Mechanical analysis of a waveguide pipe for the reflectometry diagnostic in DEMO

The reflectometry diagnostic proposed for DEMO aims to measure the radial edge electron density profile and to provide data for the feedback control of plasma position and shape. Microwave (MW) reflectometry measurements are foreseen at 16 poloidal locations (gaps), with sets of plasma-facing antennas and waveguides housed in a Diagnostic Slim Cassette (DSC) — a full, 25-cm thick poloidal sector to be integrated with the breeding blanket (BB). To route and support the waveguides through the upper port (UP), the use of enveloping hollow pipes with stiffener plates and motion compensators has been proposed. In the current design, each pipe conducts a set of nine waveguides. The waveguide pipes need to be compatible with the BB remote handling system and shall be able to compensate the relative movements between the BB and the vacuum vessel (VV), preventing plastic deformation of the waveguides. In this study, parametric ANSYS simulations were performed applying conservative estimates for the maximum relative displacement between the BB and the VV. The design optimizations guided by the simulations focused on removing material in highly stressed regions, to improve flexibility. The updated design reduces the maximum von Mises stress by half, from 1200 MPa to 600 MPa, compared to the original design. While this value, obtained under conservative assumptions, remains slightly above the yield strength of EUROFER at the expected VV temperatures, the stresses in the waveguides (where compensators are not implemented) are significantly lower, which indicates their ability to handle the relative movements.