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Future fusion power plants will require for plasma heating and noninductive current drive high-power gyrotrons, each of which generates 2 MW of continuous-wave microwave power, while another 2 MW in the spent electron beam is dissipated as heat on the collector wall. In today’s 1-MW continuous-wave gyrotrons, collector coils superimpose an ac (10–50 Hz) magnetic field to sweep the hot spots of spent electrons over a large area. However, to double today’s power, it will also be critical to reduce the hot spot dwell time preventing significant increase of material fatigue. This can be achieved by increasing the sweeping frequency. However, a higher frequency magnetic field would hardly penetrate the metallic collector vessel due to eddy currents. Instead of magnetic field, sweeping with electric fields is proposed for the first time. The presented mechanism is capable to apply several orders higher sweeping frequency to reduce the periodic variation of temperature, and thus, the device lifetime can be extended.

The multistage depressed collector (MDC) is essential to significantly increase the overall efficiency of the gyrotron. To date, a short pulse (SP) MDC prototype system has been manufactured and is currently under test. The long-term goal that makes the most sense is to use MDCs on 2-MW fusion gyrotrons in continuous wave (CW) operation. To achieve this, a CW MDC system based on the same physical principle as the SP system is designed and analyzed for the first time. The highlights of this design are the sweeping systems to keep the thermal loading within an acceptable limit when the MDC is operated in CW. In the present analysis, the expected performance and size of the new CW MDC design are compared with that of the SP MDC prototype and with the corresponding single-stage depressed collectors (SDCs) as a reference. In addition, the effects of thermal expansion in CW operation are analyzed and discussed.

The second variant of the electron cyclotron heating and current drive system in DEMO considers the deployment of 2 MW power Gaussian microwave beams to the plasma by frequency steering. Broadband optical grade chemical vapor deposition diamond windows are thus required. The Brewster-angle window represents the primary choice. However, in the case of showstoppers, the double-disk window is the backup solution. This window concept was used at ASDEX Upgrade for injection of up to 1 MW at four frequencies between 105 and 140 GHz. This paper shows computational fluid dynamics conjugated heat transfer and structural analyses of such a circumferentially water-cooled window design aiming to check whether it might be used for DEMO microwave beam scenarios. This design was then characterized with respect to different parameters. Temperature and thermal stress results showed that it is a feasible window solution for DEMO, but safety margins against limits shall be increased by introducing design features able to make the fluid more turbulent. A first design change is proposed, showing that, in combination with a higher inlet flow rate, the maximum temperature in the disks can be reduced from 238 to 186 °C, leading, therefore, to lower thermal gradients and stresses in the window.

The existing KIT 2 MW 170 GHz coaxial-cavity short-pulse pre-prototype gyrotron is being prepared to be upgraded towards multi-frequency operation at 170/204 GHz in the new FULGOR test facility. Therefore, each existing component was examined for the need for modification. A new anode, an elongation of the coaxial insert, the XY -table, and the HV oil tank have to be modified for the first operation. All required components are already manufactured and are prepared for assembly. In the first short-pulse experiments an RF output powers of 2.2 and 1.7 MW can be expected at 170 and 204 GHz, respectively. In a second experimental campaign the coaxial-cavity and the quasi-optical output system will be replaced for an optimized configuration achieving more than 2 MW RF output power at both frequencies.

In the previous divertor campaign, the Wendelstein 7-X (W7-X) device injected 3.6 MW of neutral beam heating power allowing for the achievement of densities approaching 2 × 1020 m−3, and providing the first initial assessment of fast ion confinement in a drift optimized stellarator. The neutral beam injection (NBI) system on W7-X is comprised of two beam boxes with space for four radio frequency sources each. The 3.6 MW of heating reported in this work was achieved with two sources in the NI21 beam box. The effect of combined electron-cyclotron resonance heating (ECRH) and NBI was explored through a series of discharges varying both NBI and ECRH power. Discharges without ECRH saw a linear increase in the line-integrated plasma density, and strong peaking of the core density, over the discharge duration. The presence of 1 MW of ECRH power was found to be sufficient to control a continuous density rise during NBI operation. Simulations of fast ion wall loads were found to be consistent with experimental infrared camera images during operation. In general, NBI discharges were free from the presence of fast ion induced Alfvénic activity, consistent with low beam betas. These experiments provide data for future scenario development and initial assessment of fast-ion confinement in W7-X, a key topic of the project.

The optimized superconducting stellarator device Wendelstein 7-X (with major radius , minor radius , and plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of with central electron temperatures were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of a temperature of with was transiently accomplished, which corresponds to with a peak diamagnetic energy of and volume-averaged normalized plasma pressure . The routine access to high plasma densities was opened with boronization of the first wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above line integrated density and central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.

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 and pulse-length of the tube. The ongoing gyrotron development demands a very effective cavity cooling system for optimum gyrotron operation. In this work, the experimental investigation of a mini-channel cavity cooling using a mock-up test set-up is described. The mock-up test set-up will be used to experimentally validate the predictive simulation results and verify the mini-channel cooling performance. It is crucial for validation of the mini-channel cooling properties to determine the amount of the heat load introduced in the cavity wall by an induction heater. In order to estimate that heat load, full 3D electromagnetic simulations have been performed using the CST Studio Suite® software. A suitable calibration factor for the load deposited in the mock-up inner wall is identified after numerical investigation by a 3D thermal model. Calorimetry measurements are performed and the experimental results are compared with the simulation results obtained with a 3D thermal-hydraulic model, using the commercial software STAR-CCM+. When the calibration factor is applied, the experimental calorimetry is well reproduced by the simulations.

The key for a significant increase of the gyrotron efficiency is the development of an efficient multistage depressed collector (MDC) for the annular spent electron beam. During the past years, many different design approaches based on $E \times \mathrm{B}$ drift concept have been theoretically investigated at KIT. The next step towards the experimental validation of such an MDC is the development of a prototype. The complexity of the mechanical design of the MDC is strongly dependent on the size of the electrodes, the manufacturing possibilities of individual parts, the electric field distribution, etc. Considering all those factors, an MDC system has been optimized in order to significantly reduce the manufacturing complexity of the prototype. As a result, a significant smaller and simpler conceptual design for the MDC system is presented.

The fundamental behavior of the W7-X island divertor under detached conditions, which has been theoretically predicted with the EMC3-Eirene code, is re-examined here under the experimental conditions achieved so far and compared with the first experimental results. Both simulations and experiments cover a range of divertor configurations and plasma parameters, and show the following common trends: (1) with rising impurity radiation, the target heat load decreases ‘uniformly’ over the entire target surface in the sense that both the peak and average heat loads can drop by an order of magnitude. Impurity radiation (mainly from intrinsic carbon) occurs primarily at the plasma edge and the resulting negative impact on the stored energy is less than 10%. (2) When the total radiation exceeds a critical level, the target particle flux (the recycling flux Γrecy) begins to fall and can drop by a factor of 3–5 at high radiation levels without an obvious indication of significant volume recombination. (3) While Γrecy decreases, the divertor neutral pressure continues to build up and reaches a maximum, at which point Γrecy has declined significantly. (4) During detachment, the electron temperature at the last closed flux surface falls in a way that is not quantitatively understandable from parallel classical heat conduction processes. This paper presents a physical explanation of the numerical/experimental results described above. Furthermore, using the EMC3-Eirene code as a diagnostic tool, we are able, apparently for the first time, to provide a full quantitative analysis of each transport channel in the island divertor, aiming to clarify how the island divertor plasma self-regulates to maintain particle, energy, and momentum balance under detached conditions.

An off-the-shelf Phase-Locked Loop (PLL) system is used to stabilize the free-running oscillation frequencies of the 140 GHz, 1 MW CW gyrotrons of the companies Thales, France, and CPI, USA, that both operate at the ECRH system of the Wendelstein 7-X (W7-X) stellarator at IPP Greifswald, Germany. Both tubes are equipped with diode-type Magnetron Injection Guns (MIGs). Considering each of both type of gyrotrons as a Voltage Controlled Oscillator (VCO), each individual gyrotron is controlled by the variation of the body voltage. The results of the experiments demonstrate the capability of the developed and implemented inexpensive off-the-shelf PLL system. It is possible to stabilize the oscillation frequency within a full -20 dB bandwidth of < 100 kHz. Applications such as Collective Thomson Scattering (CTS) systems to accurately diagnose the ions in fusion plasmas could benefit from this development in future.

The European fusion reactor research facility, called International Thermonuclear Experimental Reactor (ITER), is one of the most challenging projects that involves design and testing of hundreds of separately designed reactor elements and peripheric modules. One of the core elements involved in plasma heating are gyrotrons. They are used as a microwave source in electron–cyclotron resonance heating systems (ECRH) for variable injection of RF power into the plasma ring. In this work, the development and application of an alumina-titania 60/40 mixed oxide ceramic absorber coating on a copper cylinder is described. The cylinder is part of a dummy load used in gyrotron testing and its purpose is to absorb microwave radiation generated by gyrotrons during testing phase. The coating is applied by means of atmospheric plasma spraying (APS). The absorber coating is deposited on the inner diameter of a one-meter cylindrical tube. To ensure homogeneous radiation absorption when the incoming microwave beam is repeatedly scattered along the inner tube surface, the coating shows a varying thickness as a function of the tube length. By this it is ensured that the thermal power is distributed homogeneously on the entire inner tube surface. This paper describes a modeling approach of the coating thickness distribution, the manufacturing concept for the internal plasma spray coating and the coating characterization with regard to coating microstructure and microwave absorption characteristics.

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.

In anticipation of an Electron Cyclotron Resonance Heating system upgrade for the stellarator Wendelstein 7-X, a 1.5 MW – 140 GHz continuous-wave gyrotron is under development. In order to provide a first experimental verification of the scientific RF and electron beam optics design of the gyrotron with ms pulses, the Karlsruhe Intitule of Technology has developed a short-pulse pre-prototype gyrotron. In this work, we present details regarding the construction of the pre-prototype as well as measurements from the first experimental campaign delivering up to 1.6 MW in short pulses.

The Collective Thomson Scattering (CTS) diagnostic measures the scattering spectrum of incident radiation off collective fluctuations in plasmas. In Wendelstein 7-X (W7-X) the diagnostic uses a 140 GHz heating gyrotron as a source of the probing radiation. At this frequency, the CTS spectra are heavily affected by the electron cyclotron emission, and the microwave beam propagation is restricted at typical W7-X plasma parameters. The diagnostic was successfully commissioned in the last experimental campaign and demonstrated ion temperature measurements. However, the signal-to-noise ratio was too low for measuring other quantities such as the fast-ion velocity distribution function or the fuel ion ratio. Currently, the W7-X CTS diagnostic is undergoing an upgrade to a frequency of 175 GHz. This will increase the sensitivity of the diagnostic, since the noise due to electron cyclotron emission will be reduced, and it will relax the constraints on microwave beam propagation in W7-X. Here we present the salient features of the upgraded CTS system and discuss its prospects for both thermal-ion and fast-ion measurements.

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.

The KIT 170 GHz – 2 MW longer-pulse coaxial-cavity gyrotron is continuously upgraded with the goal to reach 1 s pulses with a modular construction. In this work, we present the latest experimental results that were achieved after modifications on the mirror-box, the collector and the cooling system of the gyrotron. In particular, it was possible to demonstrate pulses with output power up to 2 MW and pulse lengths up to 50 ms.

The 10 MW electron cyclotron resonance heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) currently relies on the successful operation of continuous wave (CW) 1 MW, 140 GHz gyrotrons which have chemical vapor deposition (CVD) diamond output windows cooled by the industrial silicon oil Dow Corning 200(R) 5 cSt. The window features a 1.8 mm thick diamond disk brazed to two copper cuffs with an aperture of 88 mm, which are then integrated in a steel housing. In the context of the upgrade of the ECRH system towards higher microwave power, this gyrotron design has been significantly advanced to fulfill the requirement of 1.5 MW CW operation, still at 140 GHz. A prototype of this new gyrotron is under development at Thales, France. This paper reports the computational fluid dynamics (CFD) conjugated heat transfer and structural analyses of the diamond window performed using the commercial code ANSYS V19.2 to investigate its performance at 1.5 MW operation. Furthermore, sensitivity studies were also carried out with respect to the absorbed power in the disk and the mm-wave beam radius at the window location. These analyses showed that the window design of the existing 1 MW gyrotrons still works quite well at higher power operation, thus verifying the performance of the window. Even in the worst case scenario of 1.5 kW absorbed power, the maximum temperature of 215 °C at the disk center can be safely accepted, being below the conservative limit of 250 °C for CVD diamond. In addition, the non-axial symmetric thermal gradients due to the geometry of the cooling channels lead to thermal stresses in the disk and the cuffs. However, they are much lower than the limits. The copper cuffs experience plasticity deformation in the region of the interface with the diamond disk up to a value of about 1.5 mm.

The generation of a specific high-order mode with excellent mode purity in a highly oversized cylindrical waveguide is mandatorily required for the verification of high-power components at sub-THz frequencies. An example is the verification of quasi-optical mode conversion and output systems for fusion gyrotrons. A rotating high-order mode can be excited by taking a low-power RF source (e.g. RF network analyser) and by injecting the RF power via a horn antenna into a specific adjustable quasi-optical setup, the so-called mode generator. The manual adjustment of the mode generator is typically very time-consuming. An automatized adjustment using intelligent algorithms can solve this problem. In the present work, the intelligent algorithms consist of five different mode evaluation techniques to determine the azimuthal and radial mode indices, the quality factor, the scalar mode content and the amount of the counter-rotating mode. Here, the implemented algorithms, the design of the computer-controlled mechanical adjustment and test results are presented. The new system is benchmarked using an existing TE28,8 mode cavity operating at 140 GHz. In addition, the repeatability of the algorithms has been proven by measuring a newly designed TE28,10 mode generator cavity. Using the described advanced mode generator system, the quality of the excited modes has been significantly improved and the time for the proper adjustment has been reduced by at least a factor of 10.

The proper alignment of the electron beam with the interaction cavity is essential in order to achieve efficient operation of modern high-power, high-frequency gyrotrons. However, tolerances, manufacturing, and magnetic field alignment errors may lead to the less optimal positioning of the electron beam in the gyrotron cavity. In this paper, we describe in detail the in-house design and development of a simple and low-cost prototype gyrotron-alignment XY-table, which can be used for the optimization of the alignment of the electron beam with the cavity axis. The efficient operation of the device is verified experimentally by operating the European ITER 170-GHz, 1-MW short-pulse prototype gyrotron in the Oxford Instruments superconducting magnet that is available at the Karlsruhe Institute of Technology gyrotron test-stand and is equipped with a set of dipole coils.

A quasi-optical mode converter is under development for an 170/204 GHz coaxial-cavity gyrotron at KIT. It is operated in the TE$_{\mathbf {34,19}}$ mode at 170 GHz and the TE$_{\mathbf {40,23}}$ mode at 204 GHz. A mirror-line launcher should be used for such modes with the ratio of caustic to launcher radius of approximately 0.32. The optimum value of the launcher radius has been found to allow for a high Gaussian-mode content at both frequencies.

The KIT 170 GHz $\mathbf{TE}_{\pmb{34,19}}$ -mode coaxial-cavity gyrotron has been studied for an upgrade towards a dual- or even triple frequency operation at 170/204/(238) GHz. For this reason, the electron magnetron injection gun (MIG), the cavity and the launcher are simulated for operation with the $\mathbf{TE}_{\pmb{40,23}}$ -mode and $\mathbf{TE}_{\pmb{48,26}}$ -mode at 204 GHz and 238 GHz, respectively. A modification of the coaxial-cavity midsection length leads to an increase of the theoretical RF output power from 1.6 MW to around 2.1 MW at 204 GHz. Further, first experimental cold tests using a mode generator setup, show a successful excitation of the $\mathbf{TE}_{\pmb{40,23}}$ -mode, which is the highest-order mode ever excited.

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 %.

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 reliability of gyrotron operation is significantly decreased approaching the individual maximum output power of the tube due to loss of the nominal operating mode. For the first time, this paper proposes an algorithm for an automated mode recovery (MORE) for gyrotrons exploiting the hysteretic gyrotron behaviour. The algorithm has been implemented in a field-programmable gate array (FPGA) controlling the acceleration voltage and is able to recover the nominal operating mode within ≤1 ms after a mode switch to the competing satellite mode. This allows the gyrotron to be operated closer to its stability limits with extended pulse lengths at potentially higher output power. Dedicated experiments to test MORE were conducted at Wendelstein 7-X (W7-X) with two gyrotrons using a beam dump. The nominal mode could be successfully recovered in 99% of 3755 modeloss events during 128 shots up to 20 s. MORE was operational for nine out of ten gyrotrons during the last experimental campaign OP1.2b of W7-X. The overall success rate during W7-X OP1.2b was 91% counting 464 modeloss events in 131 shots performed with seven gyrotrons. Using the mode losses in the working point plane, the cutoff region for the nominal working mode was identified, defining a minimum cathode current for a given acceleration voltage. The total achievable output power of the W7-X ECRH plant could be increased by at least 500 kW for the same pulse length using MORE, assuming a conservative increase of at least 50 kW per gyrotron. Comparing the output power for the same achievable pulse length and reliability level, the output power increase per gyrotron is likely in the order of 100 kW. Since MORE exploits the hysteretic gyrotron behaviour, it could be applied to other gyrotrons of already existing or future ECRH facilities of fusion experiments, like ITER.

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.

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 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 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).

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.

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.

The development of mode conversion waveguides (launchers) for high power gyrotrons has gone through three stages at KIT. Formerly, harmonically deformed launchers have been used in the series gyrotrons developed for the stellarator W7-X. In 2009, a numerical method for the analysis and synthesis of mirror-line launchers was developed at KIT. Such a launcher with adapted mode-converting mirrors for a 2 MW TE34,19- mode, 170GHz coaxial-cavity gyrotron has been designed and tested, and also a mirror-line launcher for the 1MW EU ITER gyrotron has been designed. Recently, based on the Helmholtz-Kirchhoff integral theorem, a novel numerical method for the synthesis of hybrid-type gyrotron launchers has been developed. As an example, TE32,9 mode launchers operating at 170GHz that have been designed using the three different methods are being compared.

The physics requirements of the heating and current (H&CD) systems in a Demonstration Fusion Power Plant (DEMO) are often beyond the actual level of design maturity and technology readiness required. The recent EU fusion roadmap advocates a pragmatic approach and favours, for the initial design integration studies, systems to be as much as possible, extrapolated from the ITER experience. To reach the goal of demonstrating the production of electricity in DEMO with a closed fuel cycle by 2050, one must ensure reliability, availability, maintainability, inspectability (RAMI) as well as performance, efficiency and optimized design for the H&CD systems. In the recent Power Plant Physics & Technology (PPP&T) Work Programme, a number of H&CD studies were performed. The four H&CD systems Neutral Beam (NB) Injection, Electron Cyclotron (EC), Ion Cyclotron (IC) and Lower Hybrid (LH) were considered. First, a physics optimization study was made assuming all technologies are available and identifying which parameters are needed to optimize the performance for given plasma parameters. Separately, the (i) technological maturity was considered (e.g. 240 GHz gyrotrons for EC) and (ii) technologies were adapted (e.g. multi-stage depressed collector for EC) or (iii) novel solutions (e.g. photo-neutralization for NB or new antennae concepts for IC) were studied to overcome the limitations of the present H&CD systems with respect to DEMO requirements. Further constraints imposed by remote maintenance or breeding blanket interactions were considered. © 2015 Max-Planck-Institute for Plasma Physics. Published by Elsevier B.V. All rights reserved.

The design of the ITER electron cyclotron launchers recently reached the preliminary design level - the last major milestone before design finalization. The ITER ECH system contains 24 installed gyrotrons providing a maximum ECH injected power of 20 MW through transmission lines towards the tokamak. There are two EC launcher types both using a front steering mirror; one equatorial launcher (EL) for plasma heating and four upper launchers (UL) for plasma mode stabilization (neoclassical tearing modes and the sawtooth instability). A wide steering angle range of the ULs allows focusing of the beam on magnetic islands which are expected on the rational magnetic flux surfaces q=1 (sawtooth instability), q=3/2 and q=2 (NTMs). In this paper the preliminary design of the ITER ECH UL is presented, including the optical system and the structural components. Highlights of the design include the torus CVD-diamond windows, the frictionless, front steering mechanism and the plasma facing blanket shield module (BSM). Numerical simulations as well as prototype tests are used to verify the design (C) 2013 Karlsruhe Institute of Technology (KIT). Published by Elsevier B.V. All rights reserved.