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• Description of the conceptual design of the ECRH launching system for DTT. • Design changes to comply with the updated reference plasma scenario. • New steering angle to limit the interaction between first wall and launched beams. • Introduction and preliminary cooling design of waveguide sections to reduce vacuum conductance between the DTT and transmission line environments. • Design of a quasi-optical ex-vessel optics unit for the matching between the transmission line and the waveguide entrance. One of the main goals of the Divertor Tokamak Test (DTT) facility is to reach a ratio of power crossing the separatrix over the major radius of about 15 MW m −1, as the one expected in DEMO. For this purpose, up to 45 MW of external additional heating power shall be coupled to the plasma, provided by Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating and Neutral Beam Injection. The foreseen total ECRH power installed at full power shall be 32 MW, generated using 1 MW/170 GHz gyrotrons, for 100 s. The present ECRH system foresees two launching antennas per DTT sector, based on the front-steering concept. The equatorial antenna is dedicated to plasma heating and current drive and the upper antenna to the stabilization of MHD instabilities. This paper focuses on the latest design concept for these two antennas and on the definition of the ex-vessel matching optics unit of the last section of the evacuated Transmission Line (TL). The design has been updated to be compatible with the insertion of CVD diamond windows, to separate the vacuum environment of DTT from the one of the TL. This choice requires adding corrugated waveguide sections between the last mirror of the TL and the first mirror inside the port, requiring some adaptation of the in-vessel optics and of the supporting structure. The possibility to modify the steering range for the launching mirror has been also investigated to be compatible with the new design of the first wall and for the upper antenna, to reach the q = 2 surface in the new plasma scenario. [ABSTRACT FROM AUTHOR]

• Design validation of components for EC-assisted breakdown of ITER first plasma. • Updated design of the mirrors to reduce the beams envelope at the dump entrance. • Multi-bounces ray-tracing model developed for the design and assessment of the dump. • Modeling of the thermal load on the plates and the fraction of escaping power. • Study of effects of misalignment of beams and variations in the absorber thickness. First plasma operation in ITER will start after completing the assembly of the tokamak vessel and the installation of the main sub-systems, but prior to the installation of the blanket modules and the divertor cassettes. Utilization of temporary limiters and divertor replacement structures will provide a poloidal and toroidal reference side position to the plasma edge to protect the vacuum vessel and other components already installed during operations. An additional set of mirrors is required to reflect the power injected from the upper launcher towards the plasma resonance for EC-assisted breakdown of the plasma up to a beam dump needed to trap and absorb the power of the beams after crossing the plasma in order to reduce the stray radiation escaping back into the vacuum chamber down to less than 10% of the total power. The quasi-optical system has been designed, with shape and size of the mirrors compliant with the requirements provided by ITER for their installation, realization and plasma performances, resulting in two standard focussing mirrors and one grating mirror. The beam dump consists of a box with five metal plates, the first providing a spreading of the high incident power and the others coated with absorbing material with thickness distribution studied to gradually reduce the power during the multiple reflections inside the box, avoiding damages to the coating itself. This work focuses on the validation of the quasi-optical design of the mirrors and the assessment of the dump performances, based on a multi-bounces model developed ad-hoc for this purpose. The study includes a tolerance analysis for the beam dump to include the effect of uncertainties in the thickness of the absorbing coating and misalignments of the mirrors, to verify the performances of the dump also when operating in different conditions with respect to the nominal ones. [ABSTRACT FROM AUTHOR]

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]

• Status of the experiments with the EU 170 GHz, 1 MW CW industrial prototype gyrotron for ITER. • Possible upgrades of the EU 170 GHz, 1 MW SP prototype gyrotron for the enhancement of the tube's efficiency. • First experimental results with a fully metallic beam tunnel. The European Gyrotron Consortium (EGYC) is developing the European 1 MW, 170 GHz Continuous Wave (CW) industrial prototype gyrotron for ITER in cooperation with Thales Electron Devices (TED) and Fusion for Energy (F4E). This conventional, hollow-cavity gyrotron, is based on the 1 MW, 170 GHz Short-Pulse (SP) modular gyrotron that has been designed and manufactured by the Karlsruhe Institute of Technology (KIT) in collaboration with TED. Both gyrotrons have been tested successfully in multiple experiments. In this work we briefly report on the results with the CW gyrotron at KIT and we focus at the experiments at the Swiss Plasma Center (SPC). In addition, we present preliminary results from various upgrades of the SP tube that are currently tested at KIT. [ABSTRACT FROM AUTHOR]

In this paper, semi-analytical and numerical models developed in our previous works to study the dynamic behaviour of natural convection are assessed against the experimental data obtained by means of the L2 Natural Circulation Loop (NCL) of DIME-Tec Labs (University of Genoa). As for the experimental campaign, reference is made to a set of nine experiments performed using water as working fluid and providing a thermal power of 2 kW. This set of data is firstly adopted for the validation of a semi-analytical linear analysis tool aimed at studying the asymptotic behaviour of NCLs through the definition of dimensionless stability maps. Then, two different numerical models (adopted in our previous work to confirm the linear analysis) are assessed, namely an Object-Oriented (O-O) one-dimensional model and a three-dimensional Computational Fluid Dynamics (CFD) model. In this regard, the O-O model represents a fast tool for the evaluation of the most important quantities, such as the velocity and the temperature fields in the loop along the axial coordinate. On the other hand, the CFD tool, which is intended as a support to the 1D analysis, is characterised by a high computational burden, but allows highlighting interesting 3D spatial effects. The validation of these tools is not secondary with respect to that of the stability maps. Actually, the numerical approach is fundamental to study the time-dependent behaviour of both stable and unstable natural circulation regimes, for which the stability maps do not provide information. As for the achieved results, the developed models are able to catch the behaviour of the experimental data. In particular, this outcome is possible if an accurate modelling of both the heat-exchanger section and the piping thermal inertia is considered. [ABSTRACT FROM AUTHOR]

In the final configuration of the Divertor Tokamak Test (DTT) facility, 32 front-steering launchers of the Electron Cyclotron Resonance Heating (ECRH) system will be distributed in 4 sectors of the machine. Two different antennas are hosted in the equatorial and in the upper port of each ECRH sector, with 6 and 2 single launchers respectively. This setup will deliver up to 35.2 MW installed power to the plasma, making such ECRH system the most powerful ever implemented at the time of its completion. To comply with compactness and performance requirements, a fully in-vessel driving system has been proposed for the steering mirrors. The system relies on UHV-compliant piezoelectric walking drives to provide biaxial steering capability. The main drawback of piezoelectric drives is their low driving force, in an environment where magnetic torques can be very high because of variable magnetic fields, as those induced by the non-axisymmetric in-vessel coils for plasma control, and especially in case of disruption events. Therefore, the materials of the water-cooled steering mirrors must be chosen in such a way as to minimize magnetic torques while guaranteeing adequate heat conduction. Numerical and analytical models of the magnetic torques acting on the steering mirror have been developed. The models have been applied to the steering mirror to quantify magnetic torques due to various sources and dynamic regimes. The currently proposed Copper–Chromium–Zirconium mirror was proven to be critical. Therefore, compliance requirements for the mirror have been computed, and different solutions have been numerically evaluated for magnetic torque mitigation and heat conduction capability, including combination of different materials and implementation of a tungsten-coated dielectric mirror. [ABSTRACT FROM AUTHOR]

• Work completed by the Heating and Current Drive Workpackage for DEMO during the pre-conceptual design phase (2014-2020) • The Electron Cyclotron System is the baseline for DEMO • Description of the Electron Cyclotron System • Description of the Ion Cyclotron System • Description of the Neutral Beam Heating System The European DEMO is a pulsed device with pulse length of 2 hours. The functions devoted to the heating and current drive system are: plasma breakdown, plasma ramp-up to the flat-top where fusion reactions occur, the control of the plasma during the flat-top phase, and finally the plasma ramp-down. The EU-DEMO project was in a Pre-Concept Design Phase during 2014-2020, meaning that in some cases, the design values of the device and the precise requirements from the physics point of view were not yet frozen. A total of 130 MW was considered for the all phases of the plasma: in the flat top, 30 MW is required for neoclassical tearing modes (NTM) control, 30 MW for burn control, and 70 MW for the control of thermal instability (TI), without any specific functions requested from each system, Electron Cyclotron (EC), Ion Cyclotron (IC), or Neutral Beam (NB) Injection. At the beginning of 2020, a strategic decision was taken, to consider EC as the baseline for the next phase (in 2021 and beyond). R&D on IC and NB will be risk mitigation measures. In parallel with progresses in Physics modelling, a decision point on the heating strategy will be taken by 2024. This paper describes the status of the R&D development during the period 2014-2020. It assumes that the 3 systems EC, IC and NB will be needed. For integration studies, they are assumed to be implemented at a power level of at least 50 MW. This paper describes in detail the status reached by the EC, IC and NB at the end of 2020. It will be used in the future for further development of the baseline heating method EC, and serves as starting point to further develop IC and NB in areas needed for these systems to be considered for DEMO. [ABSTRACT FROM AUTHOR]

The Divertor Tokamak Test (DTT) facility [1], whose construction is starting, will study a suitable solution for the power exhaust in conditions relevant for the future fusion device DEMO. DTT can achieve the value of 15 MW/m for the divertor figure of merit P SEP /R by employing 45 MW of auxiliary heating power to the plasma. To achieve this goal, the selected heating systems are Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating (ICRH) and Negative (ion based) Neutral Beam Injector (NNBI). The ECRH system relies on up to 32 gyrotrons (operating each at 170 GHz to supply from a minimum of 1MW to a maximum of 1.2 MW for 100 s), a Quasi Optical (QO) transmission line (TL), consisting of multi-beam mirrors installed under vacuum to reduce the overall transmission losses below the target of 10% and independent (single-beam) front-steering mirrors capable to direct the beams individually in real-time for assisted plasma breakdown, control of neoclassical tearing modes and sawtooth, ECCD and main electron heating. Although the ECRH system design presented here will be based mainly on existing and assessed technologies, like the 170 GHz gyrotron type developed for ITER and the QO TL installed at W7-X, challenging adaptations to the DTT case have to be made. In particular, the design of a QO TL under vacuum is novel and needs detailed analysis of the stray radiation along the line in order to set the requirements for the mirror dimensions and/or the cooling of the vacuum chamber that encloses the mirrors. A further relevant question is the reliability of the ECRH system: the development of automatic algorithms to control such a large number of gyrotrons is foreseen to provide the required amount and distribution of power into the plasma. [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]

• Design of different CAD models of the cooling channel of the M2 mirror of the DTT ECRH launcher. • Thermal analysis to choose the best geometric configuration with constant input parameters of the water. • Parametric thermal analysis as function of the water inlet temperature and mass flow rate. • Iterative calculation of the convection coefficient at the average temperature of the water. Divertor Tokamak Test (DTT) facility, the Italian fusion machine, foresees the Electron Cyclotron Resonance Heating (ECRH) system to provide additional heating power to plasma ohmic-heating. Such system will be composed of an initial set of 8 mm-wave 1-MW sources at 170 GHz, which will be raised to 16 in a second stage of installation. The power of each source will be launched into the plasma by a pair of mirrors in the port plug, while a series of mirrors is used for the transmission of microwave beams. They are subjected to microwave heating load and the mirrors closest to the plasma also to radiative and neutronic loads. In order to avoid a performance loss of the beam transmission, due to the deformations, caused by the increment of temperature on the mirrors, it is necessary to design a cooling circuit inside them. This paper aims to make a preliminary design, using FEA tools, of the cooling circuit in the steerable mirror (M2) of the ECRH Launcher. A thermal analysis was performed to study how geometrical and thermal choices can influence the dissipation of the heat and the maximum temperatures of the structure. First of all the best geometric configuration was individuated: a cooling circuit dug in the mirror with a shape of an elongated spiral with constant pitch on y -axis and the inlet not centred in the origin. Starting from this geometry, parametric analyses were carried out to establish the correct ranges of applicability and study the influence of the inlet temperature and the volume flow rate of the refrigerant fluid on the temperature of the mirror. The simulations confirmed the theoretical expected thermal trends, but also that the preliminary tested values of the input parameters could be used to realize an efficient cooling for the structure. The analyses are set up within ANSYS environment, on the basis of the provided data and constraints. [ABSTRACT FROM AUTHOR]