Thermo-structural analysis on the DTT ECH Transmission Line Single-Beam mirrors

The Electron Cyclotron Heating (ECH) of the Divertor Tokamak Test (DTT) facility is its main system to heat the plasma with an installed power of 32 MW. A cluster of the ECH consists of 1 MW/170 GHz sources (8 gyrotrons), a quasi-optical Transmission Line (TL) and two antennas. The TL efficiently transmits the microwave beams, produced by gyrotrons, up to the launchers. Based on the quasi-optical concept, the TL employs mirrors for the transmission of the beams. One of the three segments in which it is conceptually divided is a Single-Beam section in the Gyrotron hall (SBG), where the beams propagate with an independent optical path. A small fraction of beam power is absorbed by the reflecting mirror due to ohmic losses, resulting in a total power absorbed by an SBG mirror of ∼1-2 kW. The mirror temperature increases, generating deformations. The loss of the nominal shape of the mirror can contribute to reducing the transmission efficiency, compromising the nominal parameters that ensure the effective performance of the beam propagation system. The conceptual design of different SBG mirrors is carried out, simulating the thermal-structural behaviour. The mirrors, actively water-cooled through channels designed as elongated spirals with variable pitch, achieve maximum temperatures of 28 °C and low deformations (∼23 μm), demonstrating the effectiveness of the cooling system. In the end, the same method is then applied in a parametric analysis of the thickness to evaluate the best performance, if necessary. It showed that the initial assumption on the thickness appears to be a well-founded choice.