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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. [ABSTRACT FROM AUTHOR]

The future EC systems will consist of several gyrotron sources providing MW-level millimeter wave power at a frequency around or above 170 GHz. The development of matched loads is necessary to test the new sources, the components for the transmission lines and the launchers, and must ensure high qualification for compatibility with the nuclear environment. The development at IFP-CNR and LTC of several compact high-power prototypes during the last decade led to a refinement of the overall design, resulting in the present low-reflectivity vacuum-compatible loads. The activity on loads is supported by F4E in view of the development of the EU gyrotron for ITER and required high-power tests at QST (Naka, Japan). Qualification is now supported by new tests at SPC (Lausanne, Switzerland) using the FALCON test-bed designed to test components and the EU gyrotron prototype for the EC system of ITER. These high-power tests, performed on the first CW prototype load (provided with 16 + 16 cooling channels in parallel) and shown in this paper, highlighted the need to improve the mechanical, vacuum and hydraulic design to reach the final goal of 1000s at ∼1 MW. [ABSTRACT FROM AUTHOR]

• Description of the conceptual design of the quasi-optical ECRH transmission line for DTT. • New solution for joining Multi-Beam quasi-optical lines with different lengths. • Compact and safe way to transmit high-power beams under vacuum. • Preliminary cooling design of Multi-Beam mirrors. • Electromagnetic modeling and verification of coupling losses in the ideal mirror case and in presence of mirror thermal deformations. The Divertor Tokamak Test (DTT) facility is being realised in Frascati, Italy for the study of the power exhaust issues in view of DEMO tokamak. A multi-MW Electron Cyclotron Resonance Heating (ECRH) system is foreseen with a first set of 16 gyrotrons at 1 MW and 170 GHz available for the first stage of operation; additional 16 gyrotrons with power from 1 MW to 1.2 MW are foreseen in a later stage for a total of up to 33.6 MW of ECRH at plasma. ECRH system itself is part of the 45 MW of external additional heating coupled to the plasma, provided also by Ion Cyclotron Resonance Heating and Neutral Beam Injection. The Transmission Line (TL) is fully quasi-optical, between 84 m and 138 m long and will transfer power up to 1.5 MW per beam from the building hosting the ECRH sources to the Torus Hall. The main section runs in a straight elevated corridor at a few meters above the ground level. In order to have low power losses and a simpler and manageable system, the long TL run is made by four quasi-optical Multi-Beam (MB) lines each transmitting 8 beams via shared mirrors, similar to the W7-X Stellarator TL. The system is thus organised in "clusters" each one made of the 8 gyrotron sources and the respective transmission line and launchers. Avoidance of losses in air and microwave leaks is assured by a vacuum enclosure of the whole line with the use of metallic gaskets. Single-Beam (SB) transmission lines are foreseen at the beginning and end of the MB line, to cover the distance from the gyrotron to a beam-combining mirror and from a splitting mirror to the launcher. The optical design has to cope with space constraints in the building and inherent conversion losses at the reflection on metallic mirrors, whose number was minimised. First evaluations with the electromagnetic modeling tool GRASP and first concepts for the MB mirror cooling are reported. [ABSTRACT FROM AUTHOR]

To achieve efficient power transfer, the Divertor Tokamak Test (DTT) facility relies on supplementary heating systems. One of them is the Electron Cyclotron Heating (ECH). ECH employs 4 clusters, each comprising 8 gyrotrons as Radio Frequency (RF) sources, 1 quasi-optical Multi-Beam Transmission Line (MBTL), and 8 independent launchers. The generated RF power (170 GHz, 1 MW, 100 s) is delivered to the plasma through oversized flat and focusing mirrors. However, the MBTL faces challenges during operation as the microwave beams reflected from the mirror contribute to its heating, absorbing a fraction of the beam power (0.06–0.2 MW/m2). Consequently, mirrors temperature rises, causing thermal expansion-induced deformations that reduce the transmission line efficiency. To address this issue, the mirrors must be designed with careful consideration of their thermal and structural properties to maintain optimal optical performance. This work presents the conceptual design of various actively cooled flat and parabolic MBTL mirrors, using thermo-structural finite element simulations. The performed analyses aim to examine the distinct thermo-structural characteristics of different mirrors and reconstruct the deformed reflective surfaces. The studied mirrors are divided into two types, based on the position in the TL. The mirror temperatures and total deformations resulting from the simulations do not exceed 41 °C and 0.13 mm, respectively. The deformed reflective profiles can then be used to assess the TL losses, exporting the surfaces, and implementing them in the overall electromagnetic model of the line. [ABSTRACT FROM AUTHOR]

• Proposals of different CAD designs of a parabolic mirror (and its cooling channel) of the Multi-Beam Transmission Line of DTT ECH system. • Thermal and structural analyses to choose the best design configuration. • Optimisation of mirror performance on the best configuration found, in terms of thickness and hydraulic parameters. • Complete transient thermo-structural analysis on the optimized configuration. 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. [ABSTRACT FROM AUTHOR]

The pre-conceptual layout for an electron cyclotron system (ECS) in DEMO is described. The present DEMO ECS considers only equatorial ports for both plasma heating and neoclassical tearing mode (NTM) control. This differs from ITER, where four launchers in upper oblique ports are dedicated to NTM control and one equatorial EC port for heating and current drive (H&CD) purposes as basic configuration. Rather than upper oblique ports, DEMO has upper vertical ports to allow the vertical removal of the large breeding blanket segments. While ITER is using front steering antennas for NTM control, in DEMO the antennas are recessed behind the breeding blanket and called mid-steering antennas, referred to the radially recessed position to the breeding blanket. In the DEMO pre-conceptual design phase two variants are studied to integrate the ECS in equatorial ports. The first option integrates waveguide bundles at four vertical levels inside EC port plugs with antennas with fixed and movable mid-steering mirrors that are powered by gyrotrons, operating at minimum two different multiples of the fundamental resonance frequency of the microwave output window. Alternatively, the second option integrates fixed antenna launchers connected to frequency step-tunable gyrotrons. The first variant is described in this paper, introducing the design and functional requirements, presenting the equatorial port allocation, the port plug design including its maintenance concept, the basic port cell layout, the transmission line system with diamond windows from the tokamak up to the RF building and the gyrotron sources. The ECS design studies are supported by neutronic and tokamak integration studies, quasi-optical and plasma physics studies, which will be summarized. Physics and technological gaps will be discussed and an outlook to future work will be given. [ABSTRACT FROM AUTHOR]