Progress and challenges of the ECH transmission line design for DTT

The design of the Transmission Line (TL) as a part of the Electron Cyclotron Heating (ECH) system for Divertor Tokamak Test facility (DTT) is approaching the conceptual design maturity. With an ECH system of 16 MW installed for the first phase and with a total of 32 gyrotrons (170 GHz, ≥ 1 MW, 100 s) the TL design is undertaking the challenge of an evacuated Multi-Beam TL (MBTL) concept to deliver the large number of beam lines from the gyrotron hall to the torus hall buildings. The system is organized in 4 clusters, each of them including 8 beamlines. The routing consists of single-beam TL section used to connect the gyrotron output to a beam-combiner mirror unit for each cluster, a common MBTL running in a suspended corridor reaching the Tokamak building and a beam-splitter mirror unit to connect to the ex-vessel optics and launchers sections located in the equatorial and upper ports of one sector, for a total of 4 DTT sectors. The TL mirrors will be actively water cooled to cope with the heat load in long pulses due to the high power incident radiation, with the possibility to include advanced concepts for the cooling design compatible with additive manufacturing technology. The characteristics of the system and its components are presented, showing both the progress of the adopted solutions and the current design. Since the main challenge of this TL is to maintain the overall losses below 15%, in this paper we present the expected ohmic and spillover losses, including beam coupling simulations evaluating losses given by high order Transverse Electro-Magnetic modes (i.e. aberrations). We describe how the effects have been estimated with electromagnetic simulations and how losses could be mitigated, since TL efficiency could significantly drop due to the presence of non-idealities, like the deformations of mirrors surface ascribed to the microwaves heat loads and possible misalignments and aberrations effects occurring along the line.