Demonstration of self-seeding in a hard-X-ray free-electron laser


The Linac Coherent Light Source (LCLS) is an X-ray free-electron laser at the SLAC National Accelerator Laboratory, which has been operating since 2009 for a wide range of scientific research. The free-electron laser process at LCLS is based on self-amplified spontaneous emission (SASE) where spontaneous emission from the initial electron beam shot noise is amplified by its interaction with the electrons over a long magnetic undulator. Although SASE is very effective, producing tremendously powerful, ultrashort X-ray beams, the start-up from noise leaves poor temporal coherence and a broad, noisy spectrum. We present experimental results of a new method, suggested by colleagues at DESY, allowing self-seeding using X-rays from the first half of the undulator to seed the second half through a diamond-based monochromator, producing near Fourier-transform-limited X-ray pulses with 0.4–0.5 eV bandwidth at 8–9 keV. These results demonstrate self-seeding at ångstrom wavelengths with a relative bandwidth reduction of 40–50 with respect to SASE.

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Figure 1: Layout of the LCLS undulator with a self-seeding chicane, diamond monochromator, gas detector and hard-X-ray spectrometer.
Figure 2: Spectral and temporal intensity dependences of X-ray Bragg diffraction and FBD.
Figure 3: Monochromator assembly (left) and zoomed 10-inch flange assembly (right).
Figure 4: Each data point is an FEL X-ray pulse total energy measurement made using the gas detector and plotted against the vertical position of the diamond crystal (y).
Figure 5: Measured X-ray spectra.
Figure 6: Seeded FEL intensity as a function of chicane delay, measured using the intensity of just the narrow seeded line on the X-ray spectrometer.


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The authors would like to thank the SLAC controls, alignment, operations and engineering groups, and also J. Arthur, A. Brachmann, G. Decker, J. Galayda, P. Den Hartog, N. Holtkamp, J. Quintana, C. Pellegrini, E. Prat, T. Raubenheimer, T. Tanaka, J. Stein, B. Stephenson and L. Young for their support and interest in this work. The authors also thank G. Geloni, V. Kocharyan and E. Saldin for sharing their very effective self-seeding idea and also for participating in its commissioning effort. The authors are grateful for the support of the US Department of Energy, Office of Science (contract no. DE-AC02-76SF00515), and the sponsorship of the LCLS mission by the Office of Basic Energy Sciences. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (contract no. DE-AC02-06CH11357). MRCAT operations are supported by the Department of Energy and the MRCAT member institutions.

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P.E., J.H., Z.H., R.L., S.St., Y.S. and A.Z. co-wrote the manuscript. V.B. and S.T. developed, tested and provided the diamond crystal. S.St. and Y.S. characterized the diamond. Y.F., D.F., J.H. and D.Z. developed and commissioned the spectrometer. J.K. and A.L. analysed the spectra. D.S. and J.R. designed and tested the monochromator controls. E.T. and D.W. designed and fabricated the chicane magnets and vacuum chambers. V.B., F.-J.D., Y.D., P.E., Y.F., J.F., D.F., J.H., Z.H., J.K., R.L., H.L., A.L., H.-D.N., D.R., J.R., Y.S., S.Sp., S.T., J.W., J.W., A.Z. and D.Z. all performed experiments and analysed data. J.A. and W.B. were the project engineers at SLAC and ANL, respectively, and P.E. and A.Z. were the project physicists, also at SLAC and ANL, respectively.

Correspondence to P. Emma.

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