Relativistic electron beams driven by kHz single-cycle light pulses

Journal name:
Nature Photonics
Year published:
Published online

Laser–plasma acceleration1, 2 is an emerging technique for accelerating electrons to high energies over very short distances. The accelerated electron bunches have femtosecond duration3, 4, making them particularly relevant for applications such as ultrafast imaging5 or femtosecond X-ray generation6, 7. Current laser–plasma accelerators deliver 100 MeV (refs 8–10) to GeV (refs 11, 12) electrons using Joule-class laser systems that are relatively large in scale and have low repetition rates, with a few shots per second at best. Nevertheless, extending laser–plasma acceleration to higher repetition rates would be extremely useful for applications requiring lower electron energy. Here, we use single-cycle laser pulses to drive high-quality MeV relativistic electron beams, thereby enabling kHz operation and dramatic downsizing of the laser system. Numerical simulations indicate that the electron bunches are only ∼1 fs long. We anticipate that the advent of these kHz femtosecond relativistic electron sources will pave the way to applications with wide impact, such as ultrafast electron diffraction in materials13, 14 with an unprecedented sub-10 fs resolution15.

At a glance


  1. Measurements of the kHz electron beam.
    Figure 1: Measurements of the kHz electron beam.

    Data shown in a, c and d were taken with an estimated peak electron density of ne = 1.8 × 1020 cm–3 (±10%). a, Typical electron beam profile obtained by integrating over 500 shots. The total beam charge is estimated to be 147 fC per shot for this particular case. b, Dependence of the beam charge as a function of plasma density (the density was changed by varying the height of the gas jet). Vertical error bars represent r.m.s. fluctuations, and horizontal error bars represent uncertainty over the electron density. c, Electron beam filtered by a 500 µm pinhole with and without magnetic field. Deviation of the electron spot by the magnetic field indicates acceleration to multi-MeV energies. d, Electron spectrum. The grey shaded area represents the standard deviation over 20 spectra (each spectrum was obtained by accumulating over 1,000 laser shots). The horizontal error bar represents the spectrometer resolution at ∼6 MeV.

  2. Observation of dispersion effects.
    Figure 2: Observation of dispersion effects.

    a, Evolution of accelerated charge with chirp of the pulse (in fs2). The peak electron density is ne = 1.8 × 1020 cm–3 (±10%). Upper axis: estimated pulse FWHM duration. The grey area and vertical error bars represent r.m.s. fluctuations over 20 images (each averaged over 1 s = 1,000 shots). Horizontal error bars represent the uncertainty on the absolute value of the chirp. b, Normalized electron spectra for different chirp values. c, Laser electric field (in blue) and envelope (in red) for three different chirp values.

  3. Results of PIC simulations.
    Figure 3: Results of PIC simulations.

    a, Evolution of laser intensity (red) and injected charge (blue) during propagation in the plasma (density profile shown shaded in grey) for a pulse with a 4 fs2 positive chirp. b, Snapshot of the wakefield around the middle of the gas get. It shows the spatial distribution of electron density (in blue–white colour scale), the laser intensity (red–orange colour scale) and relativistic electrons (E > 1.5 MeV) trapped in the wakefield (in green). The intensity distribution of the laser pulse shows a strong modulation as it is practically split in two. c, Electron energy spectrum at the accelerator exit. The simulation was run with at the centre of the gas jet, yielding an electron density of ne= 1.6 × 1020 cm−3 after ionization.


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Author information


  1. LOA, ENSTA Paristech, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91120 Palaiseau, France

    • D. Guénot,
    • D. Gustas,
    • A. Vernier,
    • B. Beaurepaire,
    • F. Böhle,
    • M. Bocoum,
    • M. Lozano,
    • A. Jullien,
    • R. Lopez-Martens,
    • A. Lifschitz &
    • J. Faure


A.V., B.B., D.Gué, D.Gus and J.F. built the laser–plasma experiment. D.Gué and D.Gus performed the experiment and analysed the data. F.B., M.B., M.L., A.J. and R.L.-M. developed the near-single-cycle laser system. A.L. performed the modelling of the experiment. J.F. and D.Gué wrote the paper with inputs from all co-authors. J.F. directed the project.

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