Spatial coherence of synchrotron radiation degraded by grating monochromators

Fourth-generation synchrotron sources promise an enormous increase in the spatial coherence of X-ray radiation. In the EUV to soft X-ray range, the spatial coherence could reach almost 100% in both the horizontal and vertical directions. Identifying and understanding potential sources of degradation in the spatial coherence of X-rays transported along the beamline is critical to enable optimal performance for the experiments at the beamlines. Grating monochromators are an essential optical component of most EUV and soft X-ray beamlines. Recently, we have found that the spatial coherence is strongly degraded by the gratings used in these monochromators. In this work, we present a detailed physical and theoretical description of the origin and underlying effects that cause this degradation and describe the influence of the grating parameters and the exit slit of the monochromator. The theoretical analysis is presented in the framework of statistical optics. It is important to note that the described effects in the paper are distinct from the decoherence effects based on optics vibrations and the resulting virtual source broadening or wavefront degradation caused by surface irregularities and optical roughness.