Search

A novel type of frequency doubling gyrotron traveling wave amplifier (FD-GTWT) for applications that require high-power microwave in the sub-THz frequency range is presented. The proposed FD-GTWT delivers high power and high gain over a broad bandwidth and simultaneously doubling the frequency of the input signal. Simulations of a first 263GHz FD-GTWT design are presented, which show for a 10mW driving signal at 131.5GHz an RF output power of 250W at 263GHz and a gain of >40dB over a bandwidth of 17.5GHz. The basis of the FD-GTWT are two interaction circuits separated by a long drift section. In the first circuit, the electron beam is pre-bunched at the fundamental cyclotron harmonic. In the second one, high-power RF is induced by the pre-bunched electron beam at the 2nd cyclotron harmonic. Both sections consist of helically corrugated waveguides that efficiently suppress parasitic interactions and allow broad bandwidth. [ABSTRACT FROM AUTHOR]

A new self-consistent time-domain model for the simulation of gyrotron traveling-wave tubes with a helically corrugated interaction space (helical gyro-TWTs) is presented. The new model links classical methods using the approach of slowly varying variables together with an expansion of the electromagnetic field in eigenmodes and advanced full-wave particle-in-cell (PIC) solvers. The aim is to significantly reduce the required calculation time compared to full-wave PIC solvers, while less strict assumptions are introduced as in the classical approaches of slowly varying variables. For the first time, the classical theory of coupled circular waveguide modes for the description of the operating electromagnetic eigenmode in the helical interaction space is combined with a 3-D PIC representation of the electron beam. This allows the simulation of the beam–wave interaction over a broad bandwidth and at arbitrary harmonics of the cyclotron frequency. In addition, arbitrary electron beams (with spreads, offsets of the guiding center from the symmetry axis, and so on) can be investigated. The new approach is compared with the full-wave 3-D PIC code CST Microwave Studio. A good agreement of the simulation results is achieved, while the computing time is significantly reduced. [ABSTRACT FROM AUTHOR]

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]

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

Considering the recent understanding of the physics of electron trapping mechanisms taking place in the magnetron injection gun (MIG) region of gyrotrons and the sensitivity of the emitter ring manufacturing tolerances on the electron beam quality, a MIG has been designed and manufactured for the 2 MW, 170 GHz coaxial cavity gyrotron developed at Karlsruhe Institute of Technology. The new MIG has the following novelties: (i) the design satisfies the criteria for the suppression of the electron trapping mechanisms, (ii) a new type of emitter ring is used for the suppression of the influence of the manufacturing tolerances and misalignments on the quality of the generated electron beam, and (iii) the design was optimized to generate a good beam quality in a wide variety of magnetic field profiles to increase the flexibility. An additional important feature of the new triode MIG design is the possibility to operate with only two power supplies by using a special start-up scenario. The first experimental results of the coaxial cavity gyrotron with the new MIG are presented. [ABSTRACT FROM AUTHOR]

The gyrotron operational efficiency is considered as a very important factor of future fusion power plants. The development of an efficient multistage depressed collector (MDC) for the annular spent electron beam is the key for a significant increase in the operational efficiency. To achieve this target, a systematic investigation of all kinds of possible design approaches was conducted during the last decade at the Karlsruhe Institute of Technology. A promising approach using a cylindrical hollow collector design was found. In this work, another generic design approach based on the E × B drift concept for the spent electron beam is presented which uses a coaxial collector design. This new generic approach offers a significant improvement in the collector efficiency. Even more, it opens the path for the design of a large variety of new MDC collectors with different characteristics, e.g., the number of stages and the wall geometry. [ABSTRACT FROM AUTHOR]

Single-stage depressed collector systems are widely used in high-power gyrotrons in order to increase the overall efficiency. The position where the deceleration of the beam takes place influences the collector efficiency. In this paper, several deceleration schemes are numerically investigated for the short-pulse prototype of the 170-GHz, 1-MW European gyrotron for International Thermonuclear Experimental Reactor. The results of this paper are in a qualitative agreement with the results of dedicated experiments. A simple idea is also proposed for the significant increase of the overall gyrotron efficiency without an important technological modification. [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]

In this paper, the influence of stray magnetic fields (SMFs) in the ITER electron cyclotron resonance heating and current drive (ECRH-CD) building on the operation of the European (EU) 170 GHz, 1 MW, continuous wave gyrotrons is investigated. There are mainly two sources of SMF in the ITER ECRH-CD building: 1) the tokamak and 2) the neighboring gyrotrons which are placed in close proximity. The possible electron beam degradation caused by the influence of the SMF is numerically investigated. Furthermore, the collector operation under the influence of an SMF is also studied. A proposal to compensate the influence of the SMF 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]

Multistage depressed collectors are challenges for high-power, high-frequency fusion gyrotrons. Two concepts exist in the literature: (1) unwinding the spent electron beam cyclotron motion utilizing non-adiabatic transitions of magnetic fields and (2) sorting and collecting the electrons using the E x B drift. To facilitate the collection by the drift, the hollow electron beam can be transformed to one or more thin beams before applying the sorting. There are many approaches, which can transform the hollow electron beam to thin beams; among them, two approaches similar to the tilted electric field collectors of traveling wave tubes are conceptually studied in this paper: the first one transforms the hollow circular electron beam to an elongated elliptic beam, and then the thin elliptic beam is collected by the E x B drift; the second one splits an elliptic or a circular electron beam into two arc-shaped sheet beams; these two parts are collected individually. The functionality of these concepts is proven by CST simulations. A model of a three-stage collector for a 170 GHz, 1MW gyrotron using the latter approach shows 76% collector efficiency while taking secondary electrons and realistic electron beam characteristics into account. [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]

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]

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]

A sensitivity analysis of manufacturing imperfections and possible misalignments of the emitter ring in the gyrotron cathode structure on the electron beam quality has been performed. It has been shown that a possible radial displacement of the emitter ring of the order of few tens of microns can cause dramatic effects on the beam quality and therefore the gyrotron operation. Two different design approaches are proposed in order to achieve an electron beam which is less sensitive to manufacturing imperfections. [ABSTRACT FROM AUTHOR]

An inverse magnetron injection gun (MIG) has been designed for the 2-MW, 170-GHz, coaxial-cavity gyrotron built at the Karlsruhe Institute of Technology. The inverse gun design could offer the possibility for the implementation of a larger emitter ring without the need for a bigger bore hole in the magnet compared with the conventional type of MIGs. Considering the fundamental beam parameters, an excellent beam quality has been achieved in numerical simulation. Electron-trapping suppression criteria were considered during the design phase of the MIG. [ABSTRACT FROM AUTHOR]

Gyrotrons are the only RF sources providing significantly high output powers at continuous-wave operation in the sub-terahertz frequency range. A prerequisite for a proper study of gyrotron behavior is the accurate time-dependent self-consistent simulation of the interaction between the electron beam and the RF wave inside the interaction region (cavity). That requires an accurate description of the RF fields at the boundaries of the interaction region. In this work, an improved broadband boundary condition is proposed. Based on polynomial series expansion of the load impedance and of the wavenumber, this boundary condition not only features a significantly improved broadband matching, but also allows a customizable frequency-dependent reflection coefficient at the boundaries. The boundary condition has a general formulation, i.e., it does not rely on a special numerical method. For the case of perfect matching, the new formulation is validated through comparing the numerically calculated reflection coefficients with those obtained by the state-of-the-art matched gyrotron broadband boundary condition, whereas the validation in the case of a given frequency-dependent reflection is done by comparing the numerically calculated reflection with the prescribed one. Although this concept is initially intended for the simulation of gyrotron cavities, it can be easily extended to any other open-cavity resonators, for which proper definition of the boundary conditions is required. [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]

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]

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]

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]

Electron Cyclotron Resonance Heating (ECRH) systems for next step large fusion-devices operate at a Continuous Wave (CW) power well beyond 10 MWgenerated by a large number of gyrotrons with typically 1 MW power per unit. The combination of the power of two (or more) gyrotrons and switching of the power between different launchers for different physics applications is an attractive feature for such systems. The combination of beams fromdifferent gyrotrons would reduce the number of transmission lines and the requirements on port space. Fast switching between two antennas synchronously with the Magneto-Hydro Dynamic (MHD) modes frequency would increase the efficiency of mode stabilization. Both combination and switching as well as power sharing between different ports can be performed with high-power four-port diplexers using small frequency differences or small frequency-shift keying of the gyrotrons, respectively. Fast directional switches (FADIS) and beam combiners (BC) can be designed on the basis of different physical mechanisms: some selected design variants were investigated and the results are presented. Considerations on the integration of FADIS/BC's into large ECRH systems and their use in test arrangements are presented. [ABSTRACT FROM AUTHOR]

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]

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]

Two-dimensional distributed feedback is an effective method of producing ultrahigh-power spatially coherent radiation from an active medium, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. This paper describes the progress in the investigations of planar and coaxial free-electron masers (FEMs) based on a novel feedback mechanism. The theoretical analysis of these FEM schemes was conducted in the frame of the coupled-wave approach and 3-D simulations and agrees well with the experimental data obtained in "cold" and "hot" tests. As a result, the effective transverse (azimuthal) mode selection has been demonstrated under a transverse size of about 20-25 wavelengths, and narrow-frequency multimegawatt microwave pulses have been generated in the Kaand W-bands. [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]

In a 74.2-GHz 100-kW pulse gyrotron, the characteristics of parasitic low-frequency oscillations and their influence on electron energy spread have been determined. A method for suppressing parasitic oscillations by varying the magnetic field distribution in the beam compression region is studied. The results of measurements demonstrate the influence of azimuthal inhomogeneity of electron emission on the excitation of parasitic oscillations and on the gyrotron output parameters. Obtained data allows one to determine the ways of gyrotron efficiency enhancement by improved quality of the helical electron beam. [ABSTRACT FROM AUTHOR]

Operation of a continuous wave gyrotron back- ward-wave oscillator (gyro-BWO) with a helically rippled operating waveguide has been experimentally studied. The gyro-BWO exploits a dc oil-cooled magnet with magnetic field up to 0.5 T and utilizes a weakly relativistic (20 key) electron beam produced by a magnetron injection gun. Stable generation at the second cyclotron harmonic with a maximum power of 7 kW and an efficiency of 15% at a frequency of 24.7 GHz was achieved. Smooth oscillation frequency tuning by varying the magnetic field was measured to be as wide as 5% at the half-power level. The first gyro-BWO operation with a single-stage energy recovery system was realized. The use of a depressed collector provided an efficiency increase up to 23% and an opportunity for reduction of the main power supply voltage down to 10 kV. [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.

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]

Multistage Depressed Collectors (MDCs) are widely used in vacuum tubes to regain energy from the depleted electron beam. However, the design of an MDC for gyrotrons, especially for those deployed in fusion experiments and future power plants, is not trivial. Since gyrotrons require relatively high magnetic fields, their hollow annular electron beam is magnetically confined in the collector. In such a moderate magnetic field, the MDC concept based on E×B drift is very promising. Several concrete design approaches based on the E×B concept have been proposed. This paper presents a realizable design of a two-stage depressed collector based on the E×B concept. A collector efficiency of 77% is achievable, which will be able to increase the total gyrotron efficiency from currently 50% to more than 60%. Secondary electrons reduce the efficiency only by 1%. Moreover, the collector efficiency is resilient to the change of beam current (i.e., space charge repulsion) and beam misalignment as well as magnetic field perturbations. Therefore, compared to other E×B conceptual designs, this design approach is promising and fairly feasible. [ABSTRACT FROM AUTHOR]