AttoSHINE: Toward Megahertz, Terawatt-Class Attosecond X-rays An attosecond (10⁻¹⁸ seconds) corresponds to the natural timescale of electron motion in matter. In 2023, the Nobel Prize in Physics recognized experimental methods for generating and meas

An attosecond (10⁻¹⁸ seconds) corresponds to the natural timescale of electron motion in matter. In 2023, the Nobel Prize in Physics recognized experimental methods for generating and measuring attosecond light pulses, highlighting their ability to effectively “film” electrons in real time. The next frontier of attosecond science is extending these capabilities into the X-ray regime. X-rays provide element specificity and atomic-scale sensitivity, making them powerful probes of ultrafast dynamics in complex materials and chemical systems. However, generating high-power attosecond X-ray pulses at high repetition rates remains a major challenge.
Continuous-wave (CW) X-ray free-electron lasers (XFELs), driven by superconducting accelerators, offer a promising route toward this goal. These facilities can deliver XFEL pulses at megahertz repetition rates, enabling high-statistics measurements and greatly improved experimental stability, provided that attosecond pulse durations can be realized. SHINE, as one of the first hard X-ray CW XFEL facilities under construction, provides a unique platform for exploring high-repetition-rate attosecond science.
In a new study, scientists from the Shanghai Advanced Research Institute at the Chinese Academy of Sciences and Deutsches Elektronen-Synchrotron (DESY) report that a CW-XFEL could generate terawatt-class attosecond X-ray pulses at megahertz repetition rates, based on comprehensive simulations for Shanghai High-repetition-rate XFEL and Extreme Light Facility (SHINE). Notably, the proposed approach operates within the existing machine configuration and requires no additional hardware.
The scheme exploits a self-chirping effect within the electron beam, where collective beam interactions naturally imprint a strong energy chirp onto a small fraction of the bunch. The chirped beam is subsequently transported through a specially designed dogleg lattice located immediately upstream of the undulator. The lattice is optimized to provide strong longitudinal compression while suppressing transverse dispersion, producing an ultranarrow, high-current spike that emits an intense attosecond X-ray pulse. Because the method relies on beam transport optimization rather than new hardware, it provides a practical and scalable route for CW XFELs to reach the attosecond regime.
For hard X-rays at 6 keV, simulations predict pulses with an average duration of approximately 300 attoseconds and peak powers approaching 0.8 terawatts. For soft X-rays at 1 keV, pulse durations of around 470 attoseconds with comparable terawatt-level peak powers are obtained. If experimentally realized, the simultaneous combination of attosecond temporal resolution, terawatt peak power, and megahertz repetition rate would enable entirely new classes of X-ray experiments. Such sources could open the door to nonlinear attosecond X-ray science, enabling multi-photon and higher-order electronic processes to be explored at atomic length scales. They could also enable attosecond X-ray crystallography, allowing direct observation of electron-density evolution during chemical reactions and phase transitions. The high repetition rate would further enable rapid statistical averaging, significantly improving sensitivity to weak or rare ultrafast phenomena while enhancing experimental stability.
Beyond demonstrating feasibility, the study also reveals an important physical insight. In the post-saturation regime of FEL amplification, superradiant emission can simultaneously enhance peak power and further shorten the radiation pulse. The authors suggest that actively exploiting this regime may become a key strategy for optimizing next-generation high-peak-power attosecond XFEL sources.