuscript.

Competing interests

The authors declare no competing interests.

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Correspondence to Ichiro Inoue.

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