Hard X-ray free-electron laser with femtosecond-scale timing jitter


The hard X-ray free-electron laser at the Pohang Accelerator Laboratory (PAL-XFEL) in the Republic of Korea achieved saturation of a 0.144 nm free-electron laser beam on 27 November 2016, making it the third hard X-ray free-electron laser in the world, following the demonstrations of the Linac Coherent Light Source (LCLS) and the SPring-8 Angstrom Compact Free Electron Laser (SACLA). The use of electron-beam-based alignment incorporating undulator radiation spectrum analysis has allowed reliable operation of PAL-XFEL with unprecedented temporal stability and dispersion-free orbits. In particular, a timing jitter of just 20 fs for the free-electron laser photon beam is consistently achieved due to the use of a state-of-the-art design of the electron linear accelerator and electron-beam-based alignment. The low timing jitter of the electron beam makes it possible to observe Bi(111) phonon dynamics without the need for timing-jitter correction, indicating that PAL-XFEL will be an extremely useful tool for hard X-ray time-resolved experiments.

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Fig. 1: Schematic diagram of PAL-XFEL.
Fig. 2: FEL alignment.
Fig. 3: K-tuning and undulator vertical offset scanning using undulator radiation spectrum.
Fig. 4: FEL gain length measurements.
Fig. 5: Measured Bi(111) Bragg diffraction intensity modulation and its associated arrival time jitters of both the FEL beam and electron beam.


  1. 1.

    Emma, P. et al. First lasing and operation of an angstrom-wavelength free electron laser. Nat. Photon. 4, 641–647 (2010).

    ADS  Article  Google Scholar 

  2. 2.

    Emma., P. et al. Beam-based alignment for the LCLS FEL undulator. Nucl. Instrum. Methods A 429, 407–413 (1999).

    ADS  Article  Google Scholar 

  3. 3.

    Tanaka, H. et al. A compact X-ray free-electron laser emitting in the sub-angstrom region. Nat. Photon. 6, 540–544 (2012).

    ADS  Article  Google Scholar 

  4. 4.

    Tanaka, T. et al. Undulator commissioning by characterization of radiation in X-ray free electron lasers. Phys. Rev. ST Accel. Beams 15, 110701 (2012).

    ADS  Article  Google Scholar 

  5. 5.

    Kang, H.-S. et al. PAL-XFEL Technical Design Report (Pohang Accelerator Laboratory, 2014); http://pal.postech.ac.kr/paleng.

  6. 6.

    Kang, H.-S. et al. Status of the PAL-XFEL construction. In Proc. 2015 Particle Accelerator Conference 2439 (2015).

  7. 7.

    Braun, H. H. et al. SwissFEL, the X-ray free electron laser at PSI. In Proc. 2012 Free Electron Laser Conference 9–12 (JACOW, 2012).

  8. 8.

    Altarelli, M. The European X-ray free-electron laser facility in Hamburg. Nucl. Instrum. Methods B 269, 2845–2849 (2011).

    ADS  Article  Google Scholar 

  9. 9.

    Raubenheimer, T. O. et al. LCIS-II: status of the CW X-ray FEL upgrade to the SLAC LCLS facility. In Proc. 2015 Free Electron Laser Conference 618–624 (JACOW, 2015).

  10. 10.

    Fritz, D. M. et al. Ultrafast bond softening in bismuth: mapping a solid’s interatomic potential with X-rays. Science 315, 633–636 (2007).

    ADS  Article  Google Scholar 

  11. 11.

    Harmand, M. et al. Achieving few-femtosecond time-sorting at hard X-ray free-electron lasers. Nat. Photon. 7, 215–218 (2013).

    ADS  Article  Google Scholar 

  12. 12.

    Kang, H.-S. et al. Beam optics design for PAL-XFEL. In Proc. 2012 Free Electron Laser Conference 309–312 (JACOW, 2012).

  13. 13.

    Lee, H. S. et al. S-band accelerating structures for the PAL-XFEL. J. Korean Phys. Soc. 66, 340–344 (2015).

    ADS  Article  Google Scholar 

  14. 14.

    Heo, H. et al. Development of an S-band accelerating structure with quasi-symmetric single feed racetrack couplers. J. Korean Phys. Soc. 66, 905–914 (2015).

    ADS  Article  Google Scholar 

  15. 15.

    Chae, M. S. et al. Emittance growth due to multipole transverse magnetic modes in an rf gun. Phys. Rev. ST Accel. Beams 14, 104203 (2011).

    ADS  Article  Google Scholar 

  16. 16.

    Lee, J. et al. PAL-XFEL laser heater commissioning. Nucl. Instrum. Methods A 843, 39–45 (2017).

    ADS  Article  Google Scholar 

  17. 17.

    Heo, H. et al. Development of an X-band linearizer system for PAL-XFEL. In Proc. 2016 Particle Accelerator Conference 554 (JACOW, 2016).

  18. 18.

    Pflueger, J. et al. Status of the undulator system for the European X-ray Free Electron Laser. In Proc. 2013 Free Electron Laser Conference 367–371 (JACOW, 2013).

  19. 19.

    Hong, J. et al. A study on low emittance injector and undulator for PAL-XFEL. High Power Laser Sci. Eng. 3, e21 (2015).

    Article  Google Scholar 

  20. 20.

    Ko, I. S. et al. Construction and commissioning of PAL-XFEL facility. Appl. Sci. 7, 479 (2017).

    ADS  Article  Google Scholar 

  21. 21.

    Kang, H.-S. et al. Status of the PAL-XFEL. In Proc. 2016 Linac Conference 1042–1046 (JACOW, 2016).

  22. 22.

    Loos, H. Operational experience at LCLS. Proc. 2011 Free Electron Laser Conference 166–172 (JACOW, 2011).

  23. 23.

    Reiche, S. GENESIS 1.3: a fully 3D time-dependent FEL simulation code. Nucl. Instrum. Methods A 429, 243–248 (1999).

    ADS  Article  Google Scholar 

  24. 24.

    Parc, Y. W. et al. Radiation size and divergence at the hard X-ray beamline in the PAL-XFEL. J. Korean Phys. Soc. 64, 976–981 (2014).

    ADS  Article  Google Scholar 

  25. 25.

    Kang, H.-S. et al. Start to end simulation of three bunch compressor lattice for PAL-XFEL. In Proc. 2012 Particle Accelerator Conference 1738–1740 (JACOW, 2012).

  26. 26.

    Park, J.-K. et al. Design of a hard X-ray beamline and end-station for pump and probe experiments at Pohang Accelerator Laboratory X-ray Free Electron Laser facility. Nucl. Instrum. Methods A 810, 74–79 (2016).

    ADS  Article  Google Scholar 

  27. 27.

    Bionta, M. R. et al. Spectral encoding method for measuring the relative arrival time between X-ray/optical pulses. Rev. Sci. Instrum. 85, 083116 (2014).

    ADS  Article  Google Scholar 

  28. 28.

    Lee, S. et al. PAL-XFEL cavity beam position monitor pick-up design and beam test. Nucl. Instrum. Methods A 827, 107–117 (2016).

    ADS  Article  Google Scholar 

  29. 29.

    Kim, D. E. Updates of the PAL-XFEL undulator program. In Proc. 2015 Particle Accelerator Conference 1675–1677 (JACOW, 2015).

  30. 30.

    Yang, H. et al. Linac lattice optimization for PAL-XFEL hard X-ray FEL line. In Proc. 2014 Particle Accelerator Conference 2900–2902 (JACOW, 2014).

  31. 31.

    Borland, M. Elegant: a flexible SSDS-compliant code for accelerator simulation. LS-287 6th Int. Computational Accelerator Physics Conf. ICAP2000ANL (2000).

  32. 32.

    Kim, S. H. et al. PAL-XFEL pulse modulator system test results using a high precision CCPS. In Proc. 2015 Free Electron Laser Conference 89–92 (JACOW, 2015).

  33. 33.

    Hu, J. et al. LLRF development for PAL-XFEL. Proc. 2016 Particle Accelerator Conference, 2761–2764 (2016).

  34. 34.

    Min, C.-K. et al. RF timing distribution and laser synchronization commissioning of PAL-XFEL. In Proc. 2016 Particle Accelerator Conference 4234–4236 (2016).

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The authors thank K.J. Kim, P. Emma, Z. Huang, P. Krejcik, A. Young, S. Hoobler, C. Xu, T. Straumann, K. Kim, Y. Ding, F. Zhou, D. Ratner, T. Raubenheimer, J. Wu, J. Pflueger and H. Matsumoto for their help. The authors also thank S.L. Cho for providing Bi thin-film samples and H. Loos and J. Loos for their contribution to the PAL-XFEL commissioning. The authors acknowledge the support of the Ministry of Science and ICT of Korea.

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All authors designed, constructed and tested the accelerator and X-ray systems, performed the experiments and analysed the data. I.S.Ko was the PAL-XFEL project director.

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Correspondence to Heung-Sik Kang.

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The authors declare no competing financial interests.

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Kang, HS., Min, CK., Heo, H. et al. Hard X-ray free-electron laser with femtosecond-scale timing jitter. Nature Photon 11, 708–713 (2017). https://doi.org/10.1038/s41566-017-0029-8

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