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Multistage Depressed Collectors (MDCs) are widely used in vacuum tubes to regain energy from the depleted electron beam. However, the design of an MDC for gyrotrons, especially for those deployed in fusion experiments and future power plants, is not trivial. Since gyrotrons require relatively high magnetic fields, their hollow annular electron beam is magnetically confined in the collector. In such a moderate magnetic field, the MDC concept based on E × B drift is very promising. Several concrete design approaches based on the E × B concept have been proposed. This paper presents a realizable design of a two-stage depressed collector based on the E × B concept. A collector efficiency of 77% is achievable, which will be able to increase the total gyrotron efficiency from currently 50% to more than 60%. Secondary electrons reduce the efficiency only by 1%. Moreover, the collector efficiency is resilient to the change of beam current (i.e., space charge repulsion) and beam misalignment as well as magnetic fi...

Since many years KIT is strongly involved in the development of high power gyrotrons for use in ECRH. KIT is pursuing two development lines: (i) the conventional, hollow cavity gyrotron and (ii) the coaxial cavity gyrotron. KIT is pushing conventional cavity gyrotrons from 1 MW to 1.5 MW in a common project with IPP Greifswald. Coaxial cavity technology having the advantage of higher power capability, in particular at higher frequency, is used for the 2 MW 170 GHz short-pulse prototype. This gyrotron is currently being upgraded to allow pulse extension up to approximately 100 ms and up to 1 s in a second step. For a future DEMOnstration fusion power plant two challenging trends with respect to gyrotron features are recognized: (a) the operating frequency will be above 200 GHz and (b) the requested total efficiency of the gyrotron should be higher than 60%. KIT is addressing these requirements by investigating both gyrotron technologies for their performance at a frequency well above 200 GHz. We started careful analysis of multi-staged-depressed collectors.

In frame of the work package "Heating and Current Drive" within the HORIZON 2020 EUROfusion program the development of multi-megawatt gyrotrons for the DEMOnstration fusion power plant is ongoing at KIT. Additionally, possible future upgrades of gyrotrons for e.g. for W7-X and ITER, are under consideration. Using the KIT modular-type 170 GHz 2 MW short-pulse (ms-range) coaxial-cavity pre-prototype the superior performance of the coaxial-cavity gyrotron technology has been already demonstrated for pulse lengths of up to a few milliseconds, achieving an RF output power of 2.2 MW. This short-pulse pre-prototype is now being upgraded in two steps to allow investigation of long-pulse behavior. The first step allows pulse-lengths up to 100 ms. The second step shall allow pulse-lengths up to 1 s. The first step in the upgrade was finished just recently. Considering a multi-megawatt output power at around 50 % total efficiency of the tube, the upgrade of the pre-prototype did require significant developments in the areas of basic construction, in particular with regards to cooling technologies, and in the final assembly. In this paper, the basic construction, the manufacturing details and the assembly are shown. Additionally, it is planned to show the very first performance results of the long-pulse tube based on the measurements which will be achieved in the time frame before the conference.

A new magnetron injection gun has been designed and manufactured for the upgrade of the 2 MW 170 GHz modular coaxial cavity gyrotron. Many important novelties have been considered in this new gun.

After ITER, the first real prototype of a fusion power plant is foreseen. It is named Demonstration Power Plant (DEMO). Within the EUROfusion work package "Heating and Current Drive", the conceptual designs of both a hollow-cavity and coaxial-cavity gyrotron for DEMO has been suggested. For DEMO, the major target is to achieve an output power per tube which is significantly larger than 1 MW CW at an operating frequency of up to 240 GHz. In the case of the hollow-cavity gyrotron design, a very high-order TE mode (eigenvalue > 103 at a cut-off frequency of236 GHz) is the prerequisite for the targeted high output power. At this eigenvalue range, the excitation of the desired operating mode during gyrotron start-up becomes challenging due to a large number of competing modes. In this work, it is demonstrated by numerical simulation that mode excitation and mode competition control can be significantly facilitated by using special start-up scenarios, based on a triode electron gun.

At KIT, a modular 170 GHz, 2 MW TE34, i9-mode coaxial-cavity gyrotron with advanced water cooling is ready for tests. The successful operation of this tube will be a first important step towards a possible future DEMO gyrotron. Nevertheless, looking forward, there are two questions to be answered: (i) what potential does the existing coaxial cavity offer with regards to MW-class multi-frequency operation also at higher frequencies, and (ii) what could be a different mode selection to achieve an even higher output power in a more compact gyrotron design. To provide an answer to (i), based on the 170 GHz, 2 MW pre-prototype the multi-frequency operation at multiples of the resonance frequency of the diamond disc RF output window was carried out. Additionally, a slightly modified cavity design was introduced. To answer the question (ii), the TE25,22-mode was chosen and compared with the results got for the TE34, i9-mode. The extreme volume TE25,22-mode allows to reduce the beam radius by around 25 % and to increase the RF output power of the gyrotron by up to 30 %.

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 development of a highly efficient multistage depressed collector system for the recovery of the major part of the spent electron beam energy is the key to increase the gyrotron efficiency. For several years there has been a significant effort at Karlsruhe Institute of Technology (KIT) in order to design a reliable, robust and efficient multistage depressed collector system for high power gyrotrons. The progress and the recent results of these studies will be presented.

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.

The present gyrotron designs for EC plasma heating in nuclear fusion reactors require the safe exhaust of a power comparable to that injected into the plasma, in order to keep the maximum temperature below the acceptable value of 300 °C. In this paper, the commercial computational fluid dynamics (CFD) software STAR-CCM+® is used to analyze the thermal performance of the annular copper collector of a 1 MW ITER gyrotron, equipped with a hypervapotron structure made of annular fins with rectangular cavities of aspect ratio (depth/width) = 3, cooled by highly subcooled (90–100 °C) pressurized water flowing at ∼4 m/s. It is assumed that the simple vertical sweeping strategy is used to reduce the very high peak heat flux on the collector (up to 30 MW/m2 transient, 5 MW/m2 time average), due to the spent electron beam. The 2D steady-state conjugate heat transfer problem is solved assuming azimuthal symmetry and accounting for 2-phase flow. The single-cavity flow and heat transfer problem is considered first, to optimize the mesh and the selection of the turbulence model. For the operating conditions considered in this paper, the full collector (100+ cavities) solution shows that boiling occurs only in a limited number of cavities close to the peaks of the heat flux, with the vapor remaining trapped in the bottom of the cavities, i.e. no full hypervapotron regime should be achieved in these operating conditions. The steady-state analysis allows the numerical evaluation of the heat transfer coefficients between Cu and water; these are then used as input for the simplified, purely thermal (solid only) analysis of the actual transient problem for the full collector. The results of the simplified model, which allows a huge reduction of the computational effort, are successfully benchmarked against those of a comprehensive thermal–hydraulic simulation. The computed peak Cu temperature is below the acceptable limit under the steady-state (time averaged) heat load, but becomes unacceptably high if the actual transient heat load is considered, confirming the need for more effective sweeping strategies and/or optimized collector geometries with improved heat transfer capabilities.

Different configurations of water-cooled Cu collector for gyrotrons are investigated using the StarCCM + CFD code, aimed at optimizing its thermal–hydraulic (TH) performance. Although the current collectors show a good performance, the collector can be subjected to transient heat loads, due to the spent electron beam, of up to several tens of MW/m2, and there is an interest to increase the gyrotron output power in the future. Furthermore, an optimized cooling will lead to improved reliability and lifetime of the collector. Starting from a hypervapotron (HV)-like collector, characterized by 100+ deep rectangular cavities with aspect ratio (AR) = 3, we present in the first part of the paper a single-cavity steady-state parametric analysis of the effect of AR on the heat exhaust capabilities. The investigation is then extended to other collector designs, including circumferential ribs and dimples, in order to assess the options for further improvements of the TH performance. The peak Cu temperature is computed by the code and its minimization is the target of the present optimization exercise. A self-consistent estimate of the heat transfer coefficient between collector and coolant is also obtained, which could be useful for fatigue and lifetime assessments. In the second part of the paper the most promising collector geometries identified in the first part are analyzed in the case of a transient heat load (vertical sweeping), first at the level of a single spatial period of the collector structure, then at the full-collector level. The results of the TH transient analysis are compared with both the results of the first part and with the transient purely thermal analysis of the full collector, showing for all geometries considered in this study a room for cooling efficiency improvement with respect to the HV-like design with AR = 3, at least in the operating conditions considered for this study (V ∼ 4 m/s, almost 100 °C sub-cooling).

A detailed investigation of the RF oscillation frequency produced by megawatt-class continuous wave 140 GHz gyrotrons for the stellarator W7-X found discrete jumps of a few megahertz during the initial frequency downtuning phase, as well as in quasi-steady long-pulse operation. The common explanations for such behavior, such as voltage ripple and the classical long-line effect, are reviewed, and are found not to match the experimental observations properly. As alternative hypotheses, we suggest a long-line effect involving higher-order reflections inside the gyrotron, or discrete changes in the cavity radius, caused by discontinuous thermomechanical expansion.

Results of experimental studies on the frequency step-tunable ( $D$ -band) megawatt-class gyrotron, which is under development at IHM (KIT), are presented The goal of the short pulse (0.5 ms) experiments was to study the performance of the gyrotron equipped with a longer cylindrical part of the cavity (higher quality factor) in comparison with the previous results. The new design of the cavity was numerically optimized using the EURIDICE code. The results of numerical studies are presented. It was found that output power and efficiency are increased in this new configuration. The output efficiency of 37% at 1210 kW output power was achieved operating in the TE $_{23,8}$ mode at 143.4 GHz. With increased beam current, for the same cavity mode, the highest power of 1440 kW was obtained at 35% efficiency. The measurements were repeated for a number of different operating modes. The excited mode with the highest frequency is the TE $_{28,9}$ mode at an operating frequency of 169.2 GHz.

During the first hundred milliseconds, the frequency and RF output power of long pulse operating gyrotrons undergo deterministic variation. This well-known behavior is caused by the thermal expansion of the cavity and internal electrostatic processes related to the ionization of residual gas. A macroscopic analytical investigation of the gas conditions in modern gyrotrons indicates that ionization processes are unlikely to influence the overall internal gas pressure. In combination with electrostatic potential considerations, it was found that the beam depression voltage is not fully neutralized; in the case of W7-X gyrotrons, a maximum value of about 60 % neutralization is expected, in conflict with the common assumption of full neutralization in steady state. Using experimentally measured frequency shifts and the Evridiki gyrotron interaction simulation code, a fitting process was employed to further investigate these effects. The results are in very good agreement with the theoretical predictions and allow a separation of the time constants of the two processes causing the frequency tuning.

We propose to use scattered X-ray radiation for determining the energy spectrum of electrons arriving at the collector of a gyrotron. It is shown that, under conditions typical of devices of this type, the radiation spectrum is almost not affected by the Compton scattering in the diagnostic window. An advantage of the proposed variant of diagnostics (in comparison to that using the direct X-ray flux) is related to optimization of the radiation flux density for the use of energy-dispersive spectrometers. A new scheme of measurements has been tested that allows data averaged over the entire electron beam of a gyrotron to be obtained. The possibility of gaining information with optimum spatial resolution is discussed.

The development of a modular short-pulse pre-prototype as base for a 2 MW, 170 GHz, CW coaxial-cavity gyrotron for the international fusion project ITER and beyond is in progress at Karlsruhe Institute of Technology (KIT). The current modular pre-prototype configuration allows the generation of > 2.2 MW RF output power in short-pulses with an overall efficiency of approximately 48 % in depressed collector operation. Due to the excellent experimental results, KIT is developing and manufacturing a coaxial 2 MW, 170 GHz longer-pulse gyrotron with an advanced water cooling system for a pulselength of up to 1 s. In this paper the current short pulse experimental results, as well as the design and manufacturing processes of the coaxial 2 MW longer-pulse gyrotron, will be presented.

The introduction of advanced Multistage Depressed Collectors (MDC) to high-power fusion gyrotrons will be the key to increase the gyrotron overall efficiency above 60 % as required for future DEMOnstration power plant. MDCs which base on the E×B drift concept might solve the fundamental issue of handling the secondary electrons. To consider the effect of secondary electrons in a stationary state, full 3D Particle-In-Cell (PIC) simulations are performed. A novel two-stage collector designed using PIC is presented which shows an excellent collector efficiency of up to 77 %. Moreover, the undesired generation of secondary electrons only reduces the efficiency by 1 %o, compared to a typical 10 % efficiency reduction in other known classical concepts.

The first part of the experimental validation of the EU 1 MW, 170 GHz conventional (hollow-cavity) gyrotron for ITER was successfully completed at KIT. During this period two prototype tubes: (1) short-pulse (SP) gyrotron and (2) industrial continuous-wave (CW) gyrotron have been experimentally tested. The achieved RF output power was in range of 1 MW (SP) and 0.8 MW (CW) with a reasonable efficiency at pulse length up to 10 ms and 180 s, respectively. In order to study the possibility of further optimization of the gyrotron performance, additional experiments with the modified SP gyrotron setup are planned at KIT. The achieved results will be presented in this paper.

Karlsruhe Institute of Technology (KIT) is doing research and development in the field of high power gyrotrons for W7-X, ITER, and, towards a future DEMOnstration fusion power plant. While the initial installation of the ECRH system at W7-X is finished and operating very successfully, KIT is still involved in the development of gyrotrons for ITER. This work is coordinated by F4E and ongoing in frame of EGYC. Moreover, as part of EUROfusion, KIT is investing in the research for advanced fusion gyrotrons of future EU DEMO. The target is to develop gyrotrons which will fulfil the need for RF sources operating at a frequency above 200 GHz, an RF output power above 2 MW and a total efficiency above 60 %. Aditionally, multi-purpose/multi-frequency operation and frequency step-tunability are requested. That requires the research and development on advanced gyrotrons, test environments and control techniques. This contribution provides a view over ongoing and planned activities in the field of gyrotron development at KIT.

The key to increase the gyrotron efficiency to a level significantly higher than 50% is the development of a highly efficient collector system for the recovery of the spent electron beam energy. In this context, a significant effort has been dedicated at KIT during the last years in order to design a reliable, robust and efficient multistage collector system. For this purpose several design approaches have been theoretically and numerically investigated. An overview of the results of these studies will be presented.

In several gyrotron prototype tubes, harmful voltage stand-off instabilities were observed during the test campaign period1. It has been shown that the formation of magnetic potential wells in the design of the gyrotron magnetron injection gun (MIG) can initiate such instabilities2. However, it is still not clear which type of the potential wells create these problems and the detailed mechanism to explain that behavior is not known. In the context of this work, two different types of potential wells are theoretically and experimentally investigated. Numerical studies have been performed using the Ariadne code3, while a mock-up device has been designed and manufactured for the experimental investigation of both types of magnetic potential wells in several regimes of operating parameters. The deeper understanding of the magnetic potential well trap behavior is very important for the future design of simpler and robust gyrotron MIGs.

Considering the design goals and technological constraints for DEMO gyrotrons, conceptual design and tolerance analyses of a 236 GHz 1 MW conventional, hollow-cavity gyrotron and of a 237.5 GHz 2 MW coaxial-cavity gyrotron are ongoing at KIT. At such high power and frequency, very high order operating modes (eigenvalues > 100) are necessary to keep cavity wall loading at 2 kW/cm2. Both the small wavelength and the high eigenvalue make the gyrotrons susceptible to misalignment of electron beam and — if present — coaxial insert. For the hollow-cavity gyrotron, the influence of the off-axis electron beam on gyrotron operation has been very accurately investigated in interaction simulations with realistic beam parameters. For the coaxial-cavity gyrotron, the influence of insert misalignment on modes and on an aligned electron beam has been studied.

An iterative procedure is described, which models the influence of the thermal expansion of the gyrotron cavity on the expected gyrotron performance. It is a multi-physics simulation method, which involves electrodynamic, thermohydraulic, and thermo-mechanical simulations. The method is applied to the first European 170 GHz, 1 MW CW prototype gyrotron for ITER. According to calculations, a performance reduction of ∼15% is expected at nominal operating parameters because of the simulated thermal expansion of the cavity. An alternative operating point to mitigate this effect is proposed.

A quasi-optical (q.o.) mode converter has been designed for the MW-level dual-frequency gyrotron which is under development for the upgrade of the electron cyclotron (EC) system of the TCV tokamak. The gyrotron design is essentially based on modifications of the W7-X gyrotron [1]. The launcher of the output coupler has been synthesized to adapt the geometry of the quasi-elliptical mirror with same size as in the W7-X gyrotron for both operating modes (TE17,5 @ 85GHz, TE26,7 @ 126GHz). The simulation results show that high conversion efficiencies and low stray radiations in the q.o. system can be achieved.

A novel concept of a two-stage depressed collector for high-power, fusion gyrotrons using helix electrodes is investigated. The helix electrodes create an azimuthal component of the electric field, which causes a radial drift of the annular electron beam. This drift sorts the electrons according to their initial velocities. Particle-in-Cell simulations show that this concept is extremely robust against the negative effect of secondary electrons. The collector efficiency is reduced by only 1 %. Considering the space charge (including that of secondary electrons) a collector efficiency of 77 % can be achieved with only two stages. Furthermore, electron beam sweeping may be possible at the last stage, where the most kinetic energy of electrons is absorbed.

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.

ECRH&CD is one of the favorite heating system for magnetically confined nuclear fusion plasmas. KIT is strongly involved in the development of high power gyrotrons for use in ECRH systems for nuclear fusion. KIT is upgrading the sub-components of the existing 2 MW, 170 GHz coaxial-cavity short-pulse gyrotron to support long-pulse operation up to 1 s, all components will be equipped with a specific active cooling system. Two important developments for future high power, highly efficient gyrotrons will be discussed: design of gyrotrons with high operating frequency (∼ 240 GHz) and efficiency enhancement by using advanced collector designs with multi-staged voltage depression.

During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3·1019 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2·1020 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2.

The upgrade of the EC-system of the TCV tokamak has entered in its realization phase and is part of a broader upgrade of TCV. The MW-class dual-frequency gyrotrons (84 or 126GHz/2s/1MW) are presently being manufactured by Thales Electron Devices with the first gyrotron foreseen to be delivered at SPC by the end of 2017. In parallel to the gyrotron development, for extending the level of operational flexibility of the TCV EC-system the integration of the dual-frequency gyrotrons adds a significant complexity in the evacuated 63.5mm-diameter HE11 transmission line system connected to the various TCV low-field side and top launchers. As discussed in [1], an important part of the present TCV-upgrade consists in inserting a modular closed divertor chamber. This will have an impact on the X3 top-launcher which will have to be reduced in size. For using the new compact launcher we are considering employing a Fast Directional Switch (FADIS), combining the two 1MW/126GHz/2s rf-beams into a single 2MW rf-beam.

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.

The stellarator Wendelstein 7-X (W7-X) uses electron cyclotron resonance heating (ECRH) by 140 GHz highpower microwave sources (gyrotrons) for plasma generation and as primary heating method. At the limit of operational stability, parasitic and satellite modes simultaneously excited to the working mode are converted to stray radiation in the gyrotron, resulting in a reduced efficiency and mostly in a mode loss. Hence the stray radiation level by these modes is proposed as an indicator for the operational stability. A feedback control system ensuring stable operation of the gyrotrons at highest possible output power and a fast mode recovery after a mode loss, is desirable for the ECRH on W7-X. Consequently shot spectrograms and the signal of a RF detector were examined to identify a reliable mode-loss-precursor in the stray radiation at the gyrotron relief window for the targeted control system. This information then can be used to derive and implement a control algorithm on a field programmable gate array (FPGA).

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.

In quasi-optical transmission systems, it is usually required to transform a certain Gaussian beam into another one characterized by a different set of parameters. Mirrors with quadratic surface contour functions, such as paraboloidal, ellipsoidal, and toroidal mirrors, are widely used for such a transformation. However, quadratic surface contours do not provide sufficient conversion efficiencies needed for the transformation of off-axis Gaussian beams into desired Gaussian millimeter-wave field distributions. Therefore, it is necessary to develop a new method for the transformation of off-axis Gaussian beams. This paper presents such a method for the synthesis of beam-shaping mirrors for off-axis Gaussian beam transformation. Based on this new method, a matching optics unit consisting of two mirrors has been designed to transform a Gaussian-mode-like field distribution into a Gaussian beam with desired parameters.

The 170 GHz 2 MW TE34,19-mode coaxial-cavity modular short-pulse pre-prototype gyrotron at KIT was recently modified in order to verify the multi-megawatt coaxial-cavity technology at longer pulses. In parallel, theoretical investigations on a possibility to operate the 170 GHz TE34,19-mode coaxial-cavity prototype at multiple frequencies up to 238 GHz have been started, with a goal to find a configuration at which the tube could operate in the KIT FULGOR gyrotron test facility using the new 10.5 T SC magnet. This paper indicates which adjustments have to be made and show the feasibility of the multi-frequency operation. Small modifications at the gyrotron cavity will support an RF output power of more than 2 MW at 170/204 GHz. Furthermore, a new gyrotron launcher has been designed capable of producing a Gaussian microwave beam with a Gaussian mode content of more than 96% at these frequencies.

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.

In this paper, a measurement system for the detection of time-dependent effects in broadband spectra of high-power millimeter-wave sources is demonstrated. The heterodyne approach with sub-harmonic mixers enables high dynamic range and configuration flexibility, but typically also imposes severe problems on the analysis of transient or instationary phenomena through frequency ambiguity. A key feature of the presented system is an unambiguous reconstruction of the RF spectrum from the detected IF spectra. This is done by evaluating the upper and lower mixer sidebands of two parallel heterodyne receiver channels simultaneously, resulting in a comparatively high instantaneous measurement bandwidth of 7 GHz per channel pair and a high dynamic range of 50-60 dB in the frequency range 110-170 GHz. Sample measurements obtained during the occurrence of an arc at the dielectric output window of a 140-GHz megawatt-class gyrotron demonstrate the unique capabilities of the system for detecting highly transient and broadband effects in the tube's output spectrum.

Bremsstrahlung X-rays produced at the collector can be used for definition of electron energy distributions in gyrotrons and maybe in other high-power microwave devices. Such a possibility underwent early testing at the Karlsruhe Institute of Technology on two gyrotrons. Electron energy distributions calculated on the basis of X-ray spectra measured near the gyrotrons operated at different regimes have plausible shapes and demonstrate good correlation with expected characteristics of the electron beam, thus confirming the viability of the proposed diagnostic technique.

Parasitic beam tunnel oscillations have been hampering the series production of gyrotrons for W7-X. This problem has now been overcome thanks to the introduction of a specially corrugated beam tunnel. Two gyrotrons equipped with the new beam tunnel have fully passed the acceptance tests. Despite excellent power capability, the expected efficiency has not yet been achieved, possibly due to the presence of parasitic oscillations suspected to be dynamic after-cavity-oscillations (ACI's) or due to insufficient electron beam quality. Both theoretical and experimental investigations on these topics are ongoing. On previous W7-X gyrotrons collector fatigue has been observed, not (yet) leading to any failures so far. The plastic deformation occurring on the collector has now been eliminated due to the strict use (on all gyrotrons) of a sweeping method which combines the conventional 7 Hz solenoid sweeping technique with a 50 Hz transverse-field sweep system. Starting in 2013, the gyrotron test facility at KIT will be enhanced, chiefly with a new 10 MW DC modulator, capable of testing gyrotrons up to 4 MW CW output power with multi-stage-depressed collectors.

In this paper, we present the results of an experimental study on the influence of the lateral misalignment between the axes of the annular electron beam and the cavity on the performance of a 1-MW 140-GHz gyrotron. The subject of the study is of practical interest. On one hand, the discrepancy between numerical simulations of the gyrotron performance and experimental results could by partially attributed to the beam misalignment. On the other, it is in practice very difficult to reach the necessary tolerances in series production of high-power high-frequency (>100 GHz) gyrotrons. The problem actually will become more demanding with the increase of the operating frequency of the working mode because the required accuracy scales with the wavelength of the electromagnetic wave. It is shown that a large displacement of the annular beam from the cavity axis may lead to the excitation of a competing mod which is not excited under normal conditions. The excitation of the main mode is observed, however, at a beam-cavity displacement of up to 0.8 mm.

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.

An Inverse Magnetron Injection Gun (IMIG) design is proposed for future conventional-type high-power gyrotrons. The design is deduced from the design for the KIT 2-MW, 170-GHz coaxial-cavity gyrotron prototype which has been published recently. As for the coaxial-cavity gyrotron IMIG, the inverse MIG design for the conventional hollow-type gyrotron is offering the possibility for compact geometry, optimum cooling of critical parts and excellent beam parameters due to its principle of construction. Considering the fundamental beam parameters, in the theoretical analysis an excellent beam quality has been achieved which results in a very low velocity spread.

Manufacturing tolerances at the emitter ring region of a high power gyrotron cathode structure can have a dramatic effect on the electron beam quality, and, finally, are determining the gyrotron performance which will be achieved. A sensitivity analysis of manufacturing imperfections of the emitter ring in the gyrotron cathode structure on the electron beam quality has been performed. It has been shown that a possible radial displacement of the emitter ring of the order of few tens of microns can cause dramatic effects on the beam quality and therefore the gyrotron operation.

An experimental campaign with the European 170 GHz, 1 MW short-pulse prototype gyrotron for ITER took place at the Karlsruhe Institute of Technology in 2015. Following the experiments, extensive simulations of the experimental operation have been performed, focusing on the calculation of the electron beam properties and of the beam-wave interaction. A number of simulation results are presented and discussed in this paper. The comparison between simulation and experiment provides additional validation of the numerical tools and contributes to the interpretation of the experimental results.

The output power, efficiency and stability of high power CW gyrotrons depend strongly on the quality of the electron beam generated by the magnetron injection gun. To understand deteriorating effects like low frequency oscillations or the generation of an electron beam halo, the KIT in-house beam optics code ESRAY [1] has been extended from a pure 2D electrostatic raytracing code to an advanced quasi-static particle-in-cell code, capable of handling slowly varying transient effects. In addition, a new emitter surface roughness model [2] and the treatment of secondary electrons has been implemented to achieve significantly more realistic results.

In order to find the upper limit for mode eigenvalues for stable 1 MW-class gyrotron operation, the behavior of a 236 GHz cylindrical (hollow) cavity gyrotron design is theoretically analyzed for operation with even higher order modes (eigenvalues between 100 and 145). In this frame, two independent methods are used to determine the effect of neighboring modes on mode stability and to estimate the eigenvalue limit for stable operation. With consideration of realistic electron beam parameters (velocity spread and beam width) and a diode start-up regime, the results of both methods suggest that mode eigenvalues smaller than 125 should be selected to ensure the stability of operation.

Multistage Depressed Collectors (MDCs) have been successfully applied in many kinds of vacuum tubes; however, there are only few discussions about applicable MDCs for high-power gyrotrons, regarding the hollow small-orbit electron beam and relatively strong stray magnetic field in the collector region. A novel concept has been recently proposed, where the electron beam is transformed into one or more sheet beams and sorted by the E×B drift. This contribution shows a 3D simulation result of a realistic seven-stage depressed collector model based on the above mentioned concept. With ordinary optimizations, a collector efficiency of 86 % has been achieved in this simulation.

A design approach for a multistage depressed collector system for gyrotron is presented here. The new concept based on two steps: (i) the transformation of the cylindrical hollow beam to sheet beam and (ii) the sorting of the spent beam electrons on the electrodes, based on ExB approach, in order to achieve the optimal efficiency. The simulation results predict very high levels of efficiency (more than 80%) and this idea can open up new perspectives in gyrotron technology.