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Modern gyrotron beam tunnels are rather complicated structures designed to enhance the suppression of the parasitic oscillations, which may be excited there. In some beam tunnel designs, azimuthal corrugations are engraved on their walls to further improve the suppression of these oscillations. In this work, we investigate the effect of the geometrical properties of the corrugations on the propagation characteristics of TE modes for the simplified model of a smooth waveguide with an azimuthally corrugated region. For this structure, the scattering parameters are calculated and the mode conversion is investigated with the in-house FDTD code COCHLEA.

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.

For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation.

Parasitic oscillations frequently appear in several modern gyrotron beam tunnels. In this paper, the CST Studio Suite is used to study the performance of dielectric-loaded beam tunnel designs in attenuating the spurious modes. Two common designs are considered and the effect of the ceramic material’s dielectric constant on the beam tunnel’s performance is studied. For this purpose, the ohmic quality factor ${Q}_{\textsf {ohm}}$ and the normalized shunt impedance ${R}/{Q}$ are calculated to measure this performance. These studies focus on the backward propagating axisymmetric parasitic modes.

A research activity in the Institute for Plasma Science and Technology of National Research Council (ISTP-CNR, Italy, former IFP-CNR) and in L.T. Calcoli (LTC, Italy) is aimed at the design and construction of calorimetric loads for absorption and measurement of high power millimeter-waves in the electron cyclotron frequency range. Recently, two CW 170 GHz loads, one for the European ITER gyrotron test facility and the other for the FALCON test- bed, have been installed at the Swiss Plasma Center (SPC, Switzerland). One short pulse (2 s) load for 1 MW, designed and optimised to operate at two different frequencies (84 GHz and 126 GHz), was provided for testing and conditioning two new dual-frequency gyrotrons for the Tokamak a Configuration Variable (TCV, Switzerland). Two additional CW loads, designed for absorbing powers higher than 1 MW, have been delivered to the National Institutes for Quantum and Radiological Science and Technology (QST, Japan) and exploited for the acceptance tests and the conditioning of the prototype of the Japanese ITER gyrotron. The present status and the most recent experimental results achieved in the framework of this development activity are reported in the paper.

To increase the total injected Electron Cyclotron Resonance Heating (ECRH) power in the plasma of the nuclear fusion experiment Wendelstein 7-X (W7-X), an upgrade of the existing gyrotron installation is in progress. The existing ECRH system, currently equipped with ten one-MW-class, 140 GHz continuous wave (CW) gyrotrons, will be augmented by enhanced 1.5 MW tubes, which are based on the successful existing 1 MW gyrotron design.

High-power gyrotrons often suffer from parasitic oscillations in the compression region (beam tunnel) prior to the interaction cavity. A possible way to suppress the parasitic oscillations is to raise their starting currents by increasing the losses of the structure. In the present contribution we focus on the diffraction losses of the stacked copper-ceramic beam-tunnel geometry. Parametric simulations show that the diffraction losses can be significantly increased by optimizing the geometric characteristics of the structure.

In this work, we have studied the effect of the dielectric material on the properties of the modes in a smooth metallic gyrotron beam tunnel partially filled with a lossy dielectric material. Two kinds of modes have been identified, one located inside the empty region and the other inside the dielectric material. It has been seen that these kinds are not equally affected by the changes in the geometry and dielectric characteristics.

Azimuthal indentations have been proposed as an additional means to suppress the parasitic oscillations in the gyrotron stacked beam-tunnel. As a first step, a smooth waveguide with azimuthal indentations has been studied using COCHLEA code to identify their influence on the propagation characteristics. The - parameters have been calculated on the input and the output of the structure by decomposing the fields into several modes. It has been found that by increasing the indentations depth the field distributions change significantly and resonant effects are observed.

The in-house FDTD code COCHLEA was upgraded to study the trajectory of a relativistic electron beam by developing a trajectory solver. To verify its results the electron beam properties were calculated for the test case of the motion of the beam in an axially varying magnetostatic field. For the case considered, the numerical results were compared to the analytical calculated ones presenting a very good agreement. This is an intermediate, albeit important step towards the upgrade of the code to study self-consistently the beam-wave interaction.

Gyrotron R&D within the EUROfusion Work Package Heating and Current Drive (WPHCD) is addressing the challenging requirements posed on gyrotrons by the European concept for a demonstration fusion power plant (EU DEMO). These requirements, as specified within WPHCD, ask for highly reliable and robust long-pulse operation of the gyrotron, delivering 2 MW of microwave power at frequencies above 200 GHz with a high overall efficiency above 60% and the option for fast frequency step-tunability. To meet these targets, which are clearly beyond today's state-of-the-art, the R&D activities within WPHCD are organized in five main branches: these are the experimental verification of the advanced coaxial gyrotron technology at long pulses, the development of a coaxial gyrotron meeting the EU DEMO requirements, the development of multi-stage depressed collectors for enhanced energy recovery, the development of large broadband diamond windows to allow fast frequency tunability of the gyrotron, and the studies on further innovations and improvement of critical gyrotron components, in view of optimization of performance, reliability, and industrialization. The paper presents the progress made on these activities, the recent results, and the near-term planning. The major recent achievements include (i) the experimental validation of the fabrication of the new, water-cooled components for the longer-pulse coaxial gyrotron at Karlsruhe Institute of Technology, by demonstrating 2.2 MW at 170 GHz with 33% efficiency without depressed collector, (ii) the design of key components for a 2 MW, 170/204 GHz dual-frequency coaxial gyrotron, (iii) the design of a two-stage depressed collector with 77% efficiency, (iv) the first-ever production, in an industrial plasma reactor, of large chemical vapor deposition diamond wafers of 180mm diameter, (v) the procurement of an advanced electron gun with coated emitter edges, and (vi) advances in the theoretical and numerical modelling for investigating improved concepts for the gyrotron beam tunnel and cavity.

In Europe, the research and development with main focus on achieving robust industrial designs of series gyrotrons for electron cyclotron heating and current drive of today's nuclear fusion experiments and towards a future DEMOnstration fusion power plant is constantly progressing. The R&D is following two different paths. Both are complementing each other: Firstly, it is the adaption of the physical design and basic mechanical construction of the reliably operating 140 GHz, 1 MW CW (spec.: 920 kW, 1800 s) gyrotron of the stellarator Wendelstein 7-X (W7-X), Greifswald, Germany. With regards to time and costs it is the target to perform reliable developments of fusion gyrotrons with advanced specifications for today's plasma fusion experiments. Main focus is on the development of the first EU 170 GHz, 1 MW CW (3600 s) gyrotron for the installation in ITER, Cadarache, France. Another adaption is the dual-frequency 126/84 GHz 1 MW (2 s) gyrotron upgrade for the medium size TCV tokamak, Lausanne, Switzerland. Finally, it is the upgrade of the W7-X gyrotron design towards an RF output power per unit of up to 1.5 MW and possible dual-frequency operation by keeping the basic mechanical construction. Additional to the proven design it allows to fit the new 1.5 MW gyrotron into the already existing infrastructure and to reuse existing W7-X gyrotron auxiliaries, e. g. the high-power voltage supply (HV PS) and the superconducting (SC) magnet. The second R&D path is defined by the complementary approach with regards to development risks towards a future gyrotron which shall fulfil the significant more advanced specifications of a future EU DEMO. The starting point is the 2 MW EU/KIT coaxial-cavity gyrotron design. Main requirements are an RF output power of 2 MW CW at above 200 GHz, multiple operating frequencies, frequency step-tunability and a total efficiency above 60 %.

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.

Lossy dielectric materials are used in gyrotron beam tunnels to suppress possible parasitic oscillations by raising their starting currents. The dependence of the overall losses of the structure on the dielectric properties of the material is theoretically and numerically investigated and it is shown that materials with high values of the loss tangent are not suitable for the suppression of the parasitics. The beam tunnel is studied parametrically with a full-wave code, which calculates the diffraction and dielectric losses of the cold-cavity modes. A simplified geometry involving analytic relations is used to verify the results and investigate the underlying physics. It is shown that the proper selection of the material properties and its radial thickness can improve the absorption of the fields significantly.

The quality of the electron beam generated by a magnetron injection gun (MIG) is crucial for the performance of high-power high-frequency gyrotrons. The measurement of the electron properties would be very important for interpreting the experimental results and for improving the design of the gyrotron. Considering the increased power of the electron beam, this is a quite challenging procedure. In this paper, we study a diagnostic device based on the retarding field method for the measurement of the beam properties of an MIG. The influence of several factors on the measurement, such as the relativistic properties of the electrons, the magnetic field compression, and the voltage depression are theoretically and numerically investigated. In addition, the possibility to accurately extrapolate the properties of a high-power electron beam based on low-power beam measurements is studied in detail. In this way, an improved analytical methodology is proposed for the accurate estimation of beam quality from the experimental measurements.

The engineering design and R&D of auxiliary heating systems and their sub-systems are a key activities in the frame of the present conceptual design phase for a first of a kind DEMOnstration Fusion Power Plant in order to develop a system capable of achieving and controlling burning plasmas. In the frame of the EU DEMO reference design, the R&D activities consider the injection of about 50 MW of Electron Cyclotron (EC) power to support and assist different plasma phases. As the project is still in the conceptual phase, a range of options for gyrotrons, transmission lines and antennas is under assessment taking into account the guidelines for the integration of the EC system in a nuclear reactor and a maximal achievable reliability and availability of the EC power during operation.

An in-house full-wave numerical code COCHLEA based on the Finite Difference in Time Domain (FDTD) method has been developed and used to study the dispersion properties of corrugated waveguide structures. The mathematical formulation of the corresponding electromagnetic problem is presented in detail. Numerical results are derived for axially corrugated waveguides similar to those appeared in gyrotron stacked beam tunnels and are compared with those obtained by the in-house semi-analytical numerical code FISHBONE.

In this work an open-ended coaxial cavity with a corrugated insert and a relatively small number of corrugations on the outer wall is studied. In particular, the Spatial Harmonics Method (SHM) is employed in order to derive the TE modes characteristic equation, which is then solved by truncation for the calculation of the corresponding eigenvalues. Special care is given in the expansion functions used in order to avoid numerical instabilities in the calculation of high-order spatial terms. Various cases of outer wall corrugations are studied numerically in order to identify the effect of the outer corrugations and understand the mode coupling mechanism.

A planar dielectric waveguide loaded with an infinitely thin perfectly conducting diaphragm is considered. The problem of the guided mode reflection from the end of such a waveguide is analyzed with the help of the integral equation method and the variational method. Dependences of the fundamental mode reflection coefficient on the waveguide parameters and on the dimension of the aperture are calculated.

High-power high-frequency gyrotrons are required for several applications, including plasma heating, current drive, and plasmas stabilization for controlled thermonuclear fusion. Despite the use of dumping dielectric material in these gyrotrons, parasitic oscillations often appear, which significantly affect their operation and efficiency. In this paper, we perform an extended parametric study on the effect of the beam-tunnel characteristics, i.e., the outer radius of the inner waveguide and the width of the grooves, as well as the lossy material properties and the beam-radial position on the dumping of these oscillations.

The proposed Divertor Test Tokamak, DTT, aims at studying power exhaust and divertor load in an integrated plasma scenario. Additional heating systems have the task to provide heating to reach a reactor relevant power flow in the SOL and guarantee the necessary PSEP/R together adequate plasma performances. About 40 MW of heating power are foreseen to have PSEP/R 15 MW/m. A mix of the three heating systems presently proposed for ITER has been chosen, assuring the necessary flexibility in scenario development. An ECRH system at 170 GHz will provide 10 MW at plasma for several tasks, such as: bulk electron heating to bring the plasma in the high confinement regime, current profile tailoring by localized CD, avoidance of impurity accumulation, MHD control and current ramp up and ramp down assistance. Together with the EC system, 15MW of ICRH (in the range 60-90MHz) will provide the remaining bulk plasma heating power, on both electrons and ions. ICRH, in minority scheme, will produce fast ions, with an isotropic perpendicular distribution, allowing the study of fast particle driven instabilities like alphas in D-T burning plasmas. The heating schemes foreseen in DTT are 33He and H minority as well as Deuterium 2ndnd harmonic. The addition of 15 MW of NBI, later in the project, could provide a mainly isotropic parallel fast ion distribution to simulate the alpha heating scheme of a reactor. The NBI primary aim is to support plasma heating during the flat top phase when the need of central power deposition and the minimization of the shine-through risk suggest a beam energy around 300 keV. In the first phase of the DTT project the available power will be at least 25 MW, to be increased during the lifetime of the machine.

A demonstration fusion power plant (DEMO) producing electricity for the grid at the level of a few hundred megawatts is included in the European Roadmap [1]. The engineering design and R&D for the electron cyclotron (EC), ion cyclotron and neutral beam systems for the DEMO reactor is being performed by Work Package Heating and Current Drive (WPHCD) in the framework of EUROfusion Consortium activities. The EC target power to the plasma is about 50 MW, in which the required power for NTM control and burn control is included. EC launcher conceptual design studies are here presented, showing how the main design drivers of the system have been taken into account (physics requirements, reactor relevant operations, issues related to its integration as in-vessel components). Different options for the antenna are studied in a parameters space including a selection of frequencies, injection angles and launch points to get the best performances for the antenna configuration, using beam tracing calculations to evaluate plasma accessibility and deposited power. This conceptual design studies comes up with the identification of possible limits, constraints and critical issues, essential in the selection process of launcher setup solution.

The EU 1 MW, 170 GHz gyrotron with hollow cylindrical cavity have been designed within the European GYrotron Consortium (EGYC) in collaboration with the industrial partner Thales Electron Devices (TED) and under the coordination of Fusion for Energy (F4E). In the frame of the EU program, the short-pulse (SP) version of this tube was designed and manufactured by KIT in collaboration with TED. The experimental verification of the SP gyrotron prototype was successfully completed in 2015. The achieved experimental results show a very stable gyrotron operation with RF output power above 1 MW at good cavity interaction efficiency around 35%. The gyrotron was operated up to 10 ms pulse length; the nominal cavity mode TE32,9 has been excited at the frequency 170.1 GHz in agreement with the corresponding ITER specification. The Gaussian mode content of the output RF beam was about 98% and the total level of internal stray radiation was in the range of 2-3%. The manufacturing of the first industrial continuous-wave (CW) prototype gyrotron, based on the SP gyrotron design, was completed in November 2015 at TED. The tube was delivered to KIT and the experimental verification started in February 2016. Operation of the gyrotron with ms pulse duration resulted in stable excitation of the nominal mode at 170.22 GHz for a wide range of operating parameters and with a level of stray radiation comparable with the SP version. The maximum RF power achieved up to now in short-pulse operation at a level of 1 MW. The measured Gaussian mode content of the RF beam is 97%. Currently, further preparation for the CW operation of the gyrotron is in progress. (C) 2017 Published by Elsevier B.V.

The first series of experiments with the EU 1 MW, 170 GHz CW industrial prototype gyrotron1 for ITER have been completed at the KIT test facility at the end of 2016. Depressed collector operation with 180s pulses (pulse length limited by the HV power supply at KIT) at the Low Voltage Operating Point (I b ~45 A, V acc ~71 kV) resulted in 0.8 MW RF output power with 37 % total efficiency. Similar results were obtained for the High Voltage Operating Point (I b ~40 A, V acc ~80 kV) also. The experiments on the CW gyrotron with pulse duration of up to 1 hour will be continued in 2017 at SPC, Lausanne. In parallel, further experiments with the almost identical, modular short-pulse (SP) version of the gyrotron2, which delivered more than 1.2 MW, are currently ongoing at KIT in order to identify ways to further improve the performance of the CW prototype.

The EU 1 MW, 170 GHz gyrotron for ECH&CD on ITER has been developed by EGYC (European GYrotron Consortium) under the coordination of Fusion for Energy (F4E) and in cooperation with the industrial partner Thales Electron Devices (TED). The manufacturing of the first industrial continuous-wave (CW) prototype gyrotron, has been completed in November 2015 at TED. The tube was delivered to KIT and the experimental verification started in February 2016 in the short-pulse (SP) regime. Since October 2016 the gyrotron is in CW operation. In this work, details regarding the experiments at the KIT test-stand with pulses up to 180 s are presented.

In modern magnetic fusion devices, such as tokamaks and stellarators, radio frequency (RF) waves are commonly used for plasma heating and current profile control, as well as for certain diagnostics. The frequencies of the RF waves range from ion cyclotron frequency to the electron cyclotron frequency. The RF waves are launched from structures, like waveguides and current straps, placed near the wall in a very low density, tenuous plasma region of a fusion device. The RF electromagnetic fields have to propagate through this scrape-off layer before coupling power to the core of the plasma. The scrape-off layer is characterized by turbulent plasmas fluctuations and by blobs and filaments. The variations in the edge density due to these fluctuations and filaments can affect the propagation characteristics of the RF waves—changes in density leading to regions with differing plasma permittivity. Analytical full-wave theories have shown that scattering by blobs and filaments can alter the RF power flow into the ...

The European 1 MW, 170 GHz CW industrial prototype gyrotron for ECRH&CD on ITER was under test at the KIT test facility during 2016. In order to optimize the gyrotron operation, the tube was thoroughly tested in the short-pulse regime, with pulse lengths below 10 ms, for a wide range of operational parameters. The operation was extended to longer pulses with a duration of up to 180 s. In this work we present in detail the achievements and the challenges that were faced during the long-pulse experimental campaign.

Coaxial cavities with corrugated inserts and additional corrugations on the outer wall seem to have the potential for superior mode-selectivity and could be used to facilitate the development of multi-MW gyrotrons. This concept is used in this work to upgrade the design of an already manufactured 140-GHz coaxial gyrotron. A detailed sensitivity parametric study is performed to ensure mode stability. All calculations have been performed with a full-wave numerical code based on the Spatial Harmonics Method.

The possible parasitic oscillations, which frequently appear in several modern gyrotron beam tunnels, are identified and studied using the commercial simulation package CST Studio Suite. This study is based on the calculation of the Ohmic quality factor and the normalized shunt impedance R/Q and focuses on the backward propagating axisymmetric TE modes.

Recent high-power gyrotrons, capable of producing radio-frequency power above 1 MW, often suffer from parasitic oscillations in the beam tunnel, despite the presence of dielectric loading materials intended to prevent the growth of such modes. Such oscillations affect the operation and the efficiency of these devices significantly. Lately, a variety of dielectric materials have been used, with limited success, in ring-loaded beam tunnels to suppress unwanted oscillations. In this paper, we perform an extended parametric study of the effects of beam-tunnel geometry and lossy material properties on the damping of such oscillations.

Coaxial resonant cavities with longitudinal corrugations on the inner conductor are used in high-frequency high-power gyrotrons as means to reduce the number of possible competing modes. For a sufficiently large number of corrugations, the analytical approach usually treats the surface corrugation as a homogeneous surface impedance to obtain simple formulas for the characteristic equation and field components. These formulas can be introduced to interaction codes in a quite straightforward way. Full-wave approaches that account for the azimuthal periodicity of the structure and consider azimuthal spatial harmonics to describe the field distributions have been also employed, increasing though the complexity of the solution and the effort given in numerical calculations. In this paper, a full-wave code is used in an attempt to identify the way that the azimuthal spatial terms contribute to the reformation of the eigenvalue spectrum and propose a criterion for the selection of the spatial terms that should be taken into account for accurate enough calculations.

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.

We present the mathematical formulation and numerical results for the beam-wave interaction in a coaxial waveguide with rectangular slots filled with a lossy dielectric material. The formalism is based on a full-wave analysis and the linearized Vlasov equation. Numerical results are given for all kinds of modes in simplified gyrotron beam tunnels, and the effect of the losses to the parasitic oscillations is examined and discussed.

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained.

A method based on an eigenfunction expansion and the linearized Vlasov equation to study the beam-wave interaction in cylindrical surface corrugated waveguides with losses is presented. The mathematical formulation is described in detail, and numerical results are given in comparison with the literature.

The dispersion characteristics of the transverse electric modes in a waveguide with circular cross-section and periodic rectangular surface corrugations with smoothed edges are examined by the space harmonic method. The whole structure is divided into two regions, one in the propagation area and one inside the grooves. In the first region, the Floquet theorem is applied and the field distribution is expressed as a summation of spatial Bloch components, while an appropriate Fourier expansion of standing waves is used inside the grooves. Applying the appropriate interface conditions, an infinite system of equations is obtained, which is solved numerically by truncation. Numerical results are presented for several cases to check the convergence and the accuracy of the method, as well as its dependence on the corrugation profile. This formalism could be easily expanded to include all kind of waves that can in principle propagate in such slow-wave structures.

We present the mathematical formulation and numerical results for the resonance characteristics of the TMmp modes in a coaxial cavity with a longitudinally corrugated insert. Based on the spatial harmonics method, the eigenvalues of the transverse plane are calculated, and then, the Vlasov approximation is used to calculate the resonance frequency, diffractive quality factor, and longitudinal field profile of every mode. The numerical results are presented and compared with those found in the literature.

The dispersion characteristics of a circular cylindrical waveguide with periodic surface corrugations consisting of rectangular grooves with smoothing are examined using the Space Harmonic Method (SHM). The whole structure is divided into two regions, one describing the propagation volume and one inside the grooves. In the first region, the Floquet theorem is applicable and the field distribution is expressed as a summation of spatial Bloch components, whereas in the second one an appropriate Fourier expansion of standing waves is used. Applying the boundary conditions an infinite system of equations is obtained, which is solved numerically by truncation. Several cases are considered, including the limiting cases of a sinusoidal and a rectangular corrugation profile, to check the accuracy of the method proposed as well as its dependence on the corrugation profile. Numerical results are presented only for transverse magnetic modes, although the formalism can be easily extended to include all kinds of waves that can in principle propagate in such a structure.

We consider two continuous frequency tunable CW coaxial gyrotron oscillators, one 330 GHz with 3 GHz bandwidth and output power 50 – 400 W for scientific applications and one 30 GHz with 0.4 GHz bandwidth and output power 40 – 140 kW for industrial applications. The continuous tuning of both gyrotrons is achieved by moving the linearly tapered inner conductor in the axial direction in combination with the proper adjustment of the operating magnetic field. We consider also a non-linear tapering, which makes it possible to reduce the length of the insert and to improve efficiency of the device.

In the frame of a collaborative effort between the European GYrotron Consortium (EGYC), the European domestic agency Fusion for Energy (F4E) and the Industrial partner Thales Electron Devices (TED), the first continuous wave (CW) prototype of the EU gyrotron for the ITER Electron Cyclotron Heating system has been manufactured by TED and delivered to the Karlsruhe Institute of Technology (KIT) by end of 2015, where it will be tested up to pulse lengths of 180 s before being studied up to CW operation at Swiss Plasma Center (SPC, formerly CRPP) in Lausanne. The experimental campaign started early 2016. So far, the tube has been operated in ms pulses, allowing a detailed characterization and an extensive parametric study. It has been shown that the tube operates stably in the design mode up to the MW level and that the output beam profile has a high Gaussian content, compliant with the ITER requirements. This paper will report on the recent experimental results.

Optimized models for 90° polarization shift step twists for Ku, K and Ka bands are presented. The cross-section of the waveguide employed is similar to that of a rectangular one, with the difference that the walls of the shorter side are part of a circular one with the proper diameter. The optimized models have been found using the CST Microwave Studio simulation tool and in all cases the return loss is kept below -20 dB for a wide range of frequency spectrum. Two examples are given, one for Ku band and the other for K and Ka ones.

The authors present the mathematical analysis for the calculation of the dispersion relation, the field distributions, and the ohmic losses for TEm,p modes in an infinite coaxial waveguide with a longitudinally corrugated insert. The method employed is based on an appropriate eigenfunction expansion, and its main advantage is the very fast convergence with a few spatial harmonics. The analysis is properly extended to include tapered cavities with varying, in respect to the z-coordinate, outer and/or inner radius. Numerical results are presented for several tapered cavity geometries and compared with already published methods

We present the mathematical analysis, based on an eigenfunction expansion, and the main steps of the corresponding numerical code developed to calculate the dispersion relation and the field distributions for all kind of waves, which can propagate in a nonperiodic circumferentially corrugated circular waveguide with losses. The code offers fast computations with accuracy, which can be chosen by the user. Numerical results are given for several geometries and comparison with already established codes is made.

We employ a well-established method to determine the dispersion properties of transverse magnetic and transverse electric waves in re-entrant cavities. We assume that the waveguide structure consists of a large number of identical cavities in order that the Floquet theorem could be applied to describe the electromagnetic field in an empty cylinder. Furthermore, we describe the electromagnetic field in each cavity region by an eigenfunction expansion for standing waves. Inferring boundary conditions leads to a system of two infinite sets of equations, which is solved numerically by truncation. A numerical code has been developed to calculate the dispersion relation and the electromagnetic field components. Numerical results are presented for several waveguide geometries, including the case of the real gyrotron beam-tunnel geometry. A comparison with already established codes is made.

The progress in the development of the European 170 GHz, 1 MW/CW gyrotron for electron cyclotron heating & current drive (ECH&CD) on ITER is reported. A continuous wave (CW) prototype is being manufactured by Thales Electron Devices (TED), France, while a short-pulse (SP) prototype gyrotron is in parallel under manufacture at Karlsruhe Institute of Technology (KIT), with the purpose of validating the design of the CW industrial prototype components. The fabrication of most of the sub-assemblies of the SP prototype has been completed. In a first step, an existing magnetron injection gun (MIG) available at KIT was used. Despite this non-ideal configuration, the experiments provided a validation of the design, substantiated by an excellent agreement with numerical simulations. The tube, operated without a depressed collector, is able to produce more than 1 MW of output power with efficiency in excess of 30%, as expected, and compatible with the ITER requirements. (C) 2015 Karlsruhe Institute of Technology. Published by Elsevier B.V. All rights reserved.

We examine the radiation properties of an abruptly ended symmetric slab waveguide for the case of transverse electric (TE) guided modes with odd symmetry of the electric-field distribution by employing the integral equation method. This method converges very fast, especially in the case of small relative difference j 12 between the refractive indices of the core and cladding regions of the slab waveguide. The electric-field distribution at the terminal plane, the reflection coefficient of the first odd TE guided mode, and the far-field radiation pattern are computed. We present numerical results for several cases of abruptly ended waveguides and we examine the influence of the value of j 12 on the guided-mode reflectivity, on the electric-field distribution at the terminal plane, as well as on the angle of the maximum value (elevation angle) and on the beamwidth of the far-field radiation pattern.

For the development of a DEMOnstration Fusion Power Plant the design of auxiliary heating systems is a key activity in order to achieve controlled burning plasma. The present heating mix considers electron cyclotron resonance heating (ECRH), neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) with a target power to the plasma of about 50 MW for each system. The main tasks assigned to the EC system are plasma breakdown and assisted start-up, heating to L-H transition and plasma current ramp up to burn, MHD stability control and assistance in plasma current ramp down. The consequent requirements are used for the conceptual design of the EC system, from the RF source to the launcher, with an extensive R&D program focused on relevant technologies to be developed. Gyrotron: the R&D and Advanced Developments on EC RF sources are targeting for gyrotrons operating at 240 GHz, considered as optimum EC Current Drive frequency in case of higher magnetic field than for the 2015 EU DEMO1 baseline. Multi-purpose (multi-frequency) and frequency step-tunable gyrotrons are under investigation to increase the flexibility of the system. As main targets an output power of significantly above 1 MW (target: 2 MW) and a total efficiency higher than 60% are set. The principle feasibility at limits of a 236 GHz, conventional-cavity and, alternatively, of a 238 GHz coaxial-cavity gyrotron are under investigation together with the development of a synthetic diamond Brewster-angle window technology. Advanced developments are on-going in the field of multi-stage depressed collector technologies. Transmission line (TL): different TL options are under investigation and a preliminary study of an evacuated quasi-optical multiple-beam TL, considered for a hybrid solution, is presented and discussed in terms of layout, dimensions and theoretical losses. Launcher: remote steering antennas have been considered as a possible launcher solution especially under the constraints to avoid movable mirrors close to the plasma. With dedicated beam tracing calculations, the deposition locations coverage and the wave absorption efficiency have been investigated, considering a selection of frequencies, injection angles and launching points. An option for the EC system structure is proposed in clusters, in order to allow the necessary redundancy and flexibility to guarantee the required EC power in the different phases of the plasma pulse. Number and composition of the clusters are analysed to have high availability and therefore maximum reliability with a minimum number of components.

EU is presently developing a 1 MW, 170 GHz cylindrical cavity gyrotron for ITER. This activity was initiated in 2008 as a risk mitigation measure in parallel with the development of a 2 MW coaxial-cavity gyrotron, and in 2012 became the main line. In the previous year, the scientific designs of the 1 MW gyrotron components, such as the electron gun, beam tunnel, cavity, quasi-optical output coupler and single-stage depressed collector have been finalized. In addition, in collaboration with the European industrial partner Thales Electron Devices (TED), after completion of the industrial design of the technological parts of the gyrotron, the procurement and manufacturing of subassemblies for a 1 MW CW prototype will start in 2014. Its specifications correspond to the ITER requirements.