Fresh-slice multicolour X-ray free-electron lasers

Journal name:
Nature Photonics
Volume:
10,
Pages:
745–750
Year published:
DOI:
doi:10.1038/nphoton.2016.201
Received
Accepted
Published online

Abstract

X-ray free-electron lasers (XFELs) provide femtosecond X-ray pulses with a narrow energy bandwidth and unprecedented brightness. Ultrafast physical and chemical dynamics, initiated with a site-specific X-ray pulse, can be explored using XFELs with a second ultrashort X-ray probe pulse. However, existing double-pulse schemes are complicated, difficult to customize or provide only low-intensity pulses. Here we present the novel fresh-slice technique for multicolour pulse production, wherein different temporal slices of an electron bunch lase to saturation in separate undulator sections. This method combines electron bunch tailoring from a passive wakefield device with trajectory control to provide multicolour pulses. The fresh-slice scheme outperforms existing techniques at soft X-ray wavelengths. It produces femtosecond pulses with a power of tens of gigawatts and flexible colour separation. The pulse delay can be varied from temporal overlap to almost one picosecond. We also demonstrate the first three-colour XFEL and variably polarized two-colour pulses.

At a glance

Figures

  1. Fresh-slice multipulse scheme.
    Figure 1: Fresh-slice multipulse scheme.

    The electron bunch travels off-axis in the dechirper experiencing a strong transverse head–tail kick, represented by the yellow arrows. Electron bunch slices and trajectories are represented in blue for the head and red for the tail. Before the undulator line, the electron bunch orbit is steered to have the bunch tail travelling straight. A saturated FEL pulse at energy E1 (orange oval) is generated on the bunch tail in the first undulator section tuned to K1. The bunch head does not lase because of the oscillating orbit. The bunch orbit is steered with dipole correctors near the chicane so that the bunch head is travelling straight. A fully saturated photon beam at energy E2 (azure oval) is generated on the head in the second undulator section, tuned to K2. The magnetic chicane delays the electron bunch and therefore the photon pulse at E2 with respect to the pulse at E1. With additional chicanes and undulator sections, multiple pulses can be generated on different bunch slices. The current LCLS layout allows for three pulses with controlled photon energies and pulse delays.

  2. Fresh-slice two-colour single shots at 707 eV.
    Figure 2: Fresh-slice two-colour single shots at 707 eV.

    a, Electron bunch time-energy phase space measured for a 1.03 mJ single-shot X-ray pulse (left) and for a 734 µJ single-shot X-ray pulse (right). The electron bunch tail is on the right. Photon pulses on the tail are approximately 5 fs long. Photon pulses on the head are approximately 17 fs long. Low-energy spread slices visible at the centre of the electron bunch did not lase effectively. b, The single-shot spectrum corresponding to the 734 µJ shot phase space and the average spectrum for shots with electron bunch energies between 3,964.2 MeV and 3,965.2 MeV.

  3. Demonstration of the fresh-slice two-colour FEL at 640 eV.
    Figure 3: Demonstration of the fresh-slice two-colour FEL at 640 eV.

    ac, Single-shot time–energy phase space measured for the following conditions: lasing off (a), a single-shot 483 µJ pump-pulse (b) and a 933 µJ single shot with both pulses (c). The bunch head is on the left. d, Vertical undulator orbits measured for the presented single shot in the undulator section. Lasing off (magenta), pump-only (green), and pump and probe (blue) share the same y orbit. The dashed red line is the head-lasing orbit measured when the dechirper is set to the specified transverse offset. The dashed black line is a tail-lasing orbit calculated by switching sign to the head-lasing orbit. e, Horizontal undulator orbit. Transverse kicks are supplied to suppress lasing in different sections. Magnetic chicanes are represented by the grey rectangles. f, X-ray temporal reconstruction from the time-resolved electron bunch losses. g, Measured single-shot spectra (solid) and average spectra (dashed) for electron bunch energies between 3,750.3 MeV and 3,751.3 MeV.

  4. Polarization control of the probe pulse.
    Figure 4: Polarization control of the probe pulse.

    Probe single-shot and average spectra measured with the Delta undulator turned on and turned off. The average contrast between the circularly polarized light and the linearly polarized background is close to 35:1. The linearly polarized pump pulse at 715 eV is not visible because of the limited spectrometer range.

  5. Demonstration of the fresh-slice three-colour XFEL at 780 eV.
    Figure 5: Demonstration of the fresh-slice three-colour XFEL at 780 eV.

    a,b, Single-shot time–energy phase space measured when lasing is suppressed (a) and for three pulses (b). Time-axis nonlinearity that is introduced by the horizontal dechirper was neglected. c,d, Measured undulator orbits: lasing suppressed (magenta), first pulse only (green), first and second pulse (orange), and all three pulses (blue). The dashed red line is the head-lasing orbit measured with the dechirper modules set at the used transverse offsets. The dashed black line is a tail-lasing orbit calculated by switching sign to the head-lasing orbit. Magnetic chicanes are represented by the grey rectangles. c, Vertical undulator orbit. d, Horizontal undulator orbit. Lasing is suppressed for sections downstream of the large oscillating orbits. e, Measured average and single-shot spectra for trajectories perturbed at different undulator locations for electron bunches with energies between 4,135.5 MeV and 4,136.5 MeV.

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Author information

Affiliations

  1. SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

    • Alberto A. Lutman,
    • Timothy J. Maxwell,
    • James P. MacArthur,
    • Marc W. Guetg,
    • Ryan N. Coffee,
    • Yuantao Ding,
    • Zhirong Huang,
    • Agostino Marinelli,
    • Stefan Moeller &
    • Johann C. U. Zemella
  2. Physics Department, University of Connecticut, Storrs, Connecticut 06269, USA

    • Nora Berrah
  3. Stanford PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, Calfornia 94025, USA

    • Ryan N. Coffee &
    • Zhirong Huang
  4. Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, Hamburg 22607, Germany

    • Johann C. U. Zemella

Contributions

A.A.L., J.P.M and R.N.C co-wrote the manuscript with input from all co-authors. A.A.L conceived the fresh-slice schemes with the dechirper. A.A.L., T.J.M., J.P.M., M.W.G., N.B., R.N.C., Y.D., Z.H., A.M., S.M. and J.C.U.Z. participated in the experiments.

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The authors declare no competing financial interests.

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