Preliminary thermal and structural analyses on the parabolic mirror of the Multi-Beam Transmission Line of the DTT ECH system

The Italian tokamak DTT (Divertor Tokamak Test) is a facility for research on nuclear fusion with the aim of investigating alternative power exhaust solutions to be exploited in the future DEMO fusion power plant. DTT foresees three additional heating systems to provide the plasma with sufficient power: Electron Cyclotron Heating (ECH) is the main one, with up to 32 MW installed. It consists of 4 clusters, each one composed of eight radio-frequency sources (gyrotrons) with their single-beam transmission lines, one quasi-optical Multi-Beam Transmission Line (MBTL) and eight independent launchers. The power produced by the source is transmitted quasi-optically through multiple reflections on metallic mirrors, the basic component of the transmission line, and then injected into the plasma. In the MBTL the reflection of the eight microwave beams on the mirror surface leads to heating of its whole structure due to the absorbed fraction of the beam power (0.21-0.27 %) ascribed to the ohmic heating at the mirror surface. During the radio-frequency power pulse (100 s), the mirror temperature increases, generating deformations that could result in a loss of beam transmission efficiency. For this reason, these mirrors need to be carefully designed from thermal and structural points of view, to guarantee the required optical performances. In the present paper, a preliminary study concerning the design of the MBTL parabolic mirror, and its cooling is presented, resorting to a coupled thermal and structural finite element simulation. The preliminary analyses address a main objective: minimize the surface mirror temperatures and consequently the deformation associated with thermal expansion, to reduce the impact on beam transmission efficiency drop. Different design choices, namely materials, body thickness and cooling solutions, in terms of cooling channel shape and water flow, are discussed.