High-power attosecond X-ray free-electron lasers: physics and design strategy

TL;DR

This paper investigates the fundamental physics and design principles for generating high-power attosecond X-ray pulses using free-electron lasers, providing a unified framework independent of specific schemes.

Contribution

It offers a scheme-independent analysis of the physics and constraints for high-power attosecond XFELs, including guidelines for optimizing electron-beam properties.

Findings

Effective lasing length governs peak power and pulse duration.

Trade-offs exist between peak power, pulse length, and single-spike probability.

Guidelines for electron-beam optimization toward terawatt-class attosecond XFELs.

Abstract

Attosecond pulses from X-ray free-electron laser (XFEL) have opened new opportunities for probing ultrafast electronic dynamics on the Angstrom--attosecond spatiotemporal scale. Most attosecond XFEL concepts rely on generating an ultrashort high-current spike through either external laser modulation or accelerator-based beam manipulation. Despite their different implementations, these approaches share the same essential physics, namely that the XFEL amplification is confined to a short effective lasing window within the electron beam. However, existing studies are often scheme-specific and do not yet provide a unified quantitative picture of how fundamental electron-beam properties constrain high-power attosecond performance. In this work, we investigate the general physics and scheme-independent requirements for generating high-power attosecond X-ray pulses from a short current spike. From the perspective of post-saturation superradiant evolution, we show that the effective lasing length of the electron beam governs both the attainable peak power and the pulse duration. We further examine the distinct roles of slice energy spread, slice emittance, energy chirp, undulator tapering, and transverse beam tilt. Our results reveal the trade-off between peak power, pulse shortening, and single-spike probability, and provide facility-independent guidelines for optimizing electron-beam phase-space manipulation toward terawatt-class attosecond XFEL operation.