Evidence for transverse dependencies in COTR and microbunching in a SASE FEL

Using coherent optical transition radiation (COTR) techniques, we have observed transverse dependencies, which in some aspects relate to the electron-beam microbunching in a visible wavelength (540 nm) self-amplified spontaneous emission (SASE) free-electron laser (FEL). The experimental COTR observations include the z-dependent e-beam sizes, the z-dependent angular distributions, and the z-dependent spectra (which show an x-dependence). A 30–40% narrowing of the observed beam size using COTR is explainable by the mechanismʹs dependence on the square of the number of microbunched particles. However, additional effects are needed to explain beam size reductions by factors of 2–3 at different z locations. Localized e-beam structure in the gun or induced in the bunch compression process may result in microbunching transverse dependence, and hence the observed COTR effects.