Application of quasi-forbidden multilayer Bragg reflection for monochromatization of hard X-ray FEL SASE pulses

Preserving and manipulating the complex time structure of the X-ray Free Electron Laser (XFEL) pulses on subfemtosecond scales presents a new challenge for X-ray optics. Promising candidates for ultra-short X-ray pulses optics are periodic multilayer (ML) structures. We present a theoretical investigation of the XFEL pulse propagation through ML structures by an approach based on decomposition of short pulses into a set of plane waves by Fourier transform and application of known methods of X-ray optics. An analysis of coherent properties of XFEL pulse propagated through the ML structures is also presented. It is shown that bandwidth of quasi-forbidden reflections from the small d-spacing Al2O3/B4C structure can be less than pulse bandwidth with reflectivity up to 80%. Such structures can be used for hard X-ray FEL pulses monochromatization and diagnostics.