Synthesis of Electron Microbunching Rotation for Generating Isolated Attosecond Soft X-ray Free-electron Laser Pulses

Attosecond x-ray pulses play a crucial role in the study of ultrafast phenomena occurring within inner and valence electrons. Especially isolated attosecond pulses with high photon energy and high peak power are of great significance in single-shot imaging in the soft x-ray region, life sciences, and attosecond pump-probe experiments. In modern accelerators, laser manipulation of electrons can be used to tailor the ultrafast properties of free-electron laser (FEL) pulses. In this paper, we propose a novel laser manipulation technique that makes use of two laser beams with mutual delays and tilted wavefronts to synthesize microbunching rotation on the scale of infrared laser wavelengths within the electron bunch for generating isolated attosecond soft x-ray pulses. This microbunching rotation ultimately leads to an enhanced current contrast ratio between the main peak and the surrounding satellite peaks within the bunch. By properly accounting for the longitudinal space charge fields within the FEL undulator, a tapered undulator can further suppress the side peaks in the radiation pulse and enable the selection of an isolated, hundred-attosecond, GW-level soft x-ray pulse.