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Attosecond pulses at Ångström wavelengths allow us to probe atomic-scale electron dynamics, yet producing high-power hard X-ray attosecond pulses is challenging. We developed a self-chirping method that shapes the electron beam into a ramped profile with a pronounced leading spike by fine-tuning the phase and voltage of a high-harmonic RF structure. As the beam accelerates, strong space-charge forces and coherent synchrotron radiation enhance the energy chirp. An arc section then further compresses the chirped spike—ensuring higher-energy electrons travel longer paths—while a transverse kick suppresses unwanted radiation from lower-current regions, yielding shorter XFEL pulses without side peaks.
High-repetition-rate seeded free-electron laser enabled by self-modulation
Seeded free-electron lasers (FELs), which use the frequency up-conversion of an external seed laser to improve temporal coherence, are ideal for providing fully coherent pulses at EUV and soft X-ray regime. However, it is difficult to operate seeded FELs at a high repetition rate due to the limitations of present state-of-the-art laser systems. We propose and experimentally demonstrate a self-modulation scheme for enhancing laser-induced energy modulation, thereby significantly reducing the requirement of an external laser system. Driven by this scheme, we experimentally realize high harmonic generation in a seeded FEL using an unprecedentedly small external laser-induced energy modulation. The results mark a major step toward a high-repetition-rate, fully coherent x-ray FEL. Moreover, this scheme opens a new path to ultra-compact, high-average-power, and fully coherent EUV light source.
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