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Broadband hard x-ray attosecond pulses from extremely chirped electron beams

River R. Robles1,2,3,*, Veronica Guo1,2,3, David Cesar1, Paris Franz1,2,3, Aliaksei Halavanau1, Alberto Lutman1, Takahiro Sato1,3, Sanghoon Song1, Nicholas Sudar1 et al.

Yanwen Sun1, Zhen Zhang1, Diling Zhu1, and Agostino Marinelli1,3,†

  • *Contact author: riverr@slac.stanford.edu
  • †Contact author: marinelli@slac.stanford.edu
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Phys. Rev. Research 8, 033350 – Published 22 September, 2026

DOI: https://doi.org/10.1103/2w1p-jvp1

Abstract

Attosecond pulses from free-electron lasers have opened the doors to atomic site-specific studies of bound electronic dynamics on their natural, subfemtosecond timescales. Key to their success has been electron beam shaping techniques enabling the generation of subfemtosecond current spikes with peak currents on the order of 10 kA. We demonstrate in an rf linac the generation of current spikes with extreme chirps on the order of 350MeV/µm, directly competitive with the chirps expected from beam-driven plasma wakefield accelerators. Leveraging chirp-taper compensation, we use these highly chirped beams to generate hard x-ray attosecond pulses with bandwidths exceeding 30 eV, a factor of 2 beyond previous demonstrations. We simultaneously present the first explicit experimental evidence of chirp-taper compensation in an attosecond x-ray free-electron laser, finding that optimal tapering improves the bandwidth and pulse energy by factors of 2 and 5, respectively, for our conditions. In addition to the immediate utility of such broadband hard x-ray pulses, electron beams with such extreme chirps can be utilized for unique new experimental modalities by performing further compression after the undulators. Such postlasing compression can enable subsequent superradiant light emission at longer wavelengths or direct excitation of quantum systems with the beam’s intense space-charge field for unique attosecond pump-probe possibilities.

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Phys. Rev. Research 8, 033350– Published 22 September, 2026
  • Received 10 April 2026
  • Accepted 19 August 2026
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