Predicting Thermal Distortion of Synchrotron Radiation Mirrors with Finite Element Analysis,
CALIFORNIA UNIV BERKELEY LAWRENCE BERKELEY LAB
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High power and high power densities due to absorbed radiation are significant design considerations which can limit performance of mirrors receiving highly collimated synchrotron radiation from insertion devices and bending magnet sources. Although the grazing incidence angles needed for x ray optics spread the thermal load, localized, nonuniform heating can cause distortions which exceed allowable surface figure errors and limit focusing resolution. This paper discusses the suitability of numerical approximations using finite element methods for heat transfer, deformation, and stress analysis of optical elements. The primary analysis objectives are 1 to estimate optical surface figure under maximum heat loads, 2 to correctly predict thermal stresses in order to select suitable materials and mechanical design configurations, and 3 to minimize fabrication costs by specifying appropriate tolerances for surface figure. Important factors which determine accuracy of results include finite element model mesh refinement, accuracy of boundary condition modeling, and reliability of material property data. Some methods to verify accuracy are suggested. Design analysis for an x ray mirror is presented. Some specific configurations for internal water cooling are evaluated in order to determine design sensitivity with respect to structural geometry, material properties, fabrication tolerances, absorbed heat magnitude and distribution, and heat transfer approximations. Estimated accuracy of these results is discussed. accuracy of these results is discussed.