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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.
2. Self-seed free-electron laser carrying orbital angular mometum
In this work, we propose a self-seeded XFEL scheme for generating X-ray pulses carrying orbital angular momentum (OAM). For the first time, we demonstrate that the transverse mode of FEL radiation can be shaped during the SASE amplification process. Our theoretical analysis and numerical simulations reveal that introducing an OAM phase front via an optical element reshapes the transverse mode composition of the FEL field and enables selective amplification of the target OAM mode. The dominant OAM mode exhibits exponential growth and can be amplified by more than two orders of magnitude. Building on this self-seeded OAM XFEL concept, we further show the potential to generate high-power isolated attosecond XFEL pulses as well as attosecond pulse pairs carrying distinct topological charges.
3. 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.
4. Laser–electron beam interaction in a dipole magnet
The fundamental physics behind the FEL is the interaction between an electromagnetic wave and a relativistic electron beam in an undulator, which consists of hundreds or thousands of dipole magnets with an alternating magnetic field. We report the first observation of the laser–beam interaction in a pure dipole magnet in which the electron beam energy modulation with a 40-keV amplitude and a 266-nm period is measured. We demonstrate that such an energy modulation can be used to launch a seeded FEL, that is, lasing at the sixth harmonic of the seed laser in a high-gain harmonic generation scheme. The results reveal the most basic process of the FEL lasing and open up a new direction for the study and exploitation of laser–beam interactions.
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