In an Electron Cyclotron Resonance Heating (ECRH) system, to efficiently couple the signal power to the plasma, the signal wave polarization must be accurately matched to the plasma conditions at the plasma boundary. However, the millimeter-wave radiation from the power source (gyrotron) is normally linearly polarized: consequently, some kind of polarization matching is required. This study focuses on the design of a grating polarizer with sinusoidal grooves for the 170 GHz ECRH system, with an application specifically intended for the Divertor Tokamak Test (DTT), currently under construction in Frascati, Italy. To enable the generation of all possible output polarization states, a pair of polarizer mirrors will be employed and integrated into the Quasi-Optical (QO) transmission line connecting the gyrotrons to the Electron Cyclotron (EC) waves launchers. The primary objective of this study is to describe an analytical tool capable of providing detailed insights into the polarization characteristics of the reflected electric field resulting from the interaction between the incident wave and the polarizer. Additionally, the proposed program tool calculates the precise combinations of rotation angles required for the polarizers to achieve the desired output polarization states. The accuracy and reliability of the model’s prediction have been validated by comparing them with simulations conducted using commercial electromagnetic software.

The full power configuration of the Divertor Tokamak Test (DTT) facility will include 32 independent electron cyclotron resonance heating (ECRH) front-steering launching mirrors. A highly compact, 2-degree-of-freedom steering mechanism based on in-vessel piezoelectric walking drives is currently under design. This solution is intended to minimize space occupation within the ports—allowing for the launchers to fit within the limited DTT duct space—while optimizing dynamic performance and control bandwidth. Wherever feasible, flexures replace traditional hinges, with the combined advantages of eliminating wear and backlash, thus extending component lifespan and enhancing steering accuracy. At the same time, flexible joints introduce elastic resistance to the steering motion, which must be counteracted by the actuators. This reduces the force available for resisting other external disturbances, like electromagnetic (EM) loads. In order to mitigate elastic resistance, a negative-stiffness element called oblique-spring stiffness compensator (OSSC) is proposed. The static analysis of the device is presented, optimal design rules are identified, and the conceptual design of a prototype integrated in the launcher assembly is shown. The target of the study is the realization of a statically balanced steering mechanism that retains the main benefits of compliant joints, namely the absence of wear, backlash, and joint friction, without incurring the usual penalty of elastic resistance and loss of available driving force.

We explore the theoretical capabilities of Electron Cyclotron Current Drive (ECCD) in a volumetric neutron source (VNS), a high-neutron-fluence tokamak for component testing and qualification that is being considered in the frame of the EUROfusion Consortium. Two selected applications are addressed, namely bulk current drive in the plasma centre and stabilization of Neoclassical Tearing Modes (NTMs). It is shown that a current drive efficiency above 50 kA/MW can be reached, close to typical values reported for DEMO central ECCD. Suppression of NTMs should require less than 10 MW, but this power might become marginal in case of significant beam broadening caused by density fluctuations. The optimum launcher parameters for the envisaged applications will be further iterated to find the sweet spot between physics and engineering constraints.

The electron cyclotron heating (ECH) launchers for the Divertor Tokamak Test (DTT) facility are currently being designed. The aim is to inject power in different plasma locations with high flexibility to allow plasma current ramp-up and ramp-down, central heating with tailored deposition profiles, and neoclassical tearing modes (NTMs) stabilization. Two antennas are developed, one for the equatorial port with six beam lines and one for the upper port with two beam lines, with similar front-steerable mirror modules. Each beam-line module is composed of a cooled corrugated waveguide launching a beam toward a fixed shaping mirror (M1) and then to the plasma-facing plane mirror (M2), movable around two axes, both cooled to sustain the high heat load due to the ohmic losses of the incident 1 MW beam and the plasma radiation. Thermal loads also include plasma and microwave stray radiation. Main structural loads on the mirrors and their supports, including the launcher supporting baseplate, are forces and torques due to the induced electro-magnetic (EM) currents, during normal operation and disruptions, interacting with the high magnetic field in which the mirrors and supports are immersed. Thermal and structural challenges, in particular on the M1 and M2 mirrors, have opposite solutions that require a delicate trade-off: optimal cooling requires high-thermal-conductivity materials such as copper alloys, while low-electrically-conductive materials are needed to have lower induced currents during disruptions. Solutions under study for the mirror cooling for minimal deformations and stresses, and for drive mechanisms, are presented. The possible reduction of induced currents during disruptions using different materials, supporting structures, and layouts is discussed.

The steerable launcher mirrors accurately guide microwave beams into the plasma, which is a fundamental function within the Electron Cyclotron Resonance Heating (ECRH) system of the Divertor Tokamak Test (DTT) facility, currently being built in Frascati, Italy. Since the mirror is subjected to intense electromagnetic field variations, which could induce high loads leading to structural failure, it is essential to identify a trade-off between thermomechanical performance and an adequate material selection with lower electrical conductivity which limits the electromagnetic induced loads. This paper investigates the two main configurations of metallic mirrors: monolithic Inconel 718 and a bimetallic design combining Stainless Steel 316 L and CuCrZr. The study covers thermo-fluid dynamic and thermomechanical analyses to assess the mirror’s resistance under the imposed heat flux conditions. Furthermore, a fatigue analysis is conducted to evaluate the component’s life cycle. Finally, different configurations are compared, highlighting how the two main proposed solutions offer the best compromise between electromagnetic forces and thermomechanical stresses.

European DEMOnstration (EU-DEMO) fusion reactor is currently progressing through its conceptual design phase. Reliability assessments play a key role in demonstrating that the system architecture can fulfill its intended functions over a defined lifespan. In the current approach, availability assessments begin with a failure mode and effects analysis (FMEA) to systematically identify potential failure events affecting system performance. Fault tree (FT) models are developed for subsystems exhibiting multiple failure modes, while the overall system configuration is modeled using a reliability block diagram (RBD). This study focuses on the electron cyclotron (EC) heating system, which supports plasma heating during ramp-up and ramp-down phases, stabilizes neoclassical tearing modes, and mitigates radiative instabilities. The analysis targets the current design of the transmission line (TL) and launcher subsystems, excluding gyrotrons from the current scope. Availability simulations have been performed over a 20-year operational period. Parametric analyses are also conducted to quantify the sensitivity of availability to maintenance duration and restoration parameters, providing key insights into design robustness and operational planning.

Abstract This paper reports the main design keys and the challenging issues of the Heating and Current Drive System (HCD) of the Divert Tokamak Test (DTT) facility that is under construction at ENEA site in Frascati with the aim to perform studies on the power exhaust in a flexible and easily modifiable environment. The selected HCD systems for DTT are those with the most consolidated technology and expected to be relevant for the future reactor. The status of each system is reported, both in terms of design and procurement, which are well advanced for the system required in the first phase of the DTT exploitation: electron and ion cyclotron resonant heating. The third system is neutral beam injector, based on negative ion acceleration, which will be installed in DTT in a second phase, after the first five years of operation. The full heating power will be reached with the doubling of the radiofrequency power to reach the 45 MW at plasma needed to test the divertor with a power density at reactor level.

The design, manufacturing, and experimental characterization of the prototype of an electron cyclotron heating (ECH) mirror equipped with an innovative cooling system based on triply periodic minimal surfaces (TPMS) is presented here. The prototype, manufactured in AISI 316L via selective laser melting (SLM), is engineered to withstand high heat fluxes and to mitigate eddy currents induced during tokamak operation. The TPMS cooling structure features a Gyroid topology with variable cell size. Hydraulic and thermal tests were conducted in a dedicated water loop, and experimental results were compared to numerical predictions. The measured pressure drops were worse than those calculated, probably due to surface roughness and geometrical deviations, and can be reproduced numerically with a geometry with an effective porosity lower than the nominal one. The thermal measurements, on the other hand, appear to be quite far from the simulations, probably due to the influence of the experimental setup on the thermocouple readings. These findings confirm the potential of TPMS-based cooling for ECH mirrors and highlight areas for further improvement in manufacturing, modeling, and testing.

Many current and next generation fusion experiments use Electron Cyclotron Resonance Heating for current profile tailoring and mode stabilization. Millimetre-wave power is delivered to the plasma in the form of Gaussian beams, either radiated by corrugated waveguides or via quasi-optical transmission lines. The size of the beams dictates that of the ducts in the neutron shielding, with the conflicting requirements of minimizing volume -to prevent loss of shielding- and reducing power loss in the wall -to lower the cooling requirements.This paper is stating scalable criteria for estimating millimetre-wave loss, to aid in assessing a minimum duct size for given power loss requirements.

The steerable launching mirrors, essential for directing microwave beams into the plasma, play a pivotal role in the Electron Cyclotron Resonance Heating (ECRH) system for the Divertor Tokamak Test (DTT) facility, currently under construction in Frascati, Italy. Due to the substantial heat and electromagnetic induced loads acting on the mirrors, implementing internal channels for active water cooling, together with a proper choice of the materials, is necessary to keep temperature and deformation under control. Three different channel configurations are studied. First, the single-channel spiral cooling path with a constant cross-section, defined in a previous design stage, has been examined. Then, a constant-depth complementary spiral geometry that increases heat exchange area has been defined and analyzed. Finally, a variable-depth complementary spiral channel is proposed and optimized to increase heat exchange efficiency. In all cases, single-channel geometries are considered to enhance safety and malfunctioning detectability. The study is based on Computational Fluid Dynamics simulations. In order to reduce electromagnetic loads on the mirrors in case of plasma disruption to a tolerable extent, a reduced electrical conductivity of the mirror bulk material with respect to pure copper is necessary: this requires the use of material different than copper alloys, which have in turn a lower thermal conductivity. In this case, high cooling efficiency is mandatory. With this goal in mind, first, the performances of the different configurations in terms of mirror temperature and pressure drop are compared considering a reference material with 100 W/(m & sdot;K) thermal conductivity. Then, the variable-depth configuration is tested for different and more realistic mirror materials. Finally, a comparison between the developed geometry and previous solutions is provided.

The steerable launcher mirrors, essential for directing microwave beams into the plasma, play a pivotal role in the Electron Cyclotron Resonance Heating (ECRH) system of the Divertor Tokamak Test (DTT) facility, currently under construction in Frascati, Italy. Due to the substantial heat loads acting on the mirrors, internal water-cooling channels are necessary to control temperature and deformation. A variable-depth complementary spiral cooling channel was considered in this study. A dielectric material with high thermal conductivity was selected as a potential candidate to reduce eddy currents, this mitigating magnetic torques and mechanical stress, while guaranteeing adequate cooling. Thermo-structural simulations (FSI) were conducted to assess the mirror's resistance to induced stresses, its deformations, and cooling performance. A transient analysis showed that thermal steady-state is the worst-case thermal loading condition during the entire experiment. Additionally, the thermo-structural behavior of various materials was analyzed to demonstrate the superior performance of the selected dielectric material. The cooling channel was subsequently adapted to a prototype mirror, on which CFD and FSI simulations were performed to validate the numerical model against future real-world experiments. Finally, crack propagation analysis confirmed the feasibility of using technical ceramics for the launching mirror, paving the way for dielectric materials in the ECRH system of DTT.

The EUropean DEMOnstration power plant (EU-DEMO) project, a EUROfusion initiative, seeks to advance fusion power technology by developing a reliable Electron Cyclotron (EC) heating and current drive system capable of delivering up to 130 MW of auxiliary heating power presently through six equatorial ports. This paper introduces an innovative alternative to the baseline ex-vessel waveguide (EW) system design, which could be major for effective EC operations. The proposed design reduces vacuum-sealed interfaces significantly - from 173 to seven - minimizing failure points and reducing the need for extensive monitoring in confined port spaces. Enhanced for remote maintainability, the design simplifies maintenance tasks and mitigates vacuum breach risks. The vacuum dimple-plated casing, acting as the primary confinement boundary, ensures nuclear safety and enables integration with current infrastructure, while addressing challenges like dynamic loads and vessel movements to maintain structural integrity. These advancements indicate that the modular and sealless waveguide approach offers a more reliable, maintainable solution.

An overview is presented of the progress since 2021 in the construction and scientific programme preparation of the Divertor Tokamak Test (DTT) facility. Licensing for building construction has been granted at the end of 2021. Licensing for Cat. A radiologic source has been also granted in 2022. The construction of the toroidal field magnet system is progressing. The prototype of the 170 GHz gyrotron has been produced and it is now under test on the FALCON facility. The design of the vacuum vessel, the poloidal field coils and the civil infrastructures has been completed. The shape of the first DTT divertor has been agreed with EUROfusion to test different plasma and exhaust scenarios: single null, double null, X-divertor and negative triangularity plasmas. A detailed research plan is being elaborated with the involvement of the EUROfusion laboratories.

In this contribution, the tests of the pre-series gyrotron TH1509UA for the Divertor Tokamak Test facility (DTT) at the FALCON test facility are presented. This versatile test bed proves useful for testing continuous wave (CW) high-power gyrotrons, but also serves as a platform for testing components for the transmission line or the Upper Launcher of ITER and DTT. The gyrotron has demonstrated a power level of 1.02 MW at the gyrotron output window, corresponding to 980 kW at the output of the Matching Optics Unit (MOU) with a power variation during the pulse of < 2% after a stabilisation period. Additionally, an efficiency of 40% has been demonstrated during five consecutive 100 s pulses. Compared to the previous version, TH1509U, this gyrotron demonstrates the successful prevention of parasitic mode excitation over a wide range of parameters around the design operating point. The potential for even higher power performance has been shown in short pulses but not explored in long pulses yet, which instead focused on demonstrating compliance with the required specifications for the DTT project.

The European DEMO (EU-DEMO) reactor studies within EUROfusion aim to develop a fusion power plant concept. The large tokamak device needs an auxiliary heating power which, at the present stage, is provided by the Electron Cyclotron (EC) heating system with up to 130 MW foreseen to reach different regions of plasma for heating, suppression of instabilities and the possibility to support ramp-up and ramp-down phases. The present conceptual design of the system is based on 2 MW coaxial-cavity gyrotron sources, a transmission line (TL) using both circular corrugated waveguides and quasi-optical evacuated multi-beam TLs, and mirror antennas located in the Equatorial Port. In order to create a modular system, the sources are grouped in ‘clusters’, whose powers are combined in the quasi-optical TL, up to the tokamak building, where they are split and routed as single waveguides. In the launcher, they are combined together again on the launching mirrors, to save space for the apertures in the Breeding Blanket. The present EC heating system has a certain flexibility to adapt to changing design guidelines. The development status of the system is presented.

The EU DEMO Tokamak is foreseen to be equipped with an electron cyclotron (EC) system for plasma heating, magnetohydrodynamic (MHD) control, and thermal instability suppression. Up to six launchers will be installed into equatorial ports with the aim to inject a maximum of 130 MW millimeter wave power at frequencies of 136/170/204 GHz toward dedicated positions into the plasma. This article presents the current layout of the typical EC system launcher components within the available space reservation areas in the equatorial level of the DEMO baseline model. Beside the general arrangement of the EC launcher components, preliminary design features of the launcher port plug modules, an active cooling system approach, integration concepts for mirrors and waveguides (WGs) of the optical system, the shielding component formation, and safety important elements of the first containment barrier of the launcher are presented. Furthermore, relevant aspects of assembly and potential remote maintenance (RM) procedures are discussed.

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.

A dual-frequency gyrotron has been developed within the context of the recent Tokamak à Configuration Variable (TCV) upgrade. The gyrotron is designed to generate a 1 MW, 2 seconds RF wave at 84 or 126 GHz. Before integrating the gyrotrons in the TCV tokamak ECRH system, an extensive characterization of their behaviour has been performed. This paper focuses on presenting the results of these experiments at the two operating frequencies. The power measurements are systematically compared with numerical simulations. This comparison highlights the validation of numerical codes and the effect of After Cavity Interaction (ACI), a crucial factor that must be considered for achieving a good agreement between theoretical predictions and experimental results.

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.

The Divertor Tokamak Test (DTT) facility is equipped with auxiliary heating systems in order to be able to load the divertor with a power flux relevant to study the power exhaust issue in a reactor relevant range of parameter. The powerful system is the Electron Cyclotron Heating (ECH) with an installed power of 32 MW in its largest extension. Together with the bulk heating of the DTT plasma, the ECH system will cover several tasks for the plasma operation. This paper summarizes the main characteristics and design choices of the DTT ECH system and the related physics studies, based on the reference DTT plasma, to develop and control the plasma, fulfilling the functional tasks, with the support of simulation activities. Dedicated studies have been carried out to investigate the capability of EC power to assist plasma start-up, stabilize MHD activity and support current ramp up/down. In addition, it has been studied how changes of the ECH power distribution can have an impact on the plasma profiles, affecting the fueling pellet effectiveness and MHD modes.