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High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range


A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1% bandwidth (BW)–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 1035 photons s–1 mm–1 mrad–1 0.1%BW–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.

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Fig. 1: Schematic diagram of the self-seeding experiment at PAL-XFEL.
Fig. 2: Suppression of the MBI by the laser heater for 9.7 keV photon energy.
Fig. 3: Spectral intensity of the self-seeded versus SASE XFEL for 9.7 keV FEL.
Fig. 4: Spectral intensity of the self-seeded versus SASE XFEL for 3.5 keV and 14.6 keV.
Fig. 5: Data quality indicators as a function of resolution.

Data availability

The raw data CXI files and geometry files have been deposited in the Coherent X-ray Imaging Data Bank (CXIDB; The coordinates and structural factors have been deposited in the Research Collaboratory for Structural Bioinformatics (RCSB) under the accession codes 7BYO/7D01/7D04 (for lysozyme from the self-seeded mode) and 7BYP/7D02/7D05 (for lysozyme from the SASE mode). The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

Code availability

The line plots, dot plots and bar graphs reported in Fig. 5, Supplementary Fig. 6 and Supplementary Fig. 7 were made based on the calculation results using version 0.9.1 of the CrystFEL suite available at


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We gratefully acknowledge discussions with A. A. Lutman, F.-J. Decker, Z. Huang and H. Loos. The XFEL experiments were performed at the Nano Crystallography & Coherent Imaging (NCI) PAL-XFEL experimental station. This research has been supported by the Ministry of Science and Information, Communications & Technology (ICT) of Korea (grant number 2018R1A6B4023605) and in part by the Basic Science Research Program (grant numbers 2017R1A2B4007274, 2020R1F1A1075828, 2019R1C1C1003687) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT of Korea.

Author information




C.-K.M., Y.S., K.-J.K., S.J.L., J.P. and H.-S.K. conceived the idea of the experiment. B.O., D.N., Y.J.S., D.S., S.T. and V.B. contributed to the design of the self-seeding system. H.Y., M.H.C., C.K., M.-J.K., C.H.S., J.H.K. and H. Heo contributed to the machine operation and tuning for the self-seeding experiment. I.N., C.H.S., C.-K.M. and H.-S.K. acquired and analysed the self-seeding data. I.N. and C.-K.M. contributed to the optimization of a laser heater. G.K. developed the analysis tools for the self-seeding experiments. J.P., J.K., S.P., G.P., S.K., S.H.C., H. Hyun, J.H.L., K.S.K., I.E. and S.R. contributed to the SFX experiment. G.P. and S.J.L. contributed to the analysis of the SFX data. I.N., C.-K.M., S.J.L. and H.-S.K. wrote the manuscript, which was discussed and agreed by all the co-authors.

Corresponding authors

Correspondence to Yuri Shvyd’ko or Sang Jae Lee or Heung-Sik Kang.

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The authors declare no competing interests.

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Peer review information Nature Photonics thanks David Attwood and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–8, Discussion and Tables 1–2.

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Nam, I., Min, CK., Oh, B. et al. High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range. Nat. Photonics (2021).

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