Optomechanical Optimization for a Sagittaly Bent Double Crystal Monochromator, Using Finite Elements and Ray Tracing

Abstract

Designing a second crystal for a sagittally bent Double Crystal Monochromator (DCM) requires dealing with a number of conflicting requirements. Especially when working with high-energy photons, the angular aperture (Darwin width) becomes very narrow (below 10µrad for Si) while simultaneously the bending radius is increasing small (down to 1m for typical beamline dimensions at 40keV). In this situation, the cross-talk between tangential and sagittal curvature becomes a key parameter, and two strategies are generally used to overcome the issue: either using a flat crystal with a specific length/with ratio, or usage of a rib-stiffened crystal. In the frame of the upgrade of the SAMBA beamline DCM, both solutions have been explored, using a suite of scripts connecting a general purpose FEM code (ANSYS) and a ray-tracing code (SpotX). This has allowed a systematic evaluation of a wide number of configurations, giving insight in the interaction between geometric parameters, and ultimately resulting in a twofold increase in the photon throughput at 30keV without comprising neither spectral resolution nor spot size at sample location.