Production and analysis of metallic coatings on ceramic substrates for the electron cyclotron heating launching mirror of DTT

The Electron Cyclotron Resonance Heating system for the Divertor Tokamak Test (DTT) relies on a series of mirrors to inject high-frequency millimetre waves into the reactor vessel. The last launching mirrors transmit microwaves directly into the plasma while withstanding extreme conditions in the tokamak ports. Significant challenges include magnetic torques that could affect steering, intense heat loads, and flux of energetic neutrals. Developing mirrors that withstand this environment and remain effective in microwave reflection, without increasing roughness-related ohmic losses, is crucial. A current investigation involves combining a dielectric ceramic substrate with tungsten coatings to optimize performance and lifetime. In this project, three types of tungsten multilayers were deposited on ceramic substrates with radio-frequency magnetron sputtering. The coatings were analysed in their morphology, composition, adhesion, and electrical resistivity, and exposed to fusion-relevant plasmas in a linear device. Results indicated good adhesion and maintenance of the coating’s morphology post-exposure. Comparison with an established model suggested an erosion enhancement mechanism, and preliminary SRIM-2013 simulations explored the impact of impurities on sputtering. Additionally, a first estimation of ohmic losses provided insight into their magnitude. The project represents the first experimental study on this innovative approach, and the extensive characterization provided a solid assessment of the coating production effectiveness and of the coating's main properties.