Phase-Stable Self-Modulation for GHz Continuous-Wave Ultrafast X-Ray Free-Electron Lasers

TL;DR

This paper introduces a laser-free, phase-stable self-modulation scheme in ERLs that enables GHz-repetition-rate, ultrashort X-ray pulses with high peak power, advancing continuous-wave ultrafast X-ray source development.

Contribution

It proposes a novel, practical self-modulation method within ERLs to generate high-power, ultrashort X-ray pulses at unprecedented repetition rates without using lasers.

Findings

Achieves >4 GW peak power in soft X-ray pulses.

Demonstrates 1 fs pulse duration at 1.3 GHz repetition rate.

Validates feasibility through start-to-end simulations.

Abstract

High-brightness femtosecond-to-attosecond pulses are indispensable for probing electron dynamics on their fundamental temporal scales. X-ray free-electron lasers (XFELs) at high repetition rates will facilitate high-statistics measurements and time-resolved studies that were previously inaccessible. Although energy recovery linacs (ERLs) are well suited for high-repetition-rate operation, their relatively low peak current poses a major challenge for generating intense ultrashort X-ray pulses. Here, we propose a completely laser-free scheme that fundamentally overcomes this bottleneck through a continuous, phase-stable self-modulation process. By interacting with its own coherently emitted terahertz radiation within a helical wiggler, the electron bunch naturally accumulates a robust, few-cycle energy modulation in its core, even when starting with the intrinsically low peak current typical of ERLs. A downstream dispersion chicane subsequently converts this energy modulation into an isolated, exceptionally sharp current spike. Start-to-end simulations based on a 1~GeV ERL light source demonstrate the feasibility of generating isolated soft X-ray pulses with an average peak power exceeding 4~GW and a pulse duration of about 1~fs at an unprecedented 1.3~GHz repetition rate. The proposed scheme offers a highly practical pathway for advancing ultrafast X-ray generation into the true continuous-wave regime, with transformative implications for the development of next-generation coherent light sources.