Thermo-mechanical analysis of retro-reflectors for interferometry and polarimetry in W7-X

The stellarator Wendelstein 7-X (W7-X) is presently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The plasma density profile will be measured by two-colour interferometry where for each line of sight through the plasma the phase shifts of two far-infrared laser beams are compared. Due to the 3D shape of the superconducting coils in W7-X, opposite ports are not available. Instead, corner cube retro-reflectors (CCR) will be incorporated in the plasma vessel heat shield that reflect the beams back to the entrance window and diagnostic. This requires high accuracy of the reflecting surfaces which will be subjected to thermal loads from plasma radiation and therefore be heated up, displaced and deformed during operation. is used to perform coupled thermo-mechanical analyses of the CCR, including plasma radiation, thermal conduction, and radiation between all parts. The resulting displacements provide the inputs to MATLAB routines which then yield local deformations, curvature radii, and tilting of the reflecting surfaces. This information is used to optimize the design of the CCR and their fixations.