Evidence of electron microbunching in laser-driven modulated downramp injection and prospects for beam-driven implementation

Abstract: Generating electron beams pre-bunched at desired radiation wavelengths is essential for enhancing the coherence and reducing the saturation length of compact X-ray free-electron lasers. In this work, we report the first experimental evidence of such microbunching using plasma density-modulated downramp injection in a laser wakefield accelerator. By superimposing a laser-induced plasma grating onto a density downramp, we observed distinct periodic modulations in the electron energy spectra, a signature absent in unmodulated cases. Particle-in-cell simulations confirm these observations, revealing bunching factors of 0.05 at 0.4 μm. Building on this demonstration, we propose a beam-driven implementation for the FACET-II National User Facility. This design promises to generate multi-GeV beams pre-bunched at optical wavelengths with sub-micrometer emittance, capable of achieving attosecond bunch lengths and peak currents exceeding 100 kA through magnetic compression.

Recommended citation: Chaojie Zhang, Audrey Farrell, Ken A. Marsh, Qianqian Su, and Chan Joshi, "Evidence of electron microbunching in laser-driven modulated downramp injection and prospects for beam-driven implementation," Plasma Phys. Control. Fusion (2026).
Download Paper