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One way to improve the mode selection in terahertz gyrotrons is the use of stepped cavity resonators that satisfy the resonance condition for two modes with the same azimuthal but different radial indices. Such sectioned resonators, or complex cavities, were analyzed in detail by several authors, recently and in the past. One common problem for such designs is the excitation of unwanted modes by both the stepped cavity structure and the cavity input and output tapers. Mode conversion becomes especially a severe problem for higher-order resonant modes in highly oversized resonators. In this article, we introduce ways to minimize the unwanted mode conversion in both the radial step of the cavity as well as in the tapered sections of the cavity and the up-taper to the collector and output window. Frequency tuning between 391.5 and 393.5 GHz has been achieved by operation at different axial mode numbers of the mode pair TE8,4/TE8,5. Due to their intrinsic higher output mode purity, the so-called antiphase modes of the complex cavity have been chosen for this output mode purity study. [ABSTRACT FROM AUTHOR]

A multifaceted simulation procedure, addressing the electron beam properties, the beam-wave interaction, and the internal losses, has been used for the simulation of the experimental operation of a 1.5-MW 140-GHz short-pulse preprototype gyrotron. The preprototype is related to the development of 1.5-MW gyrotrons for the upgrade of the electron cyclotron resonance heating system at the stellarator W7-X. A very good reproduction of experimental results has been achieved by simulation, without resorting to arbitrary speculations. This validated the numerical tools as well as the design and fabrication of the short-pulse preprototype, which fully reached the target of efficient 1.5-MW operation in millisecond pulses. Special attention has been given to simulating the possibility of parasitic after-cavity interaction in the gyrotron launcher. Also, parasitic backward-wave excitation in the gyrotron cavity has been demonstrated by simulation, at a frequency and voltage range in agreement with experimentally observed parasitic oscillations. This offers an additional possibility with respect to the origin of deleterious parasitic oscillations in high-power gyrotrons, which are usually attributed mainly to the gyrotron beam tunnel. [ABSTRACT FROM AUTHOR]

An iterative procedure is described, which models the influence of the thermal expansion of the gyrotron cavity on the expected gyrotron performance. It is a multiphysics simulation method, which involves electrodynamic, thermal-hydraulic, and thermo-mechanical simulations. The method is applied to the first European 170-GHz, 1-MW continuous wave prototype gyrotron for the ITER. According to the simulations, a performance reduction of ~15% is expected at nominal operating parameters, because of the thermal expansion of the cavity. Alternative operating points to mitigate this effect are proposed and numerically validated. The numerical results are discussed in light of experimental findings. [ABSTRACT FROM AUTHOR]

The possibility of dynamic after cavity interaction (ACI) in a down-scaled gyrotron model has been studied using a 3-D conformal finite-difference time-domain particle-in-cell simulation. The purpose has been to benchmark this type of simulation against previous numerical results obtained by other methods and tools and to obtain additional physical insight on the effect of dynamic ACI. The benchmarking has been successful, as a good agreement with the older results has been achieved. Thus, the algorithms developed and validated can be used for studying the issue in a larger scale or more realistic model with affordable computational resources in the near future. [ABSTRACT FROM AUTHOR]

The European 1-MW, 170-GHz continuous wave industrial prototype gyrotron for electron cyclotron resonance heating and current drive on international thermonuclear experimental reactor was during 2016 under test at the Karlsruhe Institute of Technology (KIT) test facility. In order to optimize the gyrotron operation, the tube was at first thoroughly tested in the short-pulse regime, with pulses that did not exceed 10 ms, for a wide range of operational parameters. Then, and after proper conditioning of the tube, the operation was extended to longer pulses with duration up to 180 s, which is the maximum pulselength possible at the KIT test facility. In this paper, we present in detail the achievements of the long-pulse experimental campaign. [ABSTRACT FROM PUBLISHER]

A numerical synthesis method for the optimization of launchers with hybrid-type wall surface for high-power fusion gyrotrons is proposed in this paper. The profile of such a launcher is built from a novel combination of a periodic harmonic wall perturbation followed by a mirror-line-type deformation, so we named it hybrid-type launcher. The synthesis method has been tested to design a hybrid-type launcher to transform the TE32,9 mode of the ITER EU 1-MW gyrotron operating at 170 GHz. The simulation results show that the fundamental Gaussian mode content (FGMC) of the millimeter-wave beam at the radiating aperture of the hybrid-type launcher is approximately 99.1%. In comparison with the corresponding harmonically deformed launcher, the simulations results reveal that the launcher with hybrid-type surface contour provides higher conversion efficiency combined with low diffraction loss (0.7%). The computation time for the design of hybrid-type launchers is just half of that for mirror-line launchers. In the case of not very short launchers, hybrid-type launchers can provide high FGMCs of RF beams with low diffraction losses that are comparable with those of mirror-line launchers. [ABSTRACT FROM PUBLISHER]

In order to evaluate the emission inhomogeneity of thermionic gyrotron cathode emitters, a novel definition of emission inhomogeneity and evaluating method from the current–voltage characteristics is described in this paper. Results for three different types of gyrotron oscillators at different emitter temperatures based on experimental data and mathematical treatment are considered in the investigation. The influence of emission inhomogeneity on gyrotron efficiency is numerically calculated using the 3-D codes ARIADNE and EURIDICE. The emitter inhomogeneity upper limits for the distributions of work function, temperature, and electric field are given at the end of this paper. [ABSTRACT FROM AUTHOR]

Coaxial-cavity gyrotrons for electron cyclotron heating in plasma experiments for nuclear fusion can operate with very high-order modes, having reduced mode competition and decreased voltage depression compared with hollow-cavity tubes. However, since exact alignment of coaxial insert and cavity wall can only be ensured up to a certain precision, the effects of misalignment must be properly understood. In this paper, an efficient method is presented to determine the voltage depression on beam electrons for arbitrary misalignment between cavity wall and insert, and for a beam with arbitrary shape and density distribution. The method has been verified using a 3-D code, and it can be generalized to some other geometries. [ABSTRACT FROM AUTHOR]

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

Spurious oscillations, particularly dynamic after cavity interaction (ACI) in high-power gyrotrons gained attention as an influencing factor on the overall gyrotron performance. Numerical investigations are a main pillar for a deeper understanding of this phenomenon. The Karlsruhe Institute of Technology in-house self-consistent, time-dependent self-consistent multimode code was modified to accommodate the influence of an axial variation of the external magnetic field on the after cavity dynamics. Inclusion of the influence of the nonuniform distribution of the magnetic field in the time-dependent gyrotron beam-wave interaction differential equations is necessary to make proper numerical investigations in terms of the appearance of the dynamic ACI particularly in the output waveguide section (uptaper), where the nonuniform distribution of the magnetic field is more significant. We evaluate the importance of this for modeling of three different high-power gyrotron cavities, where partially undesired weak interactions in the up-taper/launcher region have been observed. [ABSTRACT FROM PUBLISHER]

In recent years, the after cavity interaction (ACI) in high-power gyrotrons operating in the 100–200 GHz range gained special attention as an influence factor on overall efficiency. While investigations concentrated on ACI as a stationary effect until now, recent simulations show that undesired interactions in the output waveguide region (uptaper) can also result in additional parasitic oscillations. In this paper, such nonstationary, dynamic processes are investigated in first simulations and results are described using the self-consistent time-dependent Karlsruhe Institute of Technology multimode code. In this paper, results are generated along with the assumption of constant Bz(z)= B0 (maximum magnetic field) as well as with the implementation of the adiabatic approximation in the case of varying magnetic field for determination of the axial velocity of each particle. The simulations show weak parasitic/spurious oscillations in the uptaper of the gyrotron cavity that may be attributed to the occurrence of dynamic ACI. [ABSTRACT FROM PUBLISHER]

Experimental results using a step-frequency tunable D-band gyrotron are reported. The short pulse (\sim3~ms) gyrotron is equipped with an elliptically brazed chemical vapor deposition (CVD) diamond Brewster angle output window. It is designed for the operation in the frequency range from 111.6 up to 165.7 GHz. Operating parameters for ten different frequencies corresponding to an equal number of different cavity operating modes has been measured. A minimum output power of 830 kW and a peak output power of 1.3 MW have been realized. For all frequencies, the parameters of the RF beam generated by the internal quasi-optical converter, such as fundamental Gaussian contents and beam waist, are sufficiently good to allow an efficient coupling of the RF power out of the window. This is the first time a diamond Brewster angle window has been used in a high power gyrotron (\sim1~MW) . Such a system offers the path to a simple and compact window solution for high power broadband applications using gyrotrons. [ABSTRACT FROM AUTHOR]

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

In this paper, the design of a 10-kW/28-GHz gyrotron is presented. The main characteristic of the new gyrotron is the emitter ring, which is assembled from twelve individually supplied emitters based on controlled-porosity reservoir (CPR) technology. The main goal is to evaluate the use of CPR emitters for gyrotron applications. In addition, the azimuthally segmented emitter ring could be used for the generation of controlled nonuniform electron beams. In this way, it is planned to experimentally study the effect of nonuniform emission on the gyrotron operation. In this context, the effect of nonuniform emission on the beam quality is numerically investigated using the 3-D, electrostatic, parallel-code Ariadne, while the effect of the degraded beam on mode coupling and stability of the wave-particle interaction in the cavity is numerically studied using the nonstationary, self-consistent multimode cavity code Euridice. [ABSTRACT FROM AUTHOR]

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

Gyrotron oscillators are millimeter wave sources, capable of reaching megawatt power levels. Such high RF power is required for electron cyclotron resonance heating and current drive systems for current and future nuclear fusion facilities. With total heating system powers in the range up to about 100 MW, these installations call for unit powers above 1 M W, to reduce cost and complexity of the complete heating system. In this paper, a mode selection process for a 4-MW 170-GHz coaxial-cavity gyrotron is presented and stable operating parameters are elaborated. The employed formalism, based on normalized variables, suggests one mode, namely the TE{-52,31}, which is sufficiently separated from its competitors and supports an advanced two-beam quasi-optical mode converter refid="ref1"/. Through the utilization of time-dependent and self-consistent approaches, a coaxial cavity is optimized and stable single-mode operation at 4.35 MW of generated output power with an interaction efficiency of 33% is predicted. This paper discusses the mode selection process under consideration of realistic technical limitations, optimization of the cavity for stable operation with a pure output mode, and finally thermo-mechanical calculations on cavity cooling and surface temperature. [ABSTRACT FROM PUBLISHER]

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

A 2-MW continuous-wave (CW) 170-GHz coaxialcavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC1), a cooperation between European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short-pulse tube (preprototype) is used at KIT Karlsruhe (former FZK) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity, and quasi-optical RF output coupler. Significant progress has been achieved recently. In particular, RF output power of up to 2.2 MW with 30% output efficiency has been obtained in single-mode operation at 170 GHz. Furthermore, a new RF output system has been designed, with an efficient conversion of the generated RF power into a Gaussian RF output beam. The results have been successful, yielding a Gaussian mode content ~96%. [ABSTRACT FROM AUTHOR]

A highly efficient quasi-optical mode converter with a bandwidth of 38 GHz has been designed and tested. The mode converter combines low-diffraction losses and a Gaussian mode content up to 97% for a set of nine modes in the range of 105 to 143 GHz for a 1-MW CW gyrotron. This was achieved using a dimpled-wall waveguide antenna (launcher), one quasi-elliptical mirror, and two toroidal mirrors. The optimization of the launcher was done using coupled-mode theory. The simulation results show a well-focused Gaussian beam for all nine operating modes. The curvature radii of the toroidal mirrors were determined by Gaussian mode transformation (ABCD-law) and subsequently optimized for a multimode operation. The simulations of the quasi-optical mode converter are based on the electric field integral equation and, thus, are 3-D. Experimental low-power measurements show close agreement with predictions. [ABSTRACT FROM AUTHOR]

The RF behavior of a 42-GHz 200-kW continuous-wave (CW) gyrotron for plasma heating in a small experimental tokamak is presented in this paper. The operating mode, as a matter of technical convenience, is the TE0,3 mode, as is required by the end user. Design constraints are carefully investigated, and starting currents are computed. Cold-cavity and self-consistent calculations are carried out; power and efficiencies are computed for a typical set of beam parameters. In addition, a triode-type magnetron injection gun (MIG) has been designed, and the realistic beam parameters obtained from these MIG calculations are used in time-dependent self-consistent calculations which are carried out before and after space-charge neutralization. These studies on RF behavior suggest that it is feasible to realize CW powers in excess of 200 kW at 42 GHz with TE0,3 as the operating cavity mode. [ABSTRACT FROM AUTHOR]

A quasi-optical mode converter for the European TE22,6 118-GHz gyrotron has been continuously improved for long pulse operation. Measurements at the last generation show a double peak structure at the output window plane, but simulations have not been able to explain this behavior in the past. The overall output system was analyzed carefully. Besides measurements, the first full 3-D electromagnetic analysis of a complete quasi-optical mode converter was performed. This gives the indication of the double-peak structure at the output window plane. An enhancement of the system for a single-peak Gaussian-like output beam will be presented. [ABSTRACT FROM AUTHOR]