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Impact of electron-beam energy spread on the generation of high harmonics in seeded free-electron lasers

Eléonore Roussel1,*, Simone Spampinati2, Primož Rebernik Ribič2, Laura Badano2, Paolo Cinquegrana2, Miltcho B. Danailov2, Giovanni De Ninno2, Alexander Demidovich2, Simone Di Mitri2 et al.

Eugenio Ferrari3, Giulio Gaio2, Luca Giannessi2, Najmeh S. Mirian4, Ivaylo Petrov Nikolov2, Giuseppe Maria Penco2, Carlo Spezzani2, and Enrico Massimiliano Allaria2

  • *Contact author: eleonore.roussel@univ-lille.fr
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Phys. Rev. Accel. Beams 29, 040701 – Published 6 April, 2026

DOI: https://doi.org/10.1103/13z8-2ktg

Abstract

In seeded free-electron lasers (FELs), harmonic up-conversion of an external seed laser can be used to generate coherent emission at short wavelengths. The harmonic generation process is sensitive to the electron beam slice energy spread, which limits the highest harmonic conversion factor. We present here an experimental study of the impact of the slice energy spread on the FEL performance in two different harmonic generation schemes: high-gain harmonic generation and echo-enabled harmonic generation (EEHG). The slice energy spread is controlled by means of a laser heater installed in the low energy region of the linear accelerator that drives the FEL. The measurements of the FEL pulse energy as a function of the electron beam slice energy spread were performed at different harmonics. Our results confirm the robustness of EEHG to the slice energy spread and open the way toward fully coherent FEL pulses in the soft x-ray domain.

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Phys. Rev. Accel. Beams 29, 040701– Published 6 April, 2026
  • Received 16 June 2025
  • Accepted 20 February 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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