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The dispersion properties of surface type electromagnetic waves are studied. The waves are considered to propagate along a slab transition layer located between two infinite homogeneous plasma regions of different particle densities. The wavelength is assumed to be short as compared with the layer width. The waves propagate across a static magnetic field, which is parallel to the layer interface. The influence of the smoothness of the gradient of the plasma particle density within the layer on the surface wave propagation/disappearance is discussed. The conclusions derived in the present paper are of interest in the fields of plasma electronics, nano-technologies, plasma-antenna systems, plasma production, and magnetic confinement fusion. [ABSTRACT FROM AUTHOR]

The method of successive approximations is applied to solve the Maxwell equations in cylindrical plasma waveguide geometry for electromagnetic waves with arbitrary azimuthal wave index and small axial wavenumber. The theory of surface flute waves is used as zeroth approximation. This study generalizes previous investigations whose results are utilized for verification of newly obtained conclusions. The influences of several plasma waveguide parameters such as magnitude and sign of the azimuthal wave index, the width of the dielectric layer between a plasma and a waveguide wall and the magnitude of its dielectric constant, the radii of the plasma column and the metal wall, and the external axial static magnetic field on the wave dispersion properties are analyzed. [ABSTRACT FROM AUTHOR]

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]

The present paper addresses numerical calculations on the eigenvalues of hybrid modes in corrugated circular waveguides with varying diameter and corrugation depth. Such calculations are essential for the numerical optimization of advanced mode converters and diameter tapers for future low-loss high-power microwave applications, like broadband high-power radar sensors for space debris observation in low earth orbit (LEO). Corresponding mode converters and diameter tapers may be synthesized based on coupled mode theory. Of particular importance here is the ability to consider varying mode eigenvalues along the perturbed waveguide. The procedure presented here is able to consider arbitrary variations of the corrugation depth as well as the waveguide diameter and therefore is highly flexible. The required computational effort is low. Limitations of the method are discussed. [ABSTRACT FROM AUTHOR]

The HE11 hybrid mode, propagating in an overmoded corrugated circular waveguide, is widely used for low loss transmission of high-power microwaves. Due to the inherent broadband frequency behaviour, this will be also essential for future broadband high-power radar applications, like space debris observation in low earth orbit (LEO). A promising amplifier concept for such radar sensors is a helical gyro-TWT. However, since the HE11 hybrid mode is not suitable for electron-beam-wave interaction in this kind of vacuum electron device, an additional mode converter is required. The present paper addresses the design procedure of a broadband high-power mode converter, designed for a helical gyro-TWT intended for future broadband high-power radar applications in the W-band. The interaction mode of the helical gyro-TWT under consideration can be easily transferred to the circular waveguide TE11 mode. Therefore, a TE11 ↦ HE11 mode converter is addressed here. The design procedure is based on a scattering matrix formalism and leads to a high HE11 mode content of ≥98.6% within the considered frequency range from 92 GHz to 100 GHz. Inside this frequency band, the mode content is even better and reaches ≈ 99.7% at ≈95 GHz. This allows broadband frequency operation of a helical gyro-TWT and is suitable for broadband high-power radar applications. [ABSTRACT FROM AUTHOR]

To allow antenna movements in azimuth and elevation in high-power radar applications, rotary joints are essential. They allow the rotation of a transmission line and therefore are important transmission line components. In the present paper, a broadband rotary joint concept for high-power W-band radar applications is proposed. To avoid a twist of the polarization plane of a linearly polarized mode, like HE11, a combination of two broadband polarizer is used. A cross polarization of Xpol ≤ − 20 dB can be achieved within the considered frequency range from 90 GHz to 100 GHz. This corresponds to a suitable value for radar applications. [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 magnetron injection gun (MIG) is the most critical part of any gyrotron. Small tolerances in the manufacturing process and alignment of the subcomponents directly affect the electron beam quality and therefore the beam wave interaction. At the Karlsruhe Institute of Technology (KIT), an innovative new Inverse MIG (IMIG) is proposed for the European 2-MW 170-GHz coaxial-cavity gyrotronwhich is under the developmentand test at KIT. The design of the IMIG has been done under strict consideration of the gun design criteria published earlier. In order to find the maximum allowed tolerances of that IMIG which allows operation within the allowed gun design criteria, systematic theoretical studies have been done. Commonly used “conventional” MIGs have been shown that a small emitter, anode, and cathodemisalignment have a significant influence to the electron beam quality. [ABSTRACT FROM PUBLISHER]

Azimuthal surface waves are eigen-electromagnetic waves which propagate in coaxial plasma-vacuum waveguides across an axial stationary magnetic field. Their excitation in the coaxial plasma layer by the flow of electrons which rotate along large Larmor orbits either around or inside the plasma layer has been well studied in the collision-less approach. The influence of weak collisions between the plasma particles on the growth rate of such waves for both cases of the electron beam placement is studied here for the frequency range above the upper-hybrid frequency and compared to the case of the electron cyclotron frequency range. Transition from the regime of beam-plasma instability to that of a beam-dissipative instability is discussed. [ABSTRACT FROM PUBLISHER]

The possibility to generate electromagnetic waves in the frequency range above the upper-hybrid frequency is demonstrated. In this frequency range, a plasma is a nontransparent medium for bulk electromagnetic waves. Effective interaction of an electron beam and extraordinarily polarized electromagnetic waves of surface type is provided by application of a gyrating beam of electrons, which move along large Larmor orbits in the gap between a cylindrical plasma layer and the inner conductor of a coaxial waveguide. These waves are composed of azimuthal and radial components of the electric field and an axial component of the magnetic field. The presence of the azimuthal electric field that is parallel to the electron velocity encourages their effective interaction. The beam excitation is most efficient in the case when the waves propagate in a coaxial waveguide whose parameters correspond to the right limit of the effective wavenumber range. Introduction of the inner metal conductor instead of the internal part of the beam-plasma system does not prevent wave excitation and allows installing there a source for the electron beam production. [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]

Excitation of extraordinarily polarized azimuthal surface eigenwaves is shown to be possible in the frequency range above the upper-hybrid resonance in coaxial waveguides with metal walls, which are partially filled by cold magnetoactive plasmas. Interaction of these waves with the flow of electrons, which rotate around the plasma column in the gap separating the plasma from the waveguide wall, is studied. By employing the numerical methods, it is shown that the conditions of resonant interaction of the electron beam with these high-frequency azimuthal surface waves can be achieved in the case of sufficiently strong external magnetic fields without passing into the regime of the ultrarelativistic velocities of the beam electrons. [ABSTRACT FROM AUTHOR]

Crystallization of lithium disilicate (LS 2 ) glass using 30 GHz microwave processing was investigated. Glass frit was used as the starting material to prepare the glass via the regular melt-quench method. The annealed glass was investigated using X-ray diffraction (XRD) and thermal analysis to ensure the glassy structure formation and to determine the nucleation and the crystallization temperatures. The glass samples were first nucleated conventionally before they were crystallized using microwaves. Characterization of the glass–ceramics was performed using XRD and scanning electron microscopy. The 30 GHz microwave processing was successfully used to crystallize LS 2 glass without the aid of susceptors in a relatively short time at lower temperature compared to conventional heating. The result of this experimental investigation was in close match with the theoretical calculations that were done based on the molecular orbital model of microwave absorption. The microwave absorption in LS 2 glass was highly dependent on the microwave frequency. [ABSTRACT FROM AUTHOR]

Megawatt-class gyrotron oscillators for electron cyclotron heating and non-inductive current drive (ECH&CD) in magnetically confined thermonuclear fusion plasmas have relatively low cavity quality factors in the range of 1000 to 2000. The effective length of their cavities cannot be simply deduced from the cavity electric field profile, since this has by far not a Gaussian shape. The present paper presents a novel method to estimate the effective length of a gyrotron cavity just from the eigenvalue of the operating TE mode, the cavity radius and the exact oscillation frequency which may be numerically computed or precisely measured. This effective cavity length then can be taken to calculate the Fresnel parameter in order to confirm that the cavity is not too short so that the transverse structure of any mode in the cavity is the same as that of the corresponding mode in a long circular waveguide with the same diameter. [ABSTRACT FROM AUTHOR]

Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964. [ABSTRACT FROM AUTHOR]

Progress in the worldwide development of high-power gyrotrons for magnetic confinement fusion plasma applications is presented. Gyrotron oscillators are used for electron cyclotron heating, electron cyclotron current drive, stability control, and plasma diagnostics. After technology breakthroughs in the research on gyrotron components in the 1990s, significant progress has been achieved in the 2000s, in the field of long-pulse and continuous wave (CW) operation for a wide range of frequencies. Currently, the development of 1-MW-class CW gyrotrons for the tokamak ITER (170 GHz), the stellarator Wendelstein 7-X (140 GHz), and the tokamaks DIII-D and JT-60SA (110 GHz) has been very successful in EU, Japan, Russia, and USA. The Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW at 1-h pulse duration). For this progress in the field of high-power long-pulse gyrotrons, innovations such as the realization of high-efficiency stable oscillation in very high-order cavity modes with low ohmic losses, the use of single-stage depressed collectors for energy recovery (efficiency enhancement and simpler power supplies), highly efficient internal quasi-optical (q.o.) mode converters (low level of internal stray radiation), and synthetic diamond windows have essentially contributed. The total tube efficiencies are around 50% and the purity of the linearly polarized fundamental Gaussian output mode is 97% and higher. Power modulation technologies for stabilization of neoclassical tearing modes have proceeded. Future prospects of advanced high-power fusion gyrotrons are in the areas of two- and three-frequency gyrotrons, fast step-wise frequency tuneability, higher unit power (coaxial cavities), and higher frequencies for more efficient plasma stabilization and noninductive current drive as well as reliability, availability, maintainability and inspectability for next step fusion power stations. The GYCOM step-tuneable 1-MW gyrotron for ASDEX Upgrade employing a broadband travelling-wave-resonator window (with two diamond disks) operates at 105, 117, 127 and 140 GHz. The EU 170 GHz coaxial-cavity gyrotron prototype achieved in millisecond pulses the power of 2.1 MW at 46% efficiency and 96% Gaussian mode purity. A new power record of second harmonic oscillation has been achieved in the subterahertz band (83 kW/389 GHz/3 ms) in Japan for application to collective Thomson scattering diagnostics. A fundamental frequency 670-GHz gyrotron with pulsed magnet generated 210 kW in 20–30-\mus pulses at 20% efficiency (collaboration of institutions in Russia and USA). [ABSTRACT FROM PUBLISHER]

Reflection of circular symmetric TE0{n} modes (n= 1–6) at an oversized, open-ended circular waveguide (C76-waveguide, \inner diameter=\27.79 mm, 70 GHz) radiating into free space has been investigated theoretically employing two scattering matrix codes (SMCs), the finite-difference time-domain code EMPIRE and the uniform geometrical theory of diffraction (UTD) as well as the first time experimentally. The measurements utilized mode converters for generation of pure TE0{n} modes and a wavenumber spectrometer for mode analysis in the oversized waveguide. The total power reflection computed by EMPIRE is 4.1 to 13.4 dB lower than calculated from free-space wave and waveguide mode impedances. In all cases, most of the reflected power is carried by the backward traveling TE06 mode, which is the mode closest to cutoff. Experiments are in very good agreement with theoretical results. [ABSTRACT FROM PUBLISHER]

The prototype of the Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW, 3600 s and 1 MW, 800 s) and the efficiency record of 57%, whereas the Russian 170-GHz ITER prototype tube achieved 0.8 MW with a pulse duration of 800 s at 55% efficiency and 1 MW at 280 s and 53%. The record parameters of the European megawatt-class 140-GHz gyrotron for the stellarator Wendelstein W7-X are as follows: 0.92-MW output power at 1800-s pulse duration, nearly 45% efficiency, and 97.5% Gaussian mode purity. These gyrotrons employ a cylindrical cavity, a quasi-optical output coupler, a synthetic diamond window, and a single-stage depressed collector (SDC) for energy recovery. In order to reduce the costs of the ITER 24-MW 170-GHz ECH&CD system, 2-MW millimeter-wave power per gyrotron tube is desirable. Cylindrical gyrotron cavities are not suitable for the 2-MW power regime because of high ohmic wall losses and mode competition problems. However, in coaxial cavities, the existence of the longitudinally corrugated inner conductor reduces the problem of mode competition, thus allowing one to use even higher order modes with lower ohmic attenuation than in cylindrical cavities. Synthetic diamond windows with a transmission capability of 2-MW CW are feasible. A 2-MW CW 170-GHz coaxial-cavity gyrotron for ECH&CD in ITER is under development in cooperation with European research institutions (EGYC, collaboration among the CRPP, Switzerland, the KIT, Germany, the HELLAS, Greece, the CNR, Italy, and the ENEA, Italy). At the Karlsruhe Institute of Technology (KIT), the short-pulse (1-ms) preprototype tube delivered 2.2 MW at 30% efficiency (without SDC) with 96% Gaussian output mode purity. Design studies for a 4-MW 170-GHz coaxial-cavity gyrotron with two synthetic diamond output windows and two 2-MW millimeter-wave output beams for future fusion reactors are currently being performed at KIT. The availability of sources with fast frequency tunability would permit the use of simple fixed nonsteerable mirror antennas for local current drive experiments and plasma stabilization. IAP Nizhny Novgorod develops in collaboration with IPP Garching and KIT an industrial multifrequency 1-MW gyrotron with approximately 50% efficiency (SDC). A four-frequency tube (105, 117, 127, and 140 GHz) delivered 0.8 MW at 105 GHz and 0.95 MW at 140 GHz in 10-s pulses. After the installation of a broadband diamond window, this gyrotron will be operated also at the two intermediate frequencies. [ABSTRACT FROM AUTHOR]

Gyrotrons with output powers of several 100 kW are mainly used as high-power millimeter (mm)-wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for generation of energy by controlled thermonuclear fusion. Other applications include ECR ion sources, materials processing and plasma chemistry, high-resolution Doppler radar, radar ranging and imaging in atmospheric and planetary science, active denial systems, as well as detection of concealed radioactive materials. The present review summarizes the status of recent developments on high-power gyrotrons and introduces this Special Issue on Gyrotrons. [ABSTRACT FROM AUTHOR]

Abstract: The cyclotron autoresonance maser (CARM) operating in a transverse-magnetic (TM) wave by employing a large-orbit relativistic electron beam shows a peculiarity that the growth rate range of coherent amplification of the TM wave is split into two branches as the electrons'' cyclotron frequency increases: the upper branch is quite favorable to terahertz operation, whereas the lower branch can lead to a backward-wave CARM mechanism which is suitable to sub-terahertz operation. Compared to a terahertz large-orbit transverse-electric-wave gyrotrons and gyro-amplifiers which are experimentally realized up to date, a terahertz large-orbit TM-wave CARM requires much lower operating magnetic field and allows the cavity to have larger transverse dimension. [Copyright &y& Elsevier]

A numerical method for the analysis of the fields in highly oversized waveguides is proposed in this paper. This method allows the simulation of the fields on waveguide walls with arbitrary surface deformations in the case that the waveguide is highly oversized, and the wall deformations are shallow and smooth. Combined with the analysis method, an algorithm has been developed for synthesizing the waveguide wall to provide a desired field distribution. As an example, a 309.6-mm-long wave- guide launcher has been designed for a 170-GHz coaxial-cavity gyrotron to transform the TE34,19 cavity mode to a fundamental Gaussian distribution. An efficiency of transformation to the desired fundamental Gaussian mode of 96.3% has been obtained at the launcher aperture, whereas the transformation efficiency is just 86% using a conventional dimpled-wall launcher with a length of 660 mm. [ABSTRACT FROM AUTHOR]

During the last years, electron cyclotron heating (ECH) was proven to be one of the most attractive heating schemes for stellarators because it provides net-current-free plasma startup and heating. Both the stellarator Wendelstein 7-X (W7-X), which is under construction at the Max-Planck-Institut für Plasmaphysik, Greifswald, Germany, and the International Thermonuclear Experimental Reactor (ITER) tokamak, which will be built in Cadarache, France, will be equipped with a strong ECH and current-drive system. Both systems are comparable in frequency and have continuous-wave capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for ITER). The commissioning of the ECH plant for W7-X is well underway; the status of the project and the first integrated full-power test results from two modules are reported and may provide valuable input for the ITER plant. The ten gyrotrons at W7-X will be arranged in two subgroups symmetrically to a central beam duct in the ECH hall. The RF wave of each subgroup will be combined and transmitted by a purely optical multibeam-waveguide (MBWG) transmission line from the gyrotrons to the torus. The combination of five 1 MW gyrotron beams to one beam line with a power of 5 MW reduces the complexity of the system considerably. The single- and MBWG mirrors and the polarizers have been manufactured. Cold tests of a full-size uncooled prototype line delivered an efficiency exceeding 90%. The microwave power will be launched to the plasma through ten chemical-vapor-deposited-diamond barrier windows and in-vessel quasi-optical plug-in launchers, allowing each 1-MW RF beam to be steered independently. The polarization, as well as the poloidal and toroidal launch angles, will be adjusted individually to provide optimum conditions for different heating and current-drive scenarios. [ABSTRACT FROM AUTHOR]

The first series tube of the gyrotrons for the 10-MW electron cyclotron resonance heating system of the stellarator W7-X was tested at Forschungszentrum Karlsruhe (FZK) and yielded a tota output power of 0.98 MW, with an efficiency of 31 % (without a single-stage depressed collector) in short-pulse operation and of 0.92 MW in pulses of 180 s (efficiency of almost 45% at a depression voltage of 29 kV). The Gaussian mode output power was 0.9 ... MW. The pulselength at full power (1 MW) is limited at FZK by the available power supply. At a reduced electron beam current, it is possible to operate at longer pulselengths. At an output power of 0.57 MW (electron beam current of 29 A), the pulselength was increased to 1893 s. There was no physical reason for a limitation of this pulse: The pressure increase during the pulse was less than a factor of two and ended up at a very low value in the 10-9 mbar range. The tube was delivered to Max-Planck-Institut für Plasmaphysik Greifswald for tests at full power and up to 30-min pulselength. The Gaussian mode RF output power, measured in e calorimetric load after a 25-in-long quasi-optical transmission ... ne (seven mirrors), was 0.87 MW at a total output power of 0.92 MW in 30-min pulses. Again, no indications for a limitation in pulselength were found. The second series tube was tested in short-pulse operation and showed a strange behavior concerning a mode hopping which has not yet been understood. The third series gyrotron delivers up to now 0.65 MW at a pulse duration of 180 s. Preliminary operation of the prototype tube as a two-frequency gyrotron delivered 0.41 MW in 10-s pulses at 103.8 GHz (TE21,6 mode). [ABSTRACT FROM AUTHOR]

Due to the requirement of a newly designed ultra-broadband chemical vapour deposition (CVD)-diamond window for a multifrequency 1-MW short-pulse gyrotron at Forschungszentrum Karlsruhe (FZK), the existing beam-forming mirror system inside the gyrotron tube has to be modified. This paper describes the design scheme for shaping the output beams from the launching antenna waveguide into the desired fundamental Gaussian profile by using adapted phase-correcting mirrors with nonquadratic surface contour function. Simulation results show these phase-correcting mirrors can be used for broadband operation of a frequency step-tunable gyrotron, which operates in nine modes from the TE17,6 mode at the frequency of 105 GHz to the TE23,8 mode at 143 GHz. Further calculations predict that efficiencies of more than 94% can be obtained for converting the high-order cylindrical cavity modes into the usable fundamental Gaussian mode. [ABSTRACT FROM AUTHOR]

A highly efficient quasi-optical mode converter system with several novel features has been designed and tested at Forschungszentrum Karlsruhe (FZK). The converter consists of a dimpled-wall waveguide launcher, one quasi-elliptical mirror and two toroidal mirrors. The coupled-mode theory has been used to analyze the operation of the prebunching waveguide launcher; the radiated fields from the cut of the launcher have been calculated by the scalar diffraction integral. Simulation results show that the advanced dimpled-wall launcher generates a well-focused Gaussian radiation pattern with low diffraction losses. In this case, toroidal mirrors are sufficient to obtain a desired output beam pattern. An efficiency of more than 98% has been achieved to convert the rotating TE28,8 cavity mode at 140 GHz into a fundamental Gaussian beam. Experimental measurements show close agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

A novel high-frequency processing technology has been developed for automated fabrication of high quality carbon-fiber-reinforced plastics. The development has been assisted by extensive numerical simulations. The use of microwaves shows new approaches for the process interaction for the fiber materials, precursors, resin systems, and layup preparation. Prepreg and injection technologies are completely supported and optimized. [ABSTRACT FROM AUTHOR]

Gyrotrons are microwave oscillators based on the Electron Cyclotron Maser (ECM) instability. The free energy is the rotational energy of weakly relativistic electrons in a longitudinal magnetic cavity field. In contrast, to klystrons the interaction circuit is a high-order-mode cavity allowing higher power at higher frequencies. At present gyrotrons 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 generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 1.0 MW gyrotrons employing open-ended cylindrical resonators and synthetic diamond output windows is 5 s at 110 GHz, 12 s at 140 GHz and 9 s at 170 GHz, with efficiencies slightly above 30%. The energy world record of 160 MJ (0.89 MW at 180 s pulse length and 140 GHz) at power levels higher than 0.8 MW has been achieved by the European FZK-CRPP-CEA-TED collaboration. Total efficiencies around 50% have been obtained using a single-stage depressed collector (SDC). To reduce the costs of the ECRH system on ITER and to make its poloidal launcher for neoclassical-tearing-mode stabilization more compact, a further increase of the output power per gyrotron is desirable. To achieve output powers in excess of around 2 MW in continuous wave (CW) operation at the ITER reference frequency 170 GHz it is necessary to use a coaxial cavity geometry. A maximum output power of 2.2 MW at 165 GHz (1 ms pulse length) was obtained at FZK with an efficiency of 28%. At the nominal output power of 1.5 MW the efficiency increases from 30 to 48% in operation with a SDC. The availability of sources with fast frequency tunability would permit the use of a simple fixed, non-steerable mirror antenna for local current drive experiments. This work reports on the progress in gyrotron development and the status of advanced coaxial cavity gyrotrons and step-wise frequency tunable gyrotrons. [Copyright &y& Elsevier]

Discusses passive high-power microwave components for applications in microwave systems for radio frequency plasma generation and heating. Information on high-power components for overmoded transmission systems such as smooth-wall waveguides; Applications of high-power microwaves; Methods in accomplishing long-distance transmission in millimeter wave systems.

The possible applications of high-power millimeter (mm) and sub-mm waves from free-electron masers (FEMs) and gyro-devices span a wide range of technologies. The plasma physics community has already taken advantage of recent advances in applying high-power mm waves generated by long pulse or continuous wave (CW) gyrotron oscillators and short pulse very high-power FEMs in the areas of RF-plasma production, heating, non-inductive current drive, plasma stabilization and active plasma diagnostics for magnetic confinement thermonuclear fusion research, such as electron cyclotron resonance heating (28–170 GHz), electron cyclotron current drive , collective Thomson scattering , microwave transmission and heat-wave propagation experiments. Continuously frequency tunable FEMs could widen these fields of applications. Another important application of CW gyrotrons is industrial materials processing, e.g. sintering of high-performance functional and structural nanostructured ceramics. Sub-mm wave sources are employed in high-frequency broadband electron paramagnetic resonance and other types of spectroscopy. Future applications which await the development of novel high-power FEM amplifiers and gyro-amplifiers include high-resolution radar ranging and imaging in atmospheric and planetary science as well as deep-space and specialized satellite communications and RF drivers for next-generation high-gradient linear accelerators (supercolliders). The present paper reviews the state-of-the-art and future prospects of these recent applications of gyro-devices and FEMs and compares their specific advantages. [Copyright &y& Elsevier]

Proposes a numerical approach of solving the eigenvalue equation of a coaxial cavity with misaligned inner rod. Analysis of the higher mode coaxial cavity employed in a 140-gigahertz/1.5-megawatt gyrotron oscillator; Effect of structural misalignment on the eigenvalue of the operating mode in a misaligned coaxial cavity.

Studies the structural eccentricity influence on the starting current of a 1.5MW/140GHz, TE[sub28,26,1] coaxial-cavity gyrotron oscillator, based on the gyrokinetics of free-electron masers. Increase in the starting current due to structural eccentricity; Influence of the structural eccentricity on the performance of the coaxial cavity gyrotron.

Mode conversion due to variation of wall impedance and diameter change in circumferentially corrugated circular waveguides is investigated in this paper. Starting from Maxwell's equations and using vector identities, general simplification integral formulae for coupling coefficients in circumferentially corrugated circular waveguides with both wall impedance and diameter changes are derived. The explicit formulae of corresponding coupling coefficients for any coupled modes are obtained. As a special case of general results, explicit expressions for the coupling coefficients of a tapered smooth-wall waveguide are deduced. The signs of the coupling coefficients in corrugated and smooth-wall waveguides are worked out. [ABSTRACT FROM AUTHOR]

Curvature coupling coefficients in circumferentially corrugated circular waveguides are examined in detail in this paper. Starting from Maxwell's equations, a strict derivation is given for the curvature coupling coefficients formulae. The general exact integral expressions and the explicit expressions for any modes are obtained. From these formulae, the expressions for the coupling coefficients for any modes in smooth waveguides are obtained in the case when the slot depth approaches zero. The signs of coupling coefficients in corrugated and smooth waveguides are worked out. [ABSTRACT FROM AUTHOR]

Efficient plasma heating by ECR-wave irradiation requires axisymmetric, narrow, pencil-like millimetre wave beams with well-defined polarization. The linearly polarized gaussian-like HE[sub 11] mode satisfies these conditions best. This quasi-optical hybrid mode can be generated from TE[sub on] gyrotron mode compositions by the two multi-step mode conversion processes: (1) ΣTE[sub 0n] to TEo@ to TE[sub 11] to HE[sub 11] or (2) ΣTE[sub 0n] to TE[sub 01] to TM[sub 11] to HE[sub 11]. The first scheme has the advantage that the converters can all be made without bends, allowing an arbitrary choice and fast change of the polarization plane. The second scheme does not exhibit this advantage, but it is more suitable at very high frequencies (e.g. 140 GHz) because efficient TE[sub 0l]-to-TM[sub 11] transducers can be made considerably shorter than serpentine TE[sub 01]-to-TE[sub 11] mode converters. This paper presents computations on mode converter systems of the first type at 70GHz and of both types at 140GHz (ID = 27·8 mm for 200kW transmission lines). The structure of wall perturbations (phase-matched superposition of 2 or 3 different geometrical periods) in the rippled wall mode converters and the curvature distribution in the bent, smooth-walled TE[sub 01]-to-TM[sub 11] mode transducer were optimized by numerically solving the corresponding coupled-mode differential equations. Computer-aided optimization of circumferentially corrugated mode converters has been achieved with a scattering matrix code employing the modal field expansion technique (modular analysis concept (MAC)). In all cases the predicted overall efficiency of the complete mode converter system from ΣTE[sub 0n] (predominantly TE[sub 02] at 70GHz or TE[sub 03] at 140GHz) to HE[sub 11] in the desired polarization is approximately 95% at 70GHz and 92% at 140GHz (ohmic attenuation is included). Low-power measurements on the conversion efficiency of the various mode transdu... [ABSTRACT FROM AUTHOR]

Focuses on the influence of the electron-beam misalignment on the energy-transfer rate and starting current of a coaxial cavity gyrotron oscillator, based on the gyrokinetics of free-electron masers. Sensitivity of the dependence of the output power on the magnetic field due to a poor alignment of the electron beam.

A continuous-wave (CW) high-harmonic gyrotron driven by a low-power electron beam is a compact radiation source demanded by terahertz applications. Its physical feasibility, however, is hampered by ohmic losses and mode competition in the gyrotron cavity. An ultralow-loss diamond loading of the cavity can give a clue to this problem. This article is concerned with theoretical aspects of mode selection and design for a gyrotron cavity loaded with coaxial rod made of chemical vapor deposition (CVD) diamond. As an example, the design of a high-Q diamond-loaded cavity for a third-harmonic 658-GHz gyrotron powered by a 0.1-A, 15-kV electron beam is presented. It is shown that the designed cavity enables the gyrotron to produce up to 116-W output power in a single oscillating mode. [ABSTRACT FROM AUTHOR]

We present a conceptual proposal for a large-orbit coaxial-structure cyclotron autoresonance maser (CARM), wherein the rf structure is a coaxial waveguide or cavity and a large-orbit electron beam encircles the axis of the coaxial structure. Both dispersion analysis and nonlinear simulations demonstrate the feasibility of the proposal. The large-orbit coaxial-structure CARM could be expected to operate in millimeter and submillimeter wave ranges with high power, low operating magnetic field, and ultrahigh gain. It is found that electron-beam velocity spread substantially decreases the power of CARMs, but this adverse influence might be offset by a proper taper of the axial magnetic field. [ABSTRACT FROM AUTHOR]

An analytical model of coaxial traveling-wave-tube-like waveguides with plasma filling has been justified and utilized to analyze the eigenmodes. Very often, introducing plasma into vacuum electronic devices leads to essential advantages as compared with evacuated tubes. The cylindrical structure under the present consideration consists of a central dielectric rod, placed inside a plasma coaxial layer with a metallic helix sheath on its outer interface, and a metal screen separated from the plasma by another dielectric layer. The dispersion properties of electromagnetic waves propagating across the external axial static magnetic field in such traveling-wave-tube-like waveguides are studied and summarized. The presence of a dense plasma coaxial layer makes the media nontransparent for waves in the electron cyclotron frequency range. However, surface type electromagnetic waves can propagate in this case. These waves are called azimuthal surface waves (ASWs). The helix sheath causes coupling of ordinarily and extraordinarily polarized ASWs. The zeroth radial ASW modes have been found to be most dangerous for parasitic wave excitation in dense plasma-loaded, coaxial traveling-wave-tube-like waveguides. [ABSTRACT FROM AUTHOR]

The influence of a coaxial dielectric rod on eigenvalues, ohmic losses, transverse field structure, and beam-wave coupling coefficients is investigated for TE modes of a gyrotron cavity. It is shown that such dielectric insert, when made from a moderate-loss material, results in strong attenuation of all cavity modes, with the exception of those having caustic radii much larger than the insert radius. It is proposed to employ such dielectric loading for selective suppression of competing modes in cavities of second-harmonic gyrotrons. The high performance and flexibility of the proposed method of mode discrimination are demonstrated for the example of the cavity of the high-power 0.39-GHz second-harmonic gyrotron developed at the University of Fukui. In addition, some fascinating capabilities enabled by coaxial inserts made of ultralow-loss dielectrics are discussed. [ABSTRACT FROM AUTHOR]

Azimuthal surface waves are well known to be eigenwaves of cylindrical waveguide structures—plasma–dielectric–metal—placed into an external axial static magnetic field. The angular velocity of energy transfer along with the angular group velocity is applied to analyze electromagnetic energy rotation in these structures. Optimal magnitudes of effective azimuthal wavenumbers are determined for which the angular velocity reaches its maximum. The angular velocity is shown to decrease with the increasing absolute value of the azimuthal wavenumber and with the decreasing strength of the external axial static magnetic field. [ABSTRACT FROM AUTHOR]

Azimuthal surface waves are well-known to be eigenwaves of coaxial metal waveguides entirely filled by plasma. The present study of electromagnetic energy transfer complements the investigation of the wave dispersion properties carried out earlier. The angular velocity of energy transfer is analyzed as a function of the coaxial-line parameters and compared with three limiting cases. One case is a metal waveguide entirely filled by plasma. The second case is a metal rod placed into infinite plasma along an external static magnetic field. Finally, the third one is a thin coaxial waveguide. [ABSTRACT FROM AUTHOR]

Electromagnetic energy rotation around the axis of cylindrical metallic waveguides partially filled by isotropic plasma is studied. Angular phase velocity and angular velocity of energy transfer are analyzed as functions of the plasma waveguide parameters: plasma column radius, plasma particle density, azimuthal wavenumber, dielectric layer width and dielectric constant, and collision frequency. This study generalizes the results obtained earlier in the case of the wave propagation in two-component plasma-metal structures (cylindrical metallic waveguides entirely filled by magnetoactive plasma) to the case of three-component plasma-dielectric-metal structures. [ABSTRACT FROM AUTHOR]

A dark photon is one of the simplest extensions of the Standard Model of particle physics and can be a dark matter candidate. Dark photons kinetically mix with ordinary photons. The mass range from 10−4 to 10−3 eV of such dark photons is underconstrained by laboratory‐based experiments and a new search is therefore motivated. In this mass range, dark photons behave like waves rather than particles and the corresponding electromagnetic waves are in the millimeter‐wave range. The technical difficulties of the millimeter waves have prevented so far dark photon experiments in this mass range. The use of coherent millimeter waves to search for dark photons in a Light‐Shining‐through‐a‐Wall (LSW) experiment is proposed. The merits and limitations of coherent wave detection are clarified and the potential of single photon sensors at microwaves is investigated. Development of millimeter‐wave technology is not only limited to dark photons. Technically, an experiment for dark photons by using electromagnetic waves resembles that for axions, another light dark matter candidate, with static magnetic fields. This paper represents an essential step toward axion LSW in the millimeter‐wave range (Sub‐THz‐AXion experiment; STAX) as a potential successor of an on‐going experiment in infrared. [ABSTRACT FROM AUTHOR]

Electromagnetic energy rotation around a cylindrical metallic rod placed into an infinite plasma, parallel to an external static uniform magnetic field, is studied. Angular phase velocity and angular velocity of energy transport are introduced and analyzed as functions of the plasma waveguide parameters: rod radius, plasma particle density, azimuthal wave number, and external static axial magnetic field. These angular velocities are compared with the case of wave propagation in cylindrical metallic waveguides entirely filled by magnetoactive plasma. [ABSTRACT FROM AUTHOR]

Energy transfer along finite curvature plasma-metal interfaces in cylindrical metallic waveguides entirely filled by magnetoactive plasma is studied. Angular phase velocity, angular velocity of energy transfer, and angular group velocity are introduced and analyzed as functions of the waveguide parameters: radius, plasma particle density, azimuthal wave number, and external static axial magnetic field. [ABSTRACT FROM AUTHOR]

Azimuthal surface waves are electromagnetic eigenwaves of cylindrical plasma-filled metallic waveguides with a stationary axial magnetic field. These waves with extraordinary polarization can effectively interact with relativistic electron beams rotating along large Larmor orbits in the gap, which separates the plasma column from the waveguide wall. Both widening the layer and increasing the beam particle density are demonstrated to cause resonance overlapping seen from the perspective of the growth rate dependence on the effective wave number. [ABSTRACT FROM AUTHOR]

Azimuthal surface waves are electromagnetic eigenwaves of cylindrical plasma-dielectric waveguides which propagate azimuthally nearby the plasma-dielectric interface across an axial external stationary magnetic field. Their eigenfrequency in particular can belong to the electron cyclotron frequency range. Excitation of azimuthal surface waves by rotating relativistic electron flows was studied in detail recently in the case of the zeroth radial mode for which the waves' radial phase change within the layer where the electrons gyrate is small. In this case, just the plasma parameters cause the main influence on the waves' dispersion properties. In the case of the first and higher radial modes, the wave eigenfrequency is higher and the wavelength is shorter than in the case of the zeroth radial mode. This gain being of interest for practical applications can be achieved without any change in the device design. The possibility of effective excitation of the higher order radial modes of azimuthal surface waves is demonstrated here. Getting shorter wavelengths of the excited waves in the case of higher radial modes is shown to be accompanied by decreasing growth rates of the waves. The results obtained here are of interest for developing new sources of electromagnetic radiation, in nano-physics and in medical physics. [ABSTRACT FROM AUTHOR]

In this paper, a complete design methodology of a triple frequency gyrotron is presented, which can also be further applied to multifrequency operations. Frequencies for operation are selected as 220, 247.5, and 275 GHz for the localized and intense heating of magnetically confined plasmas (i.e., electron cyclotron heating and current drive) for future fusion reactors. A cautious selection procedure of the mode triplet is portrayed in accordance with the all possible physical and technical constraints. Typical cold cavity (beam absent) and single-mode (beam present) calculations are performed and presented with extended interaction structure (including an optimized nonlinear taper section). A triode type configuration is adopted for magnetron injection gun to produce the electron beam with desired characteristics as required for RF behavior. Time-dependent multimode calculations are presented with nonuniform magnetic field and beam parameters optimized by gun simulations. These rigorous calculations affirm proper working of the design with $\approx 1$ -MW continuous wave power for chosen mode triplet and efficiency $\approx 35$ %. [ABSTRACT FROM PUBLISHER]