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We present the results of the development of a highly stable gyrotron (with long‐term stability of an operating frequency of 170 GHz being equal to 10−9) for experiments on frequency locking and control of megawatt power sources. The electron‐optical and electrodynamic systems of the gyrotron are calculated and optimized, including the magnetron‐injection gun, interaction space, collector, and quasioptical converter of radiation into a Gaussian wave beam. Main requirements for such systems are described along with their specific features, which allow one to develop a relatively simple, low cost, and reliable master generator for synchronization of a large number of continuous‐wave megawatt gyrotrons. This reveals new possibilities for creation of gyrotron complexes capable of generation coherent radiation of nearly unlimited power levels. [ABSTRACT FROM AUTHOR]

This article presents a design of the triode coaxial magnetron injection gun (CMIG) for a 170-GHz MW-class gyrotron. The trade-off equations have been used to initialize the design of gun geometry and beam parameters. The operating mode of gyrotron is TE34,11 at fundamental harmonic. The 3-D electron tracing code CST Particle Studio has been employed to simulate and optimize the CMIG design. An optimum electron beam with transverse to longitudinal velocity ratio $\alpha = 1.3 $ and transverse velocity spread $\delta \beta _{\bot }= 3.29\%$ has been obtained. The sensitivity analysis of electron gun parameters are also investigated including geometric deviations and misalignments. [ABSTRACT FROM AUTHOR]

Equi-distantly distributed multi-peak frequency spectrum was observed in a high-power 300-GHz band gyrotron for the case of strong reflection off the vacuum window. An oscillation mode different from the design mode can oscillate by adjusting the magnetic field in the cavity. However, a large fraction of the output wave is reflected by the vacuum window. The reflected wave returns to the cavity through an internal mode converter. The azimuthal rotating direction of the reflected wave is opposite to that of the oscillating mode. A theoretical calculation of mode competition between the oscillating co-rotating mode and the counter-rotating mode originating from the reflected wave very well reproduces the observed frequency spectrum. The multi-peak frequency spectrum stems from periodic amplitude oscillation caused by the mode competition between the co-rotating mode and the counter-rotating mode. The frequency interval between the peaks is not simply decided by the round-trip time between the cavity and the vacuum window but determined by the period of the amplitude oscillation. [ABSTRACT FROM AUTHOR]

This paper introduces the modeling, simulation, and fabrication of the electron optic system (EOS) for high‐power 105 GHz gyrotron, which includes single anode magnetron injection electron gun, magnetic system, and the collector. Based on the specific research project and the theory of electron optics, the structure with detailed parameters is modeled and simulated by using two different electromagnetic analysis softwares. The simulation results show that the electron beam velocity ratio is around 1.3, and the electron landing regions from the two softwares are almost agreed with each other. At last, the components of the EOS are fabricated for the next assembling step. [ABSTRACT FROM AUTHOR]

This article presents the effect of significant up-tapering of the resonator midsection on diffractive quality factor, resonant frequency, axial field profile shift, and axial separation between peaks of higher order axial modes (HOAMs) in a gyrotron resonator. Significant up-tapering of the resonator midsection causes the quality factor curves of HOAMs to come close to each other and eventually crossover. This allows HOAMs to have comparable or even higher quality factor value than the lower axial modes—a situation which is not possible in a conventional resonator. Due to this, the operating current for all axial modes can keep an order of magnitude. The up-tapering also shifts the axial field profile toward the output section of the resonator and leads to the loss of spatial symmetry that exists in the axial separation between the peaks of HOAMs in a conventional resonator. Computations have been performed for 0.2-, 0.5-, and 1-THz resonator using single-mode cold cavity solver code developed at CSIR-CEERI. Numerical validation with published results, for 0.2- and 0.5-THz conventional resonators, has also been carried out. [ABSTRACT FROM AUTHOR]

A sub-THz, second harmonic, multifrequency gyrotron, Gyrotron FU CW GVII, which radiates Gaussian beams, was developed. This device was designed to change the frequency in a stepwise manner in the range from 270 to 420 GHz. In this paper, we present the first experiments conducted using this gyrotron. Nine modes were selected for this study. The oscillation frequency of the selected modes was measured using a heterodyne system. The profiles of the Gaussian beams converted from the modes were assessed using an infrared camera, and the power of these beams was measured using a power meter. The oscillations of the nine second harmonic modes were successfully measured, and Gaussian beams converted from seven oscillation modes were observed. Four second harmonic modes, TE8,5, TE6,5, TE5,4, and TE2,6, oscillated in a single mode. The power of the beams ranged from 0.2 to 77 W. Even in the case of spurious oscillation of fundamental modes, the beams converted from the second harmonic modes could be separated from the fundamental mode beams by taking advantage of the different radiation directions emitted through the gyrotron window. [ABSTRACT FROM AUTHOR]

Second harmonic W-band gyrotron with room-temperature solenoid experimental results is presented. The solenoid consumes a power of ~15 kW with a cooling water flow rate of 15 L/min and an inlet water temperature of 25 °C. The gyrotron interaction mode is TE02, and with the use of Vlasov mode converter, the output mode is Gaussian-like mode. The gyrotron produces 33-kW output power in short pulses with an efficiency of 23%, and in long pulses 10 kW with an efficiency of 14% (power supply limited). The gyrotron was successfully tested for pulse duration over than 10 s. [ABSTRACT FROM AUTHOR]

A medium-power broadband-tuning terahertz (THz) gyrotron operating at low voltages is especially attractive for applications in biomedicine. This article tends to elaborate on the challenges related to startup oscillation and ohmic loss suppression of an extremely low-voltage THz gyrotron. High interaction efficiency is achievable by applying long transit time of electrons to compensate the unfavorable decline of the beam-wave coupling strength for low-energy electron beams. A cavity structure which favors backward-wave interaction is proposed to extend the frequency tuning range. The theoretical study predicts that the optimal cavity contributes to the robust frequency tunability at low voltages. The results indicate that a continuous tuning range of 10.7 GHz around the 330-GHz band with output power higher than 1 W can be expected by utilizing a 0.3 kV, 0.5 A helical electron beam. [ABSTRACT FROM AUTHOR]

To generate high-power continuous-frequency tunable sub-terahertz radiation, a quasi-optical confocal waveguide is attempted as the interaction structure of a gyrotron. In experiments, a gyrotron operated in quasi-optical TE06 mode generated a maximum output power of 20 kW when driven by a 50-kV, 3.4-A gyrating electron beam, and achieved a continuous-frequency tuning band of 205.66–208.97 GHz by adjusting the operating magnetic field. The design, experimental and particle-in-cell simulation results of this gyrotron are presented. [ABSTRACT FROM AUTHOR]

This article proposes a technique of flat-top magnetic field for the goal of high-frequency gyrotron with a pulse duration of several hundred milliseconds. A 14.8-T flat-top pulsed magnet is designed for an 800-GHz second harmonic gyrotron. The magnet is energized by a battery bank with a bypass circuit to achieve a 620-ms pulse duration. To fulfill the requirements of the gyrotron and magnet power supply, the genetic algorithm NSGA-II has been used in the multiobjective optimization of the field distribution and power dissipation of the magnet. The Helix magnet with the layered reinforcement technique is introduced to address the problems of stress and temperature rise. The proposed scheme in this article can be easily extended to most pulsed magnets for gyrotrons. [ABSTRACT FROM AUTHOR]

As an important parameter, start current reveals the excitation condition of cavity modes. Normally, the method of successive iterations with one mode associated in the calculation is used to get the start current, which just reflects the self-excitation conditions in the presence of single mode. Nevertheless, in many cases, the prior-excited mode may affect the start current of the subsequent ones. In this article, we re-derived electron motion equations with two modes involved in the calculation and calculated the start current numerically based on those equations. We analyzed start currents of operation mode TE26+ and parasitic modes of a 394-GHz gyrotron with the two methods. In the calculation using the conventional method, TE26+ satisfies self-excitation condition while TE26− does not. In the calculation using the latter method, the start current of TE26− decreases under the influence of TE26+. This nonlinear oscillation of TE26− is further observed in time domain multimode simulation. In practical application, the beam velocity spread, ohmic loss, and high-frequency loss are inevitable. We studied their effects on mode interaction and then optimized the operation parameters. When the equivalent conductivity is $1.7e^{7}~\text {s/m}$ and beam velocity spread is 0.03, the optimized output power is 149.2 W. [ABSTRACT FROM AUTHOR]

The Orbital Angular Momentum (OAM) carried by light beams with helical phase front (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses. The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n, where m > 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/ω, where ω is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/ω = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM. This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume. Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper. [ABSTRACT FROM AUTHOR]

The paper presents a series of model experiments devoted to the influence of a signal reflected from a non-resonant load on the gyrotron operation regime. The non-resonant reflector used in the experiments was in the form of an iris with remote control of its position in the waveguide. The possibility of frequency tuning in the band determined by the Q-factor of the gyrotron resonator and power modulation in a range of up to 20% was demonstrated in the experiment. [ABSTRACT FROM AUTHOR]

The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends, it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters, it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The present paper consists of two parts. In the first part, the peak values of the orbital velocities and their spread are determined in all fractions of an electron beam in a gyrotron with the standard and widened emitters; also, the effect of profiling the anode on characteristics of these electron beam fractions is considered. In the second part, the interaction efficiency of electron beams produced by thin emitter rings is described and the relationship between these efficiencies and orbital-to-axial velocity ratios in these beams is discussed. [ABSTRACT FROM AUTHOR]

The gyrotrons are powerful sources of coherent radiation that can operate in both pulsed and CW (continuous wave) regimes. Their recent advancement toward higher frequencies reached the terahertz (THz) region and opened the road to many new applications in the broad fields of high-power terahertz science and technologies. Among them are advanced spectroscopic techniques, most notably NMR-DNP (nuclear magnetic resonance with signal enhancement through dynamic nuclear polarization, ESR (electron spin resonance) spectroscopy, precise spectroscopy for measuring the HFS (hyperfine splitting) of positronium, etc. Other prominent applications include materials processing (e.g., thermal treatment as well as the sintering of advanced ceramics), remote detection of concealed radioactive materials, radars, and biological and medical research, just to name a few. Among prospective and emerging applications that utilize the gyrotrons as radiation sources are imaging and sensing for inspection and control in various technological processes (for example, food production, security, etc). In this paper, we overview the current status of the research in this field and show that the gyrotrons are promising radiation sources for THz sensing and imaging based on both the existent and anticipated novel techniques and methods. [ABSTRACT FROM AUTHOR]

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz). [ABSTRACT FROM AUTHOR]

The cathode power supply is one of the most important ancillary devices of gyrotrons. Some interesting transient phenomena about the cathode voltage and the cathode current were found in the gyrotron operation in the electron cyclotron (EC) resonance heating system of the Experimental Advanced Superconducting Tokamak. The cathode voltage drops to about 10% of the original value in about $90~\mu \text{s}$ in the overcurrent case, which is much longer than the $25~\mu \text{s}$ in the normal case. In order to explain these phenomena, an equivalent circuit model of the magnetron injection gun was proposed. The equivalent circuit is composed of parallel resistors and capacitors, and it can explain the test results very well. Using the equivalent circuit model to analyze gyrotrons can be an effective means of developing and operating an EC heating system. [ABSTRACT FROM AUTHOR]

We discuss a hybrid Free Electron Laser (FEL) architecture operating with a RF undulator provided by a powerful Cyclotron Auto-Resonance Maser (CARM). We outline the design elements to operate a compact X-ray device. We review the essential aspects of wave undulator FEL theory and of CARM devices.

IEEE 802.11 Wi-Fi networks are increasingly becoming popular for its use in industrial applications. With the availability of recent amendments to IEEE 802.11 series of standards, particularly IEEE 802.11n, the adoption of Wi-Fi networks for process automation is gaining more focus and importance. The installation of Wireless networks naturally provides reduction in cable and its maintenance related costs, provides increased flexibility and mobility to enhance performance of industrial control system. The IEEE 802.11n supports parameterization that can be set for particular industrial applications and hence it has addressed to the aspects of timeliness and criticality to some extent. This paper proposes the use of IEEE 802.11n network to interconnect field instruments with Siemens PLC controller in harsh EMI/EMC environment. An application example is shown where the alternate control system architecture is developed in which non-critical and non-safety signals are communicated over Wi-Fi. While, for critical and safety signals, traditional hardwired signals methods can be implemented.

The paper explains how to solve the Gyrotron equation system in the Single Mode Time Dependent Approach. In particular, we point out problems encountered when solving these well-known equations. The starting current estimation approach a using time model is suggested. The solution has been implemented in the Matlab code, which is attached to the article.

The operation of a gyrotron requires a strong, stable magnetic field over time. Working in the CW mode requires complex/cryogen solenoid cooling systems. Working in the pulse mode is possible with a simple solenoid, but only for short pulses, typically up to 50 $\mu \text{s}$. In this article, an intermediate working point for the gyrotron is presented: a pulsed gyrotron with a simple uncooled solenoid, which is capable of producing relatively long pulses of ~10 ms. To power a longer pulse with a simple solenoid, a supercapacitor (SC)-based system has been designed, which enables the extension of the operation of pulsed gyrotrons to tens of milliseconds, while keeping the system simplicity, without a cooling system for the solenoid. The setup was realized and a 27-ms current pulse of 1.1 kA was obtained through a gyrotron with a simple copper solenoid with a stability of ~0.3%. A 5-kW/10-ms radiation pulse at 28 GHz was measured from the gyrotron. This setup enables the use of a rather simple solenoid and a gyrotron and still obtains a rather long radiation pulse. This concept may open a new regime for pulsed gyrotrons. [ABSTRACT FROM AUTHOR]

The lack of high-power terahertz sources is a major impediment to the development of terahertz science and technology. The gyrotron has great potential to generate high-power terahertz radiation, and so attracts extensive attention. In order to alleviate the problems of limited power capacity and wall loading, the high-order modes are often chosen to be the operating modes. Due to the dense spectrum of the modes, it will cause intense mode competition, which is harmful for the stable operation of a gyrotron. Currently, a 0.68-THz second harmonic gyrotron is under development in the Terahertz Research Center of University of Electronic Science and Technology (UESTC). The experiment shows that the measured output power is 2.75 kW with the efficiency of 9.05%. In order to determine whether the operating mode is the desired mode TE−15,2, the calculation of the starting current should be more accurate. In this article, we generalize the existing linear theory for the starting current, considering the effects of beam thickness and velocity spread. The starting currents of the operating mode and its competing modes are calculated using our designed gyrotron. We find out that the velocity spread and the beam thickness have significant influence on the starting current. The calculations also show that the desired mode TE−15,2 can be triggered first at the operating point. Besides, the measured output power from the experiment is much lower than that of the simulation result. Therefore, the effects of the velocity spread and beam thickness on beam–wave interactions are also analyzed; our results show that the efficiency is sensitive to the beam quality. [ABSTRACT FROM AUTHOR]

Gyrotrons are high-frequency, high-power microwave sources with potential applications in plasma generation, energy generation, spectroscopy, and security. The Department of Science and Technology initiated the development of a 42 GHz, 200 kW, continuous wave (CW) gyrotron for the Indian Tokamak system through a multi-institutional project. The activity is targeted toward indigenous development of India’s first high-power gyrotron. In gyro-devices, frequency of operation is determined by the cyclotron frequency, and thereby the magnetic field. The development of a high-field intensity normal conducting cavity magnet was an imperative step during the testing and integration of the gyrotron tube assembly to achieve the desired beam transmission with a moderate operating cost. A high and spatially homogeneous magnetic field (1.61 T) is required to be established in the beam–wave interaction region by the cavity magnet. The desired magnetic field, as a function of axial distance from the gun to the collector, is governed by the requirements of gun optics, focusing field in the interaction region, and the adiabatic expansion of the beam into the collector. A water cooled electromagnet has been designed to produce the requisite magnetic field in the cavity region over an axial length of 40 mm with field homogeneity better than ±0.35%. Magnet design was optimized to meet the field requirements in the cavity and cathode gun. Magnetic field simulations were carried out in conjunction with the electrostatic field and space charge analysis. This paper presents finite element computations, design optimization, development and testing of a continuous duty iron core electromagnet for a first harmonic interaction mode (TE03) gyrotron cavity. Magnetic, electrical, thermal, and hydraulic design of the electromagnet is discussed in this paper. [ABSTRACT FROM AUTHOR]

A 42-GHz, 200-kW gyrotron was developed and tested for the Indian plasma fusion system. The gyrotron was designed for the TE0,3 operating mode. The configuration of the triode-type magnetron injection gun (MIG) with the linear RF output, undepressed collector, and face-cooled double disk window was adopted in this gyrotron because of the first development experience. The complete tube was developed and processed at Central Electronics Engineering Research Institute (CEERI), Pilani, India. The RF testing of the gyrotron was performed at the participating institute, Institute of Plasma Research (IPR), Gandhinagar, India. Due to some cooling issues in the collector region, the tube was tested by a resistive capacitor network-based power supply for a very short pulse in place of a long pulse, high voltage power supply. The tube was tested up to the beam voltage of 51 kV and a beam current of 12 A at the cavity magnetic field of 1.61 T for the short pulse of $500~\mu \text{s}$. The tube was inserted into the magnet system consisting of a main electromagnet, auxiliary gun coils, and collector magnets. The multi-hole and single hole couplers were used to measure the RF power in the TE0,3 mode and the overall power, respectively. The measured results were also compared with the simulated results and a close agreement of the results was found. The short pulse testing confirms the electrical design and the development approach for the 42-GHz gyrotron tube. [ABSTRACT FROM AUTHOR]

Spatial harmonic method is applied to investigate selectivity properties of longitudinally corrugated waveguides for potential application in second-harmonic gyrotrons. The effect of corrugations on frequencies, ohmic losses, and mode conversion of guiding TE modes is studied in details. Numerical results are presented for operating second-harmonic and competing first-harmonic modes of a 0.4-THz gyrotron with corrugated RF structure. It is shown that longitudinal wall corrugations of proper dimensions can ensure increase in ohmic losses and decrease in beam-wave coupling strength for the first-harmonic modes, while their effect on the operating mode is only slight. This demonstrates improved selectivity properties of corrugated waveguides for second-harmonic gyrotrons. [ABSTRACT FROM AUTHOR]

In this paper, the temperature dependence of microwave absorption in powders of various metal oxides was studied. The powders of aluminum oxide Al2O3, zinc oxide ZnO, tungsten trioxide WO3, and titanium dioxide TiO2 were the test substances. A subterahertz gyrotron setup with a frequency of 263 GHz and an adjustable output power level from watts to 1 kW was used as a radiation source. In the experiments, the samples of the powders were heated by the radiation of the gyrotron, and at the same time, the temperature of the sample was recorded and the power absorbed in the sample was measured by the calorimetric method. The measurements were carried out in a wide range of temperatures: from several tens up to 1200 °C. The results obtained may be of both fundamental and applied interest for researchers engaged in microwave heating of materials. [ABSTRACT FROM AUTHOR]

The paper explains how to solve the Gyrotron equation system in the Single Mode Time Dependent Approach. In particular, we point out problems encountered when solving these well-known equations. The starting current estimation approach a using time model is suggested. The solution has been implemented in the Matlab code, which is attached to the article. [ABSTRACT FROM AUTHOR]

• The anode power supply for the ECRH system on J-TEXT is introduced. • The pulse step modulation technology is used for the design of the anode power supply. • Two types of modules with different output voltage are adopted for the anode power supply. A 105 GHz/500 kW electron cyclotron resonance heating system on J-TEXT tokamak is under development using a diode type GYCOM gyrotron with a single-stage depressed collector. To modulate the output power of the gyrotron, we designed a 33 kV/1 A anode power supply based on the pulse step modulation technology. The power supply consists of 40 modules with a fixed output voltage of 800 V and 10 modules with a fixed output voltage of 100 V, and these modules are connected in series. In this way, the output voltage of the anode power supply can be adjusted accurately without pulse width modulation. Results of the bench test with dummy load indicates that the output capability and the protection time in fault condition of the designed power supply can meet requirements of the ECRH system on J-TEXT. [ABSTRACT FROM AUTHOR]

• Description of enhancements brought to the ITER EU EC Test Facility installed at SPC (CH) to host two new experiments. • EGYC is a grant contracted with F4E in order to complete the development, of the ITER EU gyrotron manufactured by THALES (F). • FALCON is an experiment involving the ITER RU gyrotron installed on the TF to test the ITER EC components in a real environment. A Test Facility (TF) has been designed and installed at SPC to allow for the commissioning of the EU gyrotrons developed in view of their integration to the ITER EC system. The first phase of operation of this TF was dedicated to the development of the EU 2 MW coaxial cavity gyrotron [ 1 , 2 ]. The EU gyrotron development for ITER has been reoriented since then and is presently advancing a 170 GHz/1MW/CW gyrotron developed within the EGYC consortium and manufactured by THALES. At the same time, the TF has been enhanced to host the FALCON project [ 3 ], consisting in the integration of a second 170 GHz/1MW/CW gyrotron delivered by GYCOM. This new experiment is aiming to test the main RF components of the EC Transmission Line (TL) and of the Upper Launcher system to be installed on ITER. This paper describes the technical modifications brought to the existing TF to comply with the requirements of both 1 MW gyrotrons operation. Effort has been put in switching of the auxiliaries from one tower to the other within a short time (<1 week) while maintaining operator safety and equipment protection. A detailed description of the additional implementation and of the strategy followed to share the main auxiliaries (like the HVPS, the cooling system, the control system, etc.) will be presented. Moreover, most of the new equipment, such as the control hardware, have been designed to fulfil ITER site standards. Finally, the preliminary operation results with this reconfigured TF will be presented. [ABSTRACT FROM AUTHOR]

The electron-cyclotron heating system of the TCV tokamak is being enhanced as part of a major upgrade. Two MW-class dual-frequency gyrotrons will be installed. The first has already been delivered and will be fully commissioned on-site by the end of 2018 while the second is due for delivery in 2019. This paper describes the electrical integration of these two gyrotrons. The water-cooling system is also described from the electrical point of view. Results of initial measurements on the first gyrotron are described. Based on long term experience at the Swiss Plasma Center with gyrotrons, giving special emphasis to electromagnetic compatibility, overall electrical design choices will ensure reliable operation of the gyrotrons. [ABSTRACT FROM AUTHOR]

The EC-system of the TCV tokamak is progressively being upgraded with the addition of two MW-class dual-frequency gyrotrons (84 and 126 GHz/2s/1MW). In order to connect the two gyrotrons to the existing low field side and top launchers, new waveguide routing from gyrotron hall to TCV tokamak was designed and dedicated Matching Optics Units (MOU) have been developed. The internal optics of the system have been determined aiming at optimal coupling to the HE 11 waveguides. The laws of quasi-optics were used to find quadratic surfaces to shape an incoming Gaussian beam representative of the gyrotron output into a beam matching the proper field distribution at the waveguide entrance and with HE 11 content compatible with the system requirements. A solution with one flat (movable) mirror and two shaping mirrors was found and characterized with the physical optics code GRASP. The resulting field distribution is then truncated and projected onto the HE 11 component to evaluate the design solution (coupled power at the waveguide entrance >98.4% and HE 11 content >96.8% for both frequencies). The model and the results of this analysis will be presented and compared to a model based on the Rayleigh–Sommerfeld scalar diffraction integral. GRASP was also used to evaluate preliminary misalignment effects in terms of coupled power to the waveguide and the MOU design moved to the manufacturing phase. In parallel to the gyrotron integration and to extend the level of flexibility of the TCV EC-system, a modular closed divertor chamber is developed, requiring the X3 top-launcher to be redesigned. Preliminary antenna conceptual design studies including new curvature to cope with the requirements of modularity and flexibility will be presented. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Wendelstein 7-X (W7-X) experiment is equipped with an Electron Cyclotron Resonance Heating installation consisting of 10 gyrotrons capable of delivering up to 7.5 MW of Electron Cyclotron Wave power at the 140 GHz resonance in the plasma. During the plasma discharge the gyrotron power is delivered in a narrow band of several 100 MHz around the gyrotron frequency. At plasma start-up, i.e. initially in the absence of plasma, the non-absorbed gyrotron power in the vessel ('stray radiation') is very high. During the plasma discharge the stray radiation may also rise depending on the plasma optical depth or due to the challenge to deliver the gyrotron power at the exact required polarisation. At W7-X an additional issue is that at high electron density the second harmonic extra-ordinary heating scheme (X2) is no longer possible and one switches to second harmonic ordinary (O2) heating. But the ECH absorption is not complete in the O2 heating scheme and stray radiation is significantly increased. The stray radiation issue is a problem for sensitive microwave receivers, typically measuring power levels in the sub μ W range. For protection, these generally employ notch filters such as fundamental mode interference cavities [1] , [2]. This paper focuses on the effect of non-absorbed 140 GHz gyrotron power on the Michelson Interferometer at W7-X. [ABSTRACT FROM AUTHOR]

• FPGA-based real-time gyrotron control can block faults and restart RF within 10 ms. • FPGA-based control (D-Wavegen) contributed to 95% reliability for the ECH system. • D-Wavegen improved performance of gyrotron prone to body overcurrent faults. • Collector power redistribution lead to higher power density for modulated operation. In order to increase the reliability and improve the availability of electron cyclotron heating (ECH) and current drive during a plasma shot, an adaptive gyrotron command generator has been designed and implemented in all gyrotron systems on the DIII-D tokamak. This adaptive real-time generator is able to detect imminent faults, including gyrotron body overcurrent faults that can develop in a few microseconds, block faults, and restart RF generation within 10–30 ms, faster than the DIII-D energy confinement time. The adoption of this adaptive generator has substantially improved the performance of a depressed-collector gyrotron with recurrent body current faults, and has assisted the ECH system to achieve a record reliability of 95% during the last experimental campaign. The impact of power modulation on the collector loading of a depressed-collector gyrotron has been experimentally studied using calorimetry and collector maps taken by a diagnostic developed in-house. While lowering the modulation duty cycle kept the collector power constant as a result of low beam-wave efficiency during the modulation low state, an observed power redistribution inside the collector increased the peak collector power density, which reached levels above the manufacturer power density limit for low duty cycles. [ABSTRACT FROM AUTHOR]

• Status of the experiments with the EU 170 GHz, 1 MW CW industrial prototype gyrotron for ITER. • Possible upgrades of the EU 170 GHz, 1 MW SP prototype gyrotron for the enhancement of the tube's efficiency. • First experimental results with a fully metallic beam tunnel. The European Gyrotron Consortium (EGYC) is developing the European 1 MW, 170 GHz Continuous Wave (CW) industrial prototype gyrotron for ITER in cooperation with Thales Electron Devices (TED) and Fusion for Energy (F4E). This conventional, hollow-cavity gyrotron, is based on the 1 MW, 170 GHz Short-Pulse (SP) modular gyrotron that has been designed and manufactured by the Karlsruhe Institute of Technology (KIT) in collaboration with TED. Both gyrotrons have been tested successfully in multiple experiments. In this work we briefly report on the results with the CW gyrotron at KIT and we focus at the experiments at the Swiss Plasma Center (SPC). In addition, we present preliminary results from various upgrades of the SP tube that are currently tested at KIT. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Divertor Tokamak Test (DTT) facility has been proposed in the European roadmap to study solutions to mitigate the issue of power exhaust in conditions relevant for DEMO. The Italian DTT tokamak [1] is being designed to reproduce the optimal divertor magnetic configuration and test a prototype under reactor relevant power flow in the scrape off layer (P SEP /R>15 MW/m). A mix of three heating systems will equip the machine to reach the target value of 45 MW delivered at plasma: Electron Cyclotron Heating (ECH), Ion Cyclotron Heating (ICH) and Negative Neutron Beam Injector (NNBI). The present reference design considers a capability of 20–30 MW of ECH power at plasma to support and assist different tasks. The gyrotron sources (1 MW/170 GHz/100 s) will be based on the depressed collector technology with >40% efficiency and will exploit the experience gained from the solutions developed for ITER. A Transmission Line (TL) with 90% efficiency and >1 MW/beam power handling is being considered, using Quasi-Optical (QO) multi-beam mirrors. The conceptual design of equatorial and upper launchers based on the front steering concept is being developed to reach the required deposition locations for the given application. The EC wave absorption efficiency has been investigated and is presented here with dedicated beam tracing calculations using the GRAY code [ 2 ]. [ABSTRACT FROM AUTHOR]

• The refrigeration of a gyrotron coaxial cavity is investigated. • Annular cooling channel allows operation below T limit for ∼150 ms. • Steady state condition reached in the simulation after ∼ 3 s. • Mini-channels cooling configuration reduces T peak by ∼ 150 °C. 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. [ABSTRACT FROM AUTHOR]

The future EC systems will consist of several gyrotron sources providing MW-level millimeter wave power at a frequency around or above 170 GHz. The development of matched loads is necessary to test the new sources, the components for the transmission lines and the launchers, and must ensure high qualification for compatibility with the nuclear environment. The development at IFP-CNR and LTC of several compact high-power prototypes during the last decade led to a refinement of the overall design, resulting in the present low-reflectivity vacuum-compatible loads. The activity on loads is supported by F4E in view of the development of the EU gyrotron for ITER and required high-power tests at QST (Naka, Japan). Qualification is now supported by new tests at SPC (Lausanne, Switzerland) using the FALCON test-bed designed to test components and the EU gyrotron prototype for the EC system of ITER. These high-power tests, performed on the first CW prototype load (provided with 16 + 16 cooling channels in parallel) and shown in this paper, highlighted the need to improve the mechanical, vacuum and hydraulic design to reach the final goal of 1000s at ∼1 MW. [ABSTRACT FROM AUTHOR]

The ITER ECRH system consists of 24 gyrotrons with up to 24 MW installed millimeter wave heating power at 170 GHz, power supplies, control system, transmission lines, one Equatorial and the four Upper Launchers. With its high frequency and small beam focus the ECRH has the unique capability of driving locally current. While the Equatorial Launcher mainly acts for central heating and current profile shaping, the Upper Launchers aim on suppressing MHD instabilities, especially neoclassical tearing modes (NTM) triggering plasma disruptions. The Upper Launchers inject millimeter waves through a quasi-optical section consisting of three fixed and the front steering mirror set. The eight overlapping beams have focal points optimized for suppression of the q = 3/2 and q = 2/1 NTMs. Several project change requests required the redesign of the Upper Launchers and the connected ex-vessel system. This redesign includes a new boundary geometry of the launchers as well as a newly designed cooling system for the Blanket Shield Module (BSM), a modified flange of the BSM to the structural main frame and a refined optical design. Additionally shield blocks with integrated in-vessel waveguides were added and the closure plate with waveguide and supply line feedthroughs was adapted. Further changes, not all caused by PCRs, include newly designed ex-vessel waveguide components with a reduced aperture and redesigned ultra low-loss CVD diamond windows. Finally several components originally foreseen as off-the-shelf components have become part of the design scope. The new launcher design status is presented with selected results on numerical design validation. [ABSTRACT FROM AUTHOR]

Transmission of power from 42 ± 0.2 GHz gyrotron (TE03 mode) to tokamak or dummy load requires a set of transmission line components. It includes a set of mode converters that converts circularly unpolarised TE03 mode to polarised HE11 mode. The mode conversion sequence is methodised in two steps; first from TE03 to TE01 mode and then from TE01 to TM11 mode. The proposed mode converters performance depends on the parameters such as waveguide radius, beat wavelength, number of ripples, its perturbation amplitude (η) and bending angle. These parameters are numerically optimised and verified in CST microwave studio. TE03 to TE01 mode conversion is realised by the generation of intermediate mode TE02. The mode conversion length for TE03 to TE02 mode converter is6λ0, (where λb is the beat wavelength of corresponding mode conversion) which converts 99.15% of an incident circular TE03 mode power into TE02 mode power. Next, mode converter converts TE02 mode into TE01 mode with 99.06% efficiency along the optimal length of6λb. For TE01 to TM11 mode conversion, two conversion methodologies have been proposed: first using axis arc bend (34.94°) technique to convert TE01 into its analogous degenerate mode with 99.01% conversion efficiency and second using parabolic curve with 90° bend, which gives a conversion efficiency of 97.5%. [ABSTRACT FROM AUTHOR]

This paper proposes an analyzing method for the nonlinear theory of gyrotrons based on the functional analysis theory. This method is computationally efficient and highly practical, and the results have good accuracy as that of the self-consistent theory. Moreover, the stability of the gyrotron can be easily analyzed by the trajectory of the mapped efficiency space based on the fixed point of the functional analysis theory. The validity and effectiveness of the presented method are verified by the results of self-consistent theory and experiments presented in the references, such as typical examples of gyrotrons working at various frequencies and modes. [ABSTRACT FROM AUTHOR]

This paper presents the results of the first experiments demonstrating the possibility of wireless data transmission using a subterahertz band gyrotron signal as a carrier. The output power of the subterahertz gyrotron was modulated by a rapid change in the potential of the currentless anode. The resulting amplitude modulated (AM) signal with the medium power of about 50 W was received by the ultrafast THz detector. Successful transmission of an analog sound signal and transmission of a digital signal at a rate up to 1.5 Mbps over a distance of about 2 m was demonstrated. The investigated approach confirms its potential for data transmission using gyrotron in any frequency band without loss of generality. [ABSTRACT FROM AUTHOR]

The eigenvalue problem is solved for a longitudinally inhomogeneous waveguide with impedance walls. The cross-section method is applied to reduce the problem to an infinite set of ordinary differential equations for amplitudes of basis modes. As a numerical example, a tapered metallic cavity of currently available THz gyrotron is considered. The combined effect of mode coupling (conversion) and ohmic wall losses on electromagnetic properties of the gyrotron cavity is considered and discussed. [ABSTRACT FROM AUTHOR]