Impact of Non-collinear Laser Interaction on Microbunching Efficiency in Storage Ring-based EEHG

Echo-enabled harmonic generation (EEHG) in a storage ring is a promising scheme for producing coherent short-wavelength radiation. While the conventional theory assumes a collinear propagation of the seed laser and the electron beam, practical implementation in a storage ring often necessitates a non-collinear geometry due to spatial constraints and lattice configurations. In this study, we investigate the impact of the crossing angle between the seed laser and the electron beam on the phase-space dynamics and the resulting microbunching efficiency. We demonstrate that a finite crossing angle introduces two primary effects: a reduction in the effective electric field experienced by the electrons and an induction of transverse angular modulation. Although the reduction in energy modulation can be compensated by increasing the laser power, our analysis reveals that the coupled transverse kick leads to a smearing of the longitudinal phase space, thereby irreversibly degrading the bunching factor at high harmonics. Through detailed particle tracking simulations targeting 1.49 nm soft X-ray generation, we quantify this degradation and determine the tolerance limit for the laser incidence angle. The results indicate that while optimizing the dispersion strength (R56) can mitigate some effects, minimizing the crossing angle remains critical for preserving the high-harmonic bunching efficiency in storage ring-based light sources. PACS numbers: 14.65.Ha