Thermo-mechanical analysis of a steering mirror with dielectric bulk for the ECRH system of DTT

The steerable launcher mirrors, essential for directing microwave beams into the plasma, play a pivotal role in the Electron Cyclotron Resonance Heating (ECRH) system of the Divertor Tokamak Test (DTT) facility, currently under construction in Frascati, Italy. Due to the substantial heat loads acting on the mirrors, internal water-cooling channels are necessary to control temperature and deformation. A variable-depth complementary spiral cooling channel was considered in this study. A dielectric material with high thermal conductivity was selected as a potential candidate to reduce eddy currents, this mitigating magnetic torques and mechanical stress, while guaranteeing adequate cooling. Thermo-structural simulations (FSI) were conducted to assess the mirror's resistance to induced stresses, its deformations, and cooling performance. A transient analysis showed that thermal steady-state is the worst-case thermal loading condition during the entire experiment. Additionally, the thermo-structural behavior of various materials was analyzed to demonstrate the superior performance of the selected dielectric material. The cooling channel was subsequently adapted to a prototype mirror, on which CFD and FSI simulations were performed to validate the numerical model against future real-world experiments. Finally, crack propagation analysis confirmed the feasibility of using technical ceramics for the launching mirror, paving the way for dielectric materials in the ECRH system of DTT.