Design of a variable-included-angle Monk-Gillieson monochromator with varied-line-spacing gratings.

A novel design concept is presented for a variable-included-angle Monk-Gillieson mounting monochromator with a varied-line-spacing (VLS) plane grating, in which the grating is illuminated by converging X-rays. The energy resolution for a beamline with a typical undulator source is estimated by means of the ray-tracing method. It is shown that the source-size or slope-error limited resolution is achieved in a wide energy range by properly changing the included angle. Moreover, relatively high resolution is maintained even if one scans the photon energy with a fixed included angle, which is a great advantage over the previous VLS grating monochromator with diverging illumination on the grating. A design example for a bending-magnet beamline is also presented, indicating that the new design concept is valid even for high-emittance sources.