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Continuous Wave gyrotrons are the key elements for Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) in present fusion experiments and future fusion reactors. In the frame of the EUROfusion activities, a 170 GHz, 2 MW short-pulse (∼ 1 ms), water-cooled coaxial gyrotron, already tested at Karlsruhe Institute of Technology (KIT), is being upgraded for operation at longer pulses (∼ 100–1000 ms). Here we use the MUlti-physiCs tool for the integrated simulation of the CAvity (MUCCA), recently developed in collaboration between Politecnico di Torino and KIT, to analyze the evolution of the operating condition of the coaxial gyrotron cavity, self-consistently coupling thermal-hydraulic, thermo-mechanical and electro-dynamic models. The main results are presented in terms of evolution of temperature, heat load and deformation of the heated surface of the resonator and of the coaxial insert during the first few seconds of operation. We show that the system evolves towards stable operating conditions (no beam loss), with a peak temperature strongly dependent on the cooling configuration, where a large room for the improvement of the current cavity cooling design is found.

Karlsruhe Institute of Technology (KIT) is doing research and development in the field of megawatt-class radio frequency (RF) sources (gyrotrons) for the Electron Cyclotron Resonance Heating (ECRH) systems of the International Thermonuclear Experimental Reactor (ITER) and the DEMOnstration Fusion Power Plant that will follow ITER. In the focus is the development and verification of the European coaxial-cavity gyrotron technology which shall lead to gyrotrons operating at an RF output power significantly larger than 1 MW CW and at an operating frequency above 200 GHz. A major step into that direction is the final verification of the European 170 GHz 2 MW coaxial-cavity pre-prototype at longer pulses up to 1 s. It bases on the upgrade of an already existing highly modular short-pulse (ms-range) pre-prototype. That pre-prototype has shown a world record output power of 2.2 MW already. This paper summarizes briefly the already achieved experimental results using the short-pulse pre-prototype and discusses in detail the design and manufacturing process of the upgrade of the pre-prototype toward longer pulses up to 1 s.

The DEMO-relevant gyrotron research at Karlsruhe Institute of Technology is driven by the European concept for a demonstration fusion reactor (EU DEMO). This paper reports on the recent results of the theoretical and experimental studies towards the development of gyrotrons fulfilling the DEMO needs.

High-power, high-frequency gyrotrons are the only promising sources for the electron cyclotron resonance heating and current drive (ECRH&CD) in the present thermonuclear fusion plasma experiments and in future power plants. Compared to the hollow-cavity gyrotron design, the coaxial cavity gyrotron design facilitates improved mode competition control, which eventually increases the output power per tube. At KIT-IHM, the successful operation of a 170-GHz gyrotron has been demonstrated in the short-pulse regime, and the ongoing research activities are aiming to upgrade the existing coaxial cavity gyrotron from short-pulse (1–10 ms) to long-pulse (~ 1s) operation. In this paper, with the help of multiphysics simulations, the performance of the insert cooling system is verified for the continuous wave (CW) operation and the operating limits of the insert cooling systems are determined. The perfectly aligned coaxial Glidcop insert will be able to withstand operating conditions leading to a maximal heat flux of about 0.39 kW/cm2. The influence of insert misalignment on insert loading is also studied systematically in this paper.

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 high-power fusion gyrotrons., the maximum heat-load on the wall of the interaction section is in the order of 2 kW/cm2, which is the major limiting technological factor for output power., efficiency and pulse-length of the tube. The ongoing gyrotron development demands a very effective cavity cooling system for stable and optimum gyrotron operation. In this work, the thermal performance of a mini-channel cavity cooling is numerically investigated using the ANSYS Fluent®code-package. The influence of the various physical and operating parameters on the cavity cooling efficiency is systematically studied for a 170 GHz, 2 MW coaxial-cavity gyrotron and an optimized heat sink design is proposed. A mock-up test set-up is also developed to experimentally validate the simulation results.

The maximum heat load on the cavity wall of high power fusion gyrotrons is one of the major limiting technological factors for the operation of the tube. To achieve the requested output power, efficiency and pulse length, a very efficient cooling of the interaction structure is mandatory. In this work, the performance of a mini-channel cavity cooling system for a 170 GHz, 2 MW coaxial-cavity gyrotron is numerically investigated, including the development of a mock-up test set-up for experimental validation.

The successful operation of a 2 MW 170 GHz coaxial-cavity gyrotron has been demonstrated at IHM-KIT in short(ms) pulses and the ongoing research activities are aiming to upgrade the existing tube to the long-pulse regime. In this work, the performance of the insert cooling system is systematically studied for long­pulse coaxial-cavity gyrotron operation. A multi-physics simulation model is developed with the relevant turbulence model and mesh parameters. The results suggest stable operation of the insert under continuous wave (CW) operating condition. In the case of perfectly aligned insert, the maximum temperature of the cooling water is below the boiling temperature and the deformations of the insert geometry are significantly small enough not to influence the performance of the gyrotron. Additionally, the operational limits of the insert cooling system are determined with respect to the maximum allowable heat-flux.

After ITER, a DEMOnstration power plant (DEMO) is planned to prove technical feasibility of controlled thermonuclear fusion energy. High-frequency $(\geq 0.2\mathbf{THz})$ , high-power (2 MW) gyrotrons are planned as RF sources for Electron Cyclotron Resonance Heating and Current Drive (ECRH&CD) applications in DEMO. At such a high frequency, the effect of a possible electron beam misalignment could be critical for the gyrotron performance. For this reason, the beam misalignment tolerance has been numerically studied for a 236 GHz, 1.5 MW hollow-cavity gyrotron design using a macro-electron based simulation approach. Spread in the electron perpendicular velocity and the radial width of the electron beam are considered too. The results suggest a stable excitation of the selected TE-52,18-operating mode assuming a maximum electron beam misalignment (D) of 0.225 mm $(\mathbf{D}/1=0.18)$ .

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.

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 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 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.

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.

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.

High frequency (>230 GHz) megawatt-class gyrotrons are planned as RF sources for electron cyclotron resonance heating and current drive in DEMOnstration fusion power plants (DEMOs). In this paper, for the first time, a feasibility study of a 236 GHz DEMO gyrotron is presented by considering all relevant design goals and the possible technical limitations. A mode-selection procedure is proposed in order to satisfy the multi-frequency and frequency-step tunability requirements. An effective systematic design approach for the optimal design of a gradually tapered cavity is presented. The RF-behavior of the proposed cavity is verified rigorously, supporting 920 kW of stable output power with an interaction efficiency of 36% including the considerations of realistic beam parameters.

As part of the EUROfusion project, the conceptual design of a 1 MW 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a DEMOnstration Power Plant (DEMO), along with a 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-steerable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been studied as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2–3 GHz is demonstrated in numerical simulations. A hybrid-type Quasi-Optical Launcher (QOL) has been designed for a step-frequency tunable gyrotron with sufficiently high Fundamental Gaussian Mode Content (FGMC).

A large part of the gyrotron R&D activities at Karlsruhe Institute of Technology focus on addressing challenging requirements posed on gyrotrons by the European concept for a demonstration fusion power plant (EU DEMO). This paper reports on the progress of these activities and on the recent results. Extensive theoretical and experimental investigations are ongoing at Karlsruhe Institute of Technology (KIT) within the Work Package Heating and Current Drive (WPHCD), coordinated by the Power Plant Physics and Technology (PPPT) Department of EUROfusion. The focus is on gyrotron R&D, which is necessary for meeting the needs of the envisaged Electron Cyclotron (EC) heating and current drive system for a European demonstration fusion power plant (EU DEMO) [1].