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The launcher mirrors will allow the injection of microwave power from the gyrotrons in the plasma of the Divertor Tokamak Test facility, under construction in Italy. The active cooling of the elliptical M1 (curved and not-steerable) and M2 (flat and steerable) mirrors, both subject to a significant heat load, is investigated in this article. Several cooling solutions are analyzed: a spiral cooling for M1, both in a massive and in a lightened configuration, and also a radial-azimuthal cooling for M2, both in a massive and in a lightened configuration. Figures of merit are computed for the different configurations.

In this work the Electron Cyclotron (EC) physics performances of the EC system foreseen for the new Divertor Tokamak Test facility (DTT) are investigated using the beam tracing code GRAY on the flat top phase of the most recent DTT full power scenario. The whole core plasma region can be reached by EC beams with complete absorption, assuring bulk heating and core current drive (CD) for profile tailoring, and NTM mitigation in correspondence of the rational surfaces. A detailed analysis regarding modifications of the EC propagation, absorption and CD location due to density fluctuations caused by pellet injection is performed. The compatibility between the EC system and the pellet injection system is verified: the density variations due to pellet injection are foreseen to negligibly influence the EC performances, allowing the EC beams to reach the plasma central region for bulk heating and to drive current on the rational surfaces for NTM mitigation. Finally, the polarization variations originated by the angle steering foreseen for the operational and physics tasks accomplishment during the flat top phase of the discharge are assessed. Negligible power losses have been found keeping fixed polarization during the needed steering.

The DTT tokamak, whose construction is starting in Frascati (Italy), will be equipped with an ECRH system of 16 MW for the first operation phase and with a total of 32 gyrotrons (170 GHz, ≥ 1 MW, 100 s), organized in 4 clusters of 8 units each in the final design stage. To transmit this large number of power beams from the gyrotron hall to the torus hall building a Quasi-Optical (QO) approach has been chosen by a multi-beam transmission line (MBTL) similar to the one installed at W7-X Stellarator. This compact solution, mainly composed of mirrors in “square arrangement” shared by 8 different beams, minimizes the mode conversion losses. The single-beam QOTL is used to connect each gyrotron MOU output to a beam-combiner mirror unit and, after the MBTL, from a beam-splitter mirror unit to the exvessel and launchers sections located in the equatorial and upper ports of 4 DTT sectors. A novelty introduced is that the mirrors of the TLs are embodied in a vacuum enclosure, using metal gaskets, to avoid atmospheric absorption losses and microwave leaks. The TL, designed for up to 1.5 MW per single power beam, will have a total optical path length between 84 m and 138 m from the gyrotrons to the launchers. The main straight section will travel along an elevated corridor ~10 m above the ground level. The development of the optical design reflects the constraints due to existing buildings and expected neutron flux during plasma operation. In addition, the power throughput of at least 90% should be achieved.

The EC-system of the TCV tokamak is progressively being upgraded with the addition of two MW-class dual-frequency gyrotrons (84 and 126 GHz/2s/1MW). In order to connect the two gyrotrons to the existing low field side and top launchers, new waveguide routing from gyrotron hall to TCV tokamak was designed and dedicated Matching Optics Units (MOU) have been developed. The internal optics of the system have been determined aiming at optimal coupling to the HE11 waveguides. The laws of quasi-optics were used to find quadratic surfaces to shape an incoming Gaussian beam representative of the gyrotron output into a beam matching the proper field distribution at the waveguide entrance and with HE11 content compatible with the system requirements. A solution with one flat (movable) mirror and two shaping mirrors was found and characterized with the physical optics code GRASP. The resulting field distribution is then truncated and projected onto the HE11 component to evaluate the design solution (coupled power at the waveguide entrance >98.4% and HE11 content >96.8% for both frequencies). The model and the results of this analysis will be presented and compared to a model based on the Rayleigh–Sommerfeld scalar diffraction integral. GRASP was also used to evaluate preliminary misalignment effects in terms of coupled power to the waveguide and the MOU design moved to the manufacturing phase. In parallel to the gyrotron integration and to extend the level of flexibility of the TCV EC-system, a modular closed divertor chamber is developed, requiring the X3 top-launcher to be redesigned. Preliminary antenna conceptual design studies including new curvature to cope with the requirements of modularity and flexibility will be presented.

openaire: EC/H2020/633053/EU//EUROfusion An Electron Cyclotron Wave (ECW) system will be used in the European DEMO for the stabilization of Neoclassical Tearing Modes (NTMs). In order to avoid movable mirrors in the harsh environment close to the plasma and to simplify the NTM launcher integration, the tuning of the ECW deposition location can be achieved by launching frequency-tunable ECWs from fixed mirrors while the frequency is tuned in discrete steps of 2–3GHz. An overview of the frequency step-tunable ECW system for NTM stabilization is presented. The design considerations are discussed based on the current DEMO baseline parameters and the status of technologies. A simulation of NTM stabilization with an idealized frequency tunable ECW system on an analytical NTM model is shown. The simulation takes into account a realistic tuning speed based on the present technology and considers the current NTM launcher configurations in DEMO. A simple sweeping strategy is adapted for the control of frequency. Various uncertainties, which will affect the feasibility, need to be further investigated.

Heating and current drive (H&CD) systems are being investigated for a demonstration fusion power plant DEMO to deliver net electricity for the grid around 2050. Compared to ITER, which has to show the generation of 500-MW thermal power, the target of DEMO is the successful production of 300 to 500 MW electrical power to the grid and to aim for a self-sufficient tritium fuel cycle. Three H&CD systems are under development for DEMO in Europe, the electron cyclotron (EC) system, the neutral beam injection (NBI) system, and the ion cyclotron system. Based on present studies for plasma ramp-up, ramp-down, and flat top phases, to be further validated in more detailed simulations, the assumed total launched power needed from the H&CD system in DEMO is in the range of 50-100 MW, to be provided for plasma heating and control. This paper describes the design and Research and Development status of selected H&CD systems, considered for their deployment in the EU DEMO. It was always considered that different H&CD configurations and design variants will have an impact on the performances for the whole fusion plant. It shall be noted that the basis for the H&CD integrated design and system development is the actual version of the European fusion electricity roadmap. The project also elaborates on H&CD efficiency improvements which will reduce the recirculating power fraction in the future fusion power plants. Different studies under investigation will be discussed such as for NBI the photoneutralization and for EC novel concepts for gyrotron multistage-depressed collector.

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 PSEP/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.

The feasibility of getting “D” shaped plasma configurations in FTU, with a possible X point close to the first wall, has been investigated with the aim of achieving an H-mode regime in this machine. This regime could allow both evaluating the thermal effects on the liquid lithium limiter due to the possible Edge Localized Modes and studying the L-H transition properties in low recycling conditions due to the presence of lithium.. An alternative design for the magnetic system in FTU has been also proposed, to realize an X-point inside the plasma chamber, close to the Liquid Lithium Limiter.Preliminary experiments with elongated configurations and limited ECRH additional heating power did not allowed approaching the L-H transition but they were used to develop a proper elongation control. This controller allowed guaranteeing the vertical stability in elongated configurations despite the reduced power available for the horizontal field coils in FTU. The elongation was stably keep over 1.2, while the lithium limiter was very close to the last close flux surface. Neither limiter damages nor plasma pollution were observed in these configurations.A possible alternative connection of the poloidal field coils in FTU is here proposed, with the aim of achieving a true X-point configuration with a magnetic single null well inside the plasma chamber and strike points on the lithium limiter. A preliminary assessment of this design allowed estimating the required power supply upgrade and showed its compatibility with the existing mechanical structure and cooling system, at least for plasmas with current up to 300 kA and flat-top duration up to 4s. Keywords: FTU, Liquid lithium limiter, L-H transition, X-point, Plasma elongation

A new receiving antenna for collecting signals of the Collective Thomson Scattering (CTS) diagnostics in FTU Tokamak has been recently installed. The squared corrugated section and the precisely defined length make it possible to receive from different directions by remotely steering the receiving mirrors. This type of Remote-Steering (RS) antennas, being studied on FTU for the DEMO Electron Cyclotron Heating (ECH) system launch, is already installed on the W7- X stellarator and will be tested in the next campaign. The transmission of the signal from the antenna in the tokamak hall to the CTS diagnostics hall will be mainly realized by means of oversized circular corrugated waveguides carrying the hybrid HE11 (quasi-gaussian) waveguide mode, with inclusion of a special smooth-waveguide section and a short run of reduced-size square-corrugated waveguide through the tokamak bio-shield. The coupling between different waveguide types is made with ellipsoidal focusing mirrors, using quasi-optical matching formulas between the gaussian-shaped beams in input and output to the waveguides. In this work, after a complete study of feasibility of the overall line, a design for the receiving line will be proposed, in order to realize an executive layout to be used as a guideline for the commissioning phase. © 2018 ENEA. Published by IOP Publishing Ltd on behalf of Sissa Medialab.

During the last years the Collective Thomson Scattering (CTS) system installed on the Frascati Tokamak Upgrade (FTU), operating at 140 GHz, has been exploited to investigate different kinds of phenomena: high-frequency daughter waves by Parametric Decay Instabilities, which, under certain conditions, are recently presumed to be conceivable even under routine ECRH scenarios, and bulk ions thermal emissions stimulated by a 140 GHz gyrotron beam. A new receiving line, giving rise to a second further line of sight from the plasma during the scattering experiments and integrated in the CTS antenna, has been recently completed and installed. Such new line will be useful to recognise signals originating in the volume of beams cross from the ones coupled outside or also to monitor the gyrotron's spectrum in real-time during the shot. The present status of this new installation is shown in the paper.

During the ongoing pre-concept phase (2014 – 2020) for a possible future European DEMOnstration Fusion Power Plant (DEMO) the activities within EUROfusion WP HCD EC Gyrotron R&D and Advanced Developments are focusing on options for near-term solutions, and, at the same time, on long-term even more advanced options. The near-term target for DEMO is to realize pulsed operation. According to the current baseline it will probably use an EC system operating at 170 GHz and 204 GHz is being assessed, whereas the long-term target aims for steady-state operation and frequencies for current drive up to 240 GHz. Common targets for both are an RF output power per unit of significantly above 1 MW (target: 2 MW) and a total gyrotron efficiency of significantly higher than 60 %. Frequency step-tunability and multi-purpose/multi-frequency operation have to be considered. Those targets shall be achieved by considering the coaxial-cavity gyrotron technology and advanced technologies for key components (e.g. CVD diamond-disk Brewster angle window). Advanced simulation and test tools are complementing the research and developments. Gyrotron development is additionally supported by a significant investment into a new multi-megawatt long-pulse gyrotron test stand which is under final installation at KIT currently.

The upgrade of the electron cyclotron (EC) system of the TCV tokamak has entered in its realization phase. The MW-class dual-frequency gyrotron design (84 or 126GHz/2s), based on the 140GHz W7-X gyrotron, is finalized and a contract for the manufacturing of two units has been placed with Thales Electron Devices (TED). For maintaining a high level of operational flexibility of the EC-system the integration of the dual-frequency gyrotrons adds a significant complexity in the evacuated HE11 transmission line system connected to the various TCV low-field side and top launchers. Specific aspects of the gyrotrons design and their integration into the EC-system are described. The EC-system upgrade is an essential step of a larger extent upgrade of TCV.

During the last years anomalous signals have been found in the spectra analyzed with the Collective Thomson Scattering (CTS) diagnostics in tokamak devices like TEXTOR, ASDEX and FTU. The signal frequency, down-shifted with respect to the probing one, suggested the explanation of their origin in terms of Parametric Decay Instability (PDI) processes correlated with the presence of magnetic islands and taking place even for values of the pumping wave power below the threshold predicted by conventionalmodels. A threshold lower or comparable with the power levels routinely used could pose limitations to the use of the ECRH in fusion devices. An accurate characterization of the conditions for the occurrence of this phenomenon and of its consequences is thus of primary importance. The CTS diagnostics of FTU has been recently improved for this aim with a new fast data acquisition, allowing spectra reconstruction by direct FFT of the intermediate frequency signal, added in parallel to the pre-existing multichannel spectrum analyzer. A section with polarizing mirrors has been introduced in the quasi-optical transmission line to control the polarization of the received signals. Exploiting the front-steering configuration available with the real-Time launcher of FTU, the setup allows studying the anomalous emissions in conditions of density and wave injection similar to ITER. Given the possibility of the CTS in FTU to operate in both O-and X-mode, a second radiometer is being installed, to detect simultaneously signals scattered in both polarizations. The upgrades mentioned above are presented in this work as well as the first data obtained in 2014, during the experimental activity carried out under an EUROfusion Enabling Research project.

The ITER scenarios and the project of DEMO involve stable operation above the Greenwald density, which justifies efforts to understand and overcome the density limit, this last observed as a disruptive termination of tokamak discharges and a thermal crash (with no disruption) of stellarator and reversed-field pinch (RFP) ones. Both in the tokamak and the RFP, new findings show that the high density limit is not governed by a unique, theoretically well-determined physical phenomenon, but by a combination of complex mechanisms involving two-fluid effects, electrostatic plasma response to magnetic islands and plasma-wall interaction. In this paper we will show new evidence challenging the traditional picture of the 'Greenwald limit', in particular with reference to the role of thermal instabilities and the edge radial electric field Er in the development of this limit. © 2015 EURATOM.

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Since 2014 under the umbrella of EUROfusion Consortium the Work Package Heating and Current Drive (WPHCD) is performing the engineering design and R&D for the electron cyclotron (EC), ion cyclotron and neutral beam systems of the future fusion power plant DEMO. This presentation covers the activities performed in the last two years on the EC system conceptual design, as part of the WPHCD, focusing on launchers, transmission lines, system reliability and architecture.

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The oblique electron cyclotron emission (ECE) diagnostic installed at JET allows simultaneous analysis of the ECE spectra along three lines of sight (with toroidal angles of 0°, ~ 10°, and ~ 20°) and two linear polarizations for each oblique line of sight. The diagnostic is capable of measuring EC emission over the band of 75-800 GHz with 5 ms time resolution and 7.5 GHz spectral resolution, and it is designed to investigate the features of ECE spectra related to electron distribution in the thermal velocity range. Instrumental accuracy was assessed using sources at different temperatures (77-900 K) and with plasma emission. ECE from high temperature plasmas and in the presence of fast ions has been compared to simulations performed with the modeling code SPECE, setting an upper limit to possible discrepancies from thermal emission.

Deviations of the electron function velocity distribution from Maxwellian behavior in the high energy range have been extensively studied during the past two decades. A brief review of the experimental techniques and findings on the subject is given in this paper. There are indications that the electron distribution function in thermonuclear plasmas could be significantly different from the Maxwellian one even near the thermal velocity range when particular circumstances occur. These reasons motivate a renewed effort in the measurements that could detect low energies distortions, among which is the new Oblique Electron Cyclotron Emission diagnostics that has entered operations during 2006 experimental campaign at JET and aims to resolve relatively small differences in the ECE spectra taken at three toroidal angles. Such measurements allow insight into the characteristics of the electron distribution function. First results of such experimental system are discussed in this paper, along with emission simulations.

Experiments on real time control of magneto-hydrodynamic (MHD) instabilities using injection of electron cyclotron waves (ECW) are being performed with a control system based on only three real time key items: an equilibrium estimator based on a statistical regression, a MHD instability marker (SVDH) using a three-dimensional array of pick-up coils and a fast ECW launcher able to poloidally steer the EC absorption volume with dρ/dt = 0.1/30 ms maximum radial speed. The MHD instability, usually a tearing mode with poloidal mode number m and toroidal mode number n such that m/n = 2/1 or 3/2 is deliberately induced either by neon gas injection or by a density ramp hitting the density limit. No diagnostics providing the radial localization of the instabilities have been used. The sensitivity of the used MHD marker allows to close the control loop solely on the effect of the actuator's action with little elaboration. The nature of the instability triggering mechanism in these plasma prevents that the stabilization lasts longer than the ECW pulse. However when the ECW power is switched on, the instability amplitude shows a marked sensitivity to the position of the absorption volume with an increase or decrease of its growth rate. Moreover the suppression of the dominant mode by ECRH performed at high plasma density even at relatively low power level facilitates the development of a secondary mode. This minimized set of control tools aim to explore some of the difficulties which can be expected in a fusion reactor where reduced diagnostic capabilities and reduced actuator flexibility can be expected. © 2015 EURATOM.

A Martin Puplett interferometer for electron cyclotron emission (ECE) measurements from JET tokamak plasmas was extended to multichannel operation for simultaneous radial and oblique ECE measurements. This paper describes the new optics and the instrument's performance. [doi:10.1063/1.3660276]

JT-60SA envisions electron cyclotron wall conditioning (ECWC), as wall conditioning method in the presence of the toroidal field to control fuel and impurity recycling and to improve plasma performance and reproducibility. This paper reports on Helium ECWC experiments on TCV in support of JT-60SA operation. Nearly sixty Helium conditioning discharges have been successfully produced in TCV, at a toroidal field B T = 1.3 or 1.54 T, with gyrotrons at 82.7 GHz in X2 mode, mimicking ECWC operation in JT-60SA at the second harmonic of the EC wave. Discharge parameters were tuned in order to (i) minimize the time for the onset of ECWC plasmas, thus minimizing absorption of stray radiation by in-vessel components, (ii) improve discharge homogeneity by extending the discharge vertically and radially, and wall coverage, in particular of inboard surfaces where JT-60SA plasmas will be initiated, (iii) assess the efficiency of He-ECWC to deplete carbon walls from fuel. An optimized combination of vertical and radial magnetic fields, with amplitudes typically 0.1 to 0.6% of that of B T, has been determined, which resulted in lowest breakdown time, improved wall coverage and enhanced fuel removal. A standard ohmic D 2-plasma could be then sustained, whereas it would not have been possible without He-ECWC.

In the frame of the TCV Tokamak upgrade, two 84/126 GHz/2 s dual frequency gyrotrons designed by SPC and KIT and manufactured by THALES will be added to the existing EC-System. The first unit has been delivered to EPFL-SPC and tested. In the commissioning configuration, a matching optics unit (MOU) is connected to the gyrotron window. The RF is then coupled to the HE11 mode of a 63.5mm corrugated waveguide and dissipated in a load procured by CNR after 4m of waveguide and 2 miter bends. Owing to the flexible triode gun design giving the possibility to adjust the pitch angle parameter, the specifications were met at both frequencies. At 84 GHz (TE17,5 mode), a power of 0.930 MW was measured in the calorimeter, with a pulse duration of 1.1 s. At the high frequency (126 GHz, TE26,7 mode), a power of 1.04 MW was reached for a pulse length of 1.2 s. Accounting for the load reflection and the ohmic losses in the various subcomponents of the transmission line and the tube, it is estimated that the output power at the gyrotron window is in excess of 1 MW at both frequencies, with an electronic efficiency of 32% and 34% at 84 GHz and 126 GHz respectively. The gyrotron behavior is remarkably robust and reproducible, and the pulse length is limited by external systems that will be improved shortly.

In the framework of EUROfusion Consortium the Work Package Heating and Current Drive addresses the engineering design and R&D for the electron cyclotron, ion cyclotron and neutral beam systems. This paper reports the activities performed in 2014, focusing on the work done regarding the input for the conceptual design of the EC system, particularly for the gyrotron, the transmission line and the launchers.