Millimeter-Wave HF Relativistic Electron Oscillators

A review of the experimental study of single-mode oscillators based on stimulated bremsstrahlung and Cerenkov radiation of high-current relativistic electron beams is given. Three types of Cerenkov oscillators are investigated in detail: orotrons, surface wave oscillators and a flimatron (free electron maser (FEM) based on Smith-Purcell radiation). The bremsstrahlung oscillators studied are gyrotrons with TM modes, a ubitron operating at a quasi-critical frequency and cyclotron autoresonance masers. Electrodynamic and electron methods of mode selection provide stable radiation with a reproducible space structure of radiation in all oscillators under study. The radiation power attained 50-100 MW for long and 10-30 MW for short millimeter wavelengths at the efficiency up to 5-10 percent. Various types of oscillators are compared. Promising methods for increasing power and radiation frequency are discussed.     

Self consistent code for a 150 GHz gyrotron

A self consistent calculation for gyrotron oscillators is described and applied to a TE₀₃ mode gyromonotron. The RF field profile satisfies a wave equation in which the AC beam current appears as a source. The wave equation is solved simultaneously with the electron equation of motion.     

Millimeter-Wave Relativistic Electron Devices

We briefly review advances and new ideas in the field of obtaining superpowerful coherent millimeter-wave radiation. Devices based on the stimulated bremsstrahlung and the Cherenkov radiation of curved and rectilinear relativistic electron beams, respectively, are considered, in particular, gyrodevices (including traditional gyrotrons, large-orbit gyrotrons, gyro-TWTs, and gyro-BWTs), free-electron masers (cyclotron-autoresonance masers and ubitrons), backward-wave oscillators (BWOs), traveling-wave oscillators (TWOs), and orotrons. Some of the above-mentioned devices are developed to the level ensuring their efficient use for a number of promising applications.     

Mode competition, suppression, and efficiency enhancement in overmoded gyrotron oscillators

Gyrotron oscillators are of great interest as sources of high power mm wave radiation for electron cyclotron resonance heating and current drive in magnetic fusion research devices. Gyrotrons capable of efficiently generating cw power 1 megawatt will be required in future magnetic fusion studies. However, as gyrotron power approaches the megawatt level, a very large, overmoded cavity must be employed in order to keep ohmic power losses in the cavity at an acceptable level, and the problem of mode overpopulation becomes severe. Also, it becomes increasingly important to optimize gyrotron efficiency for a number of important reasons including minimizing the problem of collecting the electron beam energy. In the present paper, a detailed experimental and theoretical study of mode competition and mode locking in an overmoded gyrotron is presented. Efficiency enhancement (to 60%) and high peak power (430 kW) were achieved in the TE_2,4,1 mode using magnetostatic profiling in the cavity. With selective mode suppression, peak power of 475 kW was generated in the TE_0,4,1 mode.This work was performed at the Naval Research Laboratory. Some of the authors have affiliations with other institutions, as indicated:     

Gyrokinetics of Coaxial-Cavity Electron Cyclotron Maser

Based on the gyrokinetics of free-electron masers, the energy-transfer rate and starting current are derived for a coaxial-cavity electron cyclotron maser (gyrotron oscillator) with the misalignment of the inner-rod axis to the outer-cavity axis taken into account. The effect of the finite cavity length is included in the derivation. The stable term of the instability which results from the contribution of the guiding-center distribution is of significance, although it is usually neglected in a cylindrical-cavity gyrotron. It is found that the structure misalignment has more serious influence on the performance of the coaxial cavity gyrotron than the beam misalignment.     

Comparison of Electron-Beam-Misalignment Effect on Starting Current and Output Power in Coaxial-Cavity and Cylindrical-Cavity Gyrotron Oscillators

Comparative study is presented to the effect of the electron-beam misalignment on the starting current and output power of the coaxial-cavity and cylindrical-cavity gyrotron oscillators operating in the millimeter wave ranges. The numerical analysis is based on the gyrokinetic formulas for a TE_28,16,1 mode at a frequency of 140 GHz. Results show that the coaxial-cavity gyrotron oscillator has lower starting current and less power loss than the cylindrical-cavity gyrotron oscillator when the electron-beam axis has a misalignment to the cavity axis.     

Kinetic theory of electron-cyclotron resonance masers withasymmetry of the electron beam in a cavity

On the basis of kinetic theory, general relations for the starting current and frequency shift in electron-cyclotron resonance masers are derived in the case of eccentricity of the electron beam in a cavity. The influence of a misalignment of the beam and cavity on the gyrotron operation is discussed as an example     

Resonant reflectors for free electron masers

A new type of a waveguide reflector is discussed. Its operating principle is based on the effect of resonant reflection of a wave from a cavity when there is coupling between the wave and an eigenmode of the cavity. The resonant reflectors are very attractive for the use in microwave systems of free electron masers due to their high reflection coefficients and narrow frequency bands at comparatively short lengths. Results of calculations and measurements of some resonant reflectors and a microwave system for KA-band cyclotron autoresonance masers are presented.A part of results of this paper was presented at 9th International Conference on High-Power Beams, Washington, 1992 [1].     

Microwave system for free electron masers based on a curved open waveguide

A new microwave system for free electron masers of millimeter and submillimeter wavelength ranges on the basis of an open quasi-optical waveguide in the form of two slightly deformed metallic planes is discussed. The system gives the possibility to control the wavebeam trajectory and, in particular, to separate wave and electron beams.Some basic solutions and results of demonstration experiment are presented.     

有限引导磁场下相对论环形电子注色散特性的研究

采用等效媒质处理方法来研究有限引导磁场下沿纵向运动的相对论环形电子注.首先建立运动坐标系以电子注纵向速度匀速运动,在运动坐标系中电子注可以被考虑成静止的磁化等离子体,再通过四维空间的洛伦兹变换得到电子注在静止的实验室坐标系下可以被等效为双各向异性媒质,其不仅具有张量形式的电导率和磁导率,还具有手征特性.在此基础上同时考虑了由于电子注表面波动所引起的表面电流密度.采用该方法研究了有限引导磁场下圆柱波导中沿纵向运动的相对论环形电子注,推导出该模型的色散方程,并进行了数值计算.计算结果表明该研究方法能够得到更准 关键词： 相对论环形电子注 磁化等离子体 色散特性     

Mode competition and control in higher-power gyrotron oscillators

One of the most important problems in the design of high-power millimeter-wave sources such as gyrotron oscillators is insuring that the device operates in the desired mode. For high-power and short-wavelength devices the effective mode density is high, in that the current is above threshold for many modes. One then is led to ask whether operation in a single mode is possible and what steps must be taken to maximize the electronic efficiency of the device while ensuring single-mode operation. The answer to the first question has been determined to be yes. Provided that certain conditions are met, single-mode operation is stable. The present results emphasize time-dependent multimode simulations showing how these stable states can be accessed. In particular, the accessibility to the stable single mode with maximum efficiency is studied. Regions of parameter space for which stable single-mode operation is possible are plotted for an annular beam for a closed-cavity gyrotron operating at a high-order whispering-gallery mode (TE_80.4). These results also apply to the quasioptical gyrotron with a pencil electron beam     

Common properties of free electron lasers

A general theory of microwave devices (in particular, ubitron, gyrotron) based on the ultrarelativistic Doppler up-conversion of electron oscillation frequency is developed. The wave growth rate for the amplifier and the start current for the generator are obtained. A universal relation between the frequency conversion factor and the efficiency of the laser is derived.     

W波段回旋行波管放大器的模拟与设计

回旋行波管放大器是高功率毫米波雷达发射系统最重要的候选者.通过对回旋行波管放大器中的绝对不稳定性、回旋返波振荡以及电子注-波互作用的研究，讨论了回旋行波管的稳定性、寄生模式的抑制和工作参数的优化等问题，给出了W波段TE₀₁模回旋行波管放大器的模拟设计结果.PIC粒子模拟结果表明，在电子注电压100kV、电流10A、工作磁场3.52T时，94GHz的基波回旋行波管放大器可获得大于250kW的输出功率、40dB的增益、大于25％的效率和约5％的带宽. 关键词： W波段 回旋行波管放大器 模拟 设计     

Powerful millimeter-wave generators based on the stimulated Cerenkov radiation of relativistic electron beams

Effective relativistic Cerenkoy generators of various types were studied experimentally: orotrons, surface wave oscillators, and Smith-Purcell free-electron masers that were first realized. Single-mode generation was obtained in multimode systems using electrodynamic and electron methods of mode selection. The power obtained in this regime was 100 MW at 8.6 mm with the efficiency 10%, 50 MW at 5.5 mm with the efficiency 5%, and 8 MW at 2.4 mm with the efficiency 3%. In a multimode regime the power of 0.5 MW was obtained at 0.9 mm. The study of the dependence of the generation power on the value of the focusing magnetic field revealed the presence of zones of cyclotron power absorption, that were earlier observed in centimeter-wave generators.     

Review of the theory of mode interaction in gyrodevices

The present paper contains a brief overview of the studies of mode and wave interaction in gyrodevices. The main part of it is devoted to the discussion of physical effects caused by mode interaction in the free-running and phase-locked gyrotron oscillators. Also, the problem of the stability of large-signal operation of gyro-forward-wave amplifiers with respect to excitation of parasitic backward waves is discussed     

W波段连续波30 kW回旋振荡管高频系统和收集极的设计

 根据回旋管的电子回旋脉塞理论,借助于编写的回旋振荡管自洽非线性注-波互作用计算程序,设计出了工作频率94 GHz、工作电压30 kV、工作电流3 A的基次谐波连续波单腔回旋振荡管,工作模式为TE02模。设计的回旋振荡管在电压30.0 kV、电流3.0 A、速度横纵比1.5的条件下,获得了31.8 kW的输出功率,电子效率约35％。利用粒子模拟仿真软件对设计的回旋管收集极辅助线包散焦系统进行了粒子模拟仿真分析,模拟结果表明：借助于辅助线包散焦系统可以有效缩短回旋振荡管的轴向尺寸,并使回旋管收集极上的电子束功率密度低于500 W/cm2; W波段回旋振荡管收集极的热测试验结果表明：利用粒子模拟仿真获得的收集极上的电子束功率密度分布与其试验测量结果比较吻合。     

光子带隙谐振腔回旋管振荡器的自洽非线性理论

定义了光子带隙谐振腔(photonic-band-gap cavity, PBGC)的等效半径,论证了使用该半径将PBGC等效为具有模式选择性的金属圆柱谐振腔的有效性,揭示了其在PBGC设计过程中的指导性作用.基于等效半径的运用,建立起光子带隙谐振腔回旋管振荡器(PBG回旋管)的自洽非线性理论,并对工作于TE₃₂模的PBG回旋管作了理论分析和数值计算.目前的研究表明:高频电磁场沿角向呈行波或驻波的不同极化形式对PBG回旋管的注-波互作用过程具有较大的影响;较之电子回旋基波,该器件中二 关键词： 光子带隙谐振腔 等效半径 回旋管 自洽非线性理论     

高功率回旋振荡管Denisov型辐射器的研究

研究了一种用于高功率回旋振荡管准光模式变换系统的高效率Denisov型辐射器. 基于Bessel函数的积分展开及几何光学理论,系统地分析了圆波导中电磁波的传播过程及Denisov型辐射器降低衍射损耗的机理;分析了在圆波导内壁上工作模式与耦合模式叠加形成准高斯型场分布的过程. 从Bessel函数导数的本征值出发,给出了不同工作模式在Denisov型辐射器预聚束波导段选择目标耦合模式的一种普适方法,同时给出了Denisov型辐射器的设计方法. 依据上述理论分析编写了计算程序,并对一支140 GHz,工作模式为TE_28,8,1 MW长脉冲回旋振荡管进行了对比验证,计算结果与文献报道的结果具有很好的一致性. 关键词： 回旋管 准光辐射器 耦合模理论 模式变换     

Instability Entanglement in the Electron Cyclotron Maser

Investigation of absolute instability (AI) and convective instability (CI) of the electron cyclotron maser has independently led to the birth of gyrotron oscillators and amplifiers. Here, it is demonstrated that these instabilities can form a cooperative relationship owing to the nonlinear behavior of the stimulated electron beam. The CI can be induced in a zero-drive system with the assistance of AI, and the ohmic losses of all the excited waves inside the system are greatly reduced, which is called “instability entanglement” here. According to the theoretical and experimental study of a 167/330 GHz gyrotron, when instability entanglement occurs, the ohmic dissipation decreases to one-ninth of the AI-only condition, and the output power is enhanced by 20%. This discovery is promising for surpassing the ceiling of output power and frequency of gyrodevices placed by ohmic losses.