All-optical longitudinal fine structure reconstruction for pre-bunched electron beams

This paper proposes an all-optical scheme for generating attosecond-scale pre-bunched electron beams integrated with attosecond-resolution diagnostics. An electron beam from an LPA (523.8 MeV, 1\% energy spread) is simultaneously injected with a 266 nm laser into a modulator to induce energy modulation, which is subsequently converted into density modulation through a dispersive section.

A microbunching beam with full width at half maximum (FWHM) of 41.5 nm (138 as) sub-structures and a peak current of 20 kA is achieved and then focused onto an aluminum foil to generate coherent transition radiation (CTR) signals. With a temporal retrieval algorithm, a longitudinal diagnostic resolution of 68 nm (227 as) is successfully demonstrated. Further analysis reveals that the resolution of CTR diagnostic is fundamentally limited by the plasma frequency of metallic materials (15.3 eV for aluminum, cutoff wavelength at 67 nm), making it difficult to reach the sub-attosecond regime.