Abstract
Echo-enabled harmonic generation free-electron lasers (EEHG FELs) are promising candidates to produce fully coherent soft x-ray pulses by virtue of efficient high harmonic frequency up-conversion from UV lasers. The ultimate spectral limit of EEHG, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam. We present a mathematical description of the impact of incoherent (broadband) energy modulations on the bunching spectrum produced by the microbunching instability through both the accelerator and the EEHG line. The model is in agreement with a systematic experimental characterization of the FERMI EEHG FEL in the photon energy range $130-210$ eV. We find that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reduction of the FEL spectral brightness that is proportional to the EEHG harmonic number. Local maxima of the FEL spectral brightness and of the spectral stability are found for a suitable balance of the dispersive sections' strength and the first seed laser pulse energy. Such characterization provides a benchmark for user experiments and future EEHG implementations designed to reach shorter wavelengths.
Highlights
Free electron lasers (FELs) have enabled a new way for researchers to explore electronic dynamics at molecular and atomic scales via femtosecond pulses, gigawatt peak powers, and tunable wavelengths in the range of extreme ultraviolet to hard x rays [1]
We find that amplification of electron beam energy distortions primarily in the enabled harmonic generation (EEHG) dispersive sections explains an observed reduction of the FEL spectral brightness proportional to the EEHG harmonic number
While EEHG is predicted to be more robust than other external seeding schemes to energy distortions that occur upstream, it is anticipated that distortions that occur between the EEHG chicanes can significantly impact the FEL spectrum
Summary
Free electron lasers (FELs) have enabled a new way for researchers to explore electronic dynamics at molecular and atomic scales via femtosecond pulses, gigawatt peak powers, and tunable wavelengths in the range of extreme ultraviolet to hard x rays [1]. These energy structures, accumulated during beam manipulation in the accelerator, can hardly be removed completely They can introduce extra frequencies into the FEL gain bandwidth that deteriorate the longitudinal coherence promised by external seeding [21,22,23,24,25]. We report on results of a systematic investigation at the FERMI FEL operating in EEHG mode [10] where we find good agreement between theory and experimental data. These studies help to benchmark the analytic model which provides a practical tool for the design and optimization of EEHG sources at even shorter wavelengths.
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