Gyrotron travelling wave amplifier: III. A proposed wide-band fast wave amplifier

A simple scheme is proposed for the construction of a wide-band fast wave gyrotron travelling wave amplifier. The key features of this scheme include a contoured waveguide, a tapered magnetic field, and injection of input signals in opposite direction of the streaming electrons. Bandwidths at tens of percents, and gain at tens of decibals, are projected.     

The Linear and Self-Consistent Nonlinear Theory of the Electron Cyclotron Maser Instability

In this paper the linear and nonlinear theory of the electron cyclotron maser instability is considered. The configuration used to study the maser instability consists of relativistic electrons gyrating about and drifting along a uniform magnetic field within a parallel plate waveguide. Relativistic effects associated with the gyrating electrons are responsible for excitation of the transverse electric mode in the waveguide. Linear theory shows that the growth rate maximizes when the axial beam velocity coincides with the axial wave group velocity of the excited electromagnetic wave. This allows us to perform the nonlinear analysis in a frame where both the axial wave number and axial beam velocity vanish. We have found that the maser instability exists only if the perpendicular beam energy exceeds a threshold value. Our analysis also describes the temporal nonlinear evolution of the field amplitude and frequency of a single excited wave. The nonlinear wave dynamics are self-consistently determined from the nonlinear particle orbits through the force and wave equations. The nonlinear analysis shows that there are two possible mechanisms for the saturation of the unstable wave: 1) depletion of the available free energy associated with the rotating particles and 2) phase trapping of the gyrating electrons in the wave. The initial beam parameters determine which of the two mechanisms is responsible for saturation. Competition between the two saturation mechanisms leads to a peaking in the energy conversion efficiency as a function of beam energy. Numerical results of the nonlinear formalism show that energy conversion efficiencies from the particles to the wave can be as high as 60 percent in the beam frame. Furthermore, by appropriately contouring the external magnetic field, among other things, efficiencies as high as 70 percent can be realized.     

Mode Cooperation in a Submillimeter Wave FU Series Gyrotron

At certain gyrotron operating conditions, mode cooperation instead of mode competition takes place between a fundamental and a second harmonic mode. This means the phase bunching of a gyrating electron beam under the second harmonic operation reduces the starting current for the fundamental operation and increases total output power as well as beam efficiency. Such mode cooperation is observed in experiments and confirmed by computer simulations for submillimeter wave Gyrotron FU II.     

Analytical treatment of linearized self-consistent theory of a gyromonotron with a non-fixed structure

The linearized self-consistent theory of gyrotron is analytically treated for a self-excited oscillator with non-fixed structure. The self-consistent RF axial field profile consists of four waves, namely the forward and backward travelling waves of the waveguide, and two electron cyclotron waves. According to the magnetic field value (or the detuning parameter), the electron cyclotron wave couples strongly with the forward wave, or weakly with backward wave of the waveguide mode. Also, the starting current and the axial field profile are significantly different for the forward versus the backward interaction.     

W波段回旋行波管放大器速度零散的分析

通过包含速度零散影响的回旋行波管放大器的非线性理论模型,以W波段两段结构回旋行波管放大器为例,详细分析了速度零散对放大器电子注—波互作用的影响.模拟结果表明,通过合理地调整互作用长度,减小电子横纵速度比、调节工作磁场等方法可以有效地减小速度零散的影响,对回旋行波管放大器的优化设计提供了理论依据.     

Characteristics and Optimum Operating Parameters of a Gyrotron Traveling Wave Amplifier

Characteristics and optimum operating parameters are determined for a new type of high-power high-efficiency generator of millimeter waves known as a gyrotron traveling wave amplifier. In the example consided, wave amplification results from the interaction of a TE/sub 01/ waveguide mode with the fundamental cyclotron harmonic of an electron beam. The parameter optimization involves the determination of the point of maximum device efficiency as a function of beam density, beam energy, beam positioning, and external magnetic field for the output power required. An analytical linear theory and a numerical simulation code form the basis of theoretical calculations. As a result of the extensive survey in parameter space, the peak efficiency in the beam frame has been found to exceed 70 percent. This result has been applied to the specific design of a 35-GHz amplifier with output power ~340 kW, a power gain of 2 dB/cm, and a laboratory frame efficiency of 51 percent.     

Enhancement of high-harmonic gyrotron gain by a dielectric rod

A dielectric rod is shown to reduce the energy requirements of the electron beam for high-harmonic gyrotron operation. A large interaction filling factor can occur for moderate-energy electrons (～80 keV) spiralling around the rod within the evanescent region of a dielectric waveguide mode. Since the magnetic field strength is reduced by an order of magnitude in an interaction at the tenth harmonic of the cyclotron frequency, a submillimeter-wave gyrotron is now truly feasible because the acceleration voltages can be conveniently produced by DC power supplies.     

140 GHz共焦波导结构回旋行波管放大器

为减少太赫兹回旋器件模式密度和降低模式竞争问题,利用具有模式选择特点的共焦波导结构作为140 GHz回旋行波管(Gyro-TWT)的高频互作用系统。在理论分析基础上,建立注波互作用计算模型并对其进行数值计算;通过对共焦波导高频场分布、衍射损耗、耦合系数以及注波互作用效率等输出参量的分析,选择HE06作为工作模式,确定了140 GHz Gyro-TWT放大器的基本结构和工作参数,并利用注波互作用非线性理论进行分析。模拟结果表明：在注电压为35 kV,注电流2 A,速度比为0.75时,该高频结构在140 GHz频点获得12 kW峰值输出功率,17.1%电子效率和38 dB饱和增益,3 dB带宽达到6 GHz。     

损耗介质加载结构Ka波段TE01模回旋行波管放大器稳定性的分析

该文从分析损耗介质加载金属圆波导中电磁波传输特性出发,应用回旋行波管放大器小信号色散方程,研究损耗介质加载结构TE₀₁模回旋行波管放大器绝对不稳定性振荡和回旋返波振荡对器件稳定性的影响。结果表明：损耗介质的加载,可以提高绝对不稳定性起振电流,提高的幅度依赖于工作磁场偏离饱和磁场的程度、电子束横纵速度比等;增加波导损耗介质加载的厚度,可以提高竞争模式的回旋返波起振长度。合理选择波导的损耗层厚度、介电常数以及回旋行波管的工作电压、工作磁场和电子束横纵速度比,可以有效兼顾带宽和抑制不稳定性,保证回旋行波管放大器稳定工作。     

Quasi-optical gyro-peniotron at high cyclotron harmonics

This paper proposes a new kind of device operating at short millimetre wavelength, in which RF radiation generation is based on the interaction of a beam of sub-relativistic gyrating electrons and the RF standing-wave field of a quasi-optical resonator. This device can be called ‘quasi-optical gyro-peniotron’ because it is similar in electron-beam configuration, microwave circuitry and cyclotron resonance properties to the quasi-optical gyrotron and in its operational mechanism to the peniotron. Starting from relativistic equations of motion for the electrons, the calculation formulae are derived. Using computer simulation, a beam-wave interaction efficiency of 43% is obtained for the third cyclotron harmonic at frequency 100 GHz, beam voltage 80 kV and the applied magnetic field 13·3 kG, indicating that this tube will be potentially a high-efficiency high-power source not requiring a superconduction magnet system even within the 150 GHz band.     

Gyrotron-TWT Operating Characteristics

The gyrotron traveling-wave tube (gyro-TWT) is a new type of millimeter amplifier which employs the electron cyclotron maser instability as a basis for the electron-electromagnetic wave interaction. A C-band gyro-TWT, employing the fundarnental cyclotron resonance interaction with the circularly polarized TE/sub 11/ dominant waveguide mode, has been constructed and tested. Initial power measurements yielded an output power of 50 kW at 60-kV beam voltage with 16.6-percent efficiency and 6-percent bandwidth. These measurements were recorded with a flat magnetic field. Subsequent experimental testing yieIded, for a magnetic field increasing in magnitude towards the output portion of the tube, 128-kW and 65-kV beam voltage at 24-percent electronic efficiency. The maximum efficiency was 26 percent at 120.5-kW peak power, with an instantaneous bandwidth of 7.25 percent as measured in a high-beam power mode (65 kV, 7 A). In the low-beam power mode (40 kV, 4 A), the efficiency was 9.8 percent at 18.8-kW peak power at 9.3-percent instantaneous bandwidth. Additional experimental results of AM and PM modulation coefficients, spectral purity, phase Iinearity, and noise figure are presented.     

Self-consistent large-signal calculation on the gyrotron-TWA

By means of a ballistic method a self-consistent large-signal calculation on the gyrotron traveling wave amplifier (gyrotron-TWA) with uniform waveguide and magnetic field has been accomplished. The theory is valid for arbitrary harmonics of the cyclotron frequency and arbitrary TEmn modes. Computer simulation shows that under defined conditions a beam efficiency ～50 %, an output power ?300 kW and a gain～28 dB at a central frequency of 35 GHz with a 3 dB bandwidth of 2,7 % could be obtained. This method will be effective for the theoretical analysis and the parameter-optimization on the gyrotron-TWA.     

Marginal stability design criterion for gyro-TWTs and comparison of fundamental with second harmonic operation

Stability properties of both the fundamental and second harmonic gyrotron travelling wave amplifier (gyro-TWT) are examined with multi-mode particle simulations. The second harmonic cyclotron interaction with an axis-encircling electron beam is found to be more stable to oscillations and can yield significantly greater power than the fundamental harmonic gyro-TWT. A multiple stage interaction structure based on a marginal stability criterion is proposed and illustrated with examples of a 128kW fundamental gyro-TWT and a 532 kW second harmonic gyro-TWT, Stable amplification at much higher power levels is in principle possible.     

Ka波段二次谐波回旋行波管放大器的研究

二次谐波回旋行波管放大器的互作用磁场比基波回旋行波管放大器的磁场降低了一半,从而降低了设计难度,具有广阔的应用前景。通过对周期介质加载结构的Ka波段二次谐波回旋行波管电子枪、高频结构、模式竞争以及注波互作用研究,确定了Ka波段TE02模二次谐波回旋行波放大器的基本工作参数,通过PIC模拟计算,在电子注电压为90 kV,注电流为25 A时,获得了大于200 kW的输出功率,超过40 dB的增益。     

Nonlinear analysis of the solid-state gyrotron oscillator by the Monte Carlo method

A nonlinear analysis of the solid-state gyrotron oscillator is described to calculate the efficiency (η) and output power (Pw) of the device. Electron trajectories are calculated numerically. The motion of the electrons inside the cavity consists alternately of drift along helical trajectories followed by scattering. A Monte Carlo method has been used to treat scattering. Scattering processes included in the calculation are polar optic phonon, acoustic phonon, impurity, and impact ionization. Nonparabolicity and wave vector dependence of the periodic part of block functions are also used since phase bunching of electrons occurs due to the variation of effective mass with energy in the conduction band. η and Pware calculated as functions of frequency, temperature, impurity concentration, applied magnetic field, and other physical parameters ot the electron beam and the cavity made from InSb. At 500-GHz frequency and 4 K, output power of about 100 µW (assuming that Q of the cavity is 10) can be obtained with an efficiency of 5 percent.     

Dependence of Output Power on Cavity Length in a Gyrotron Backward Wave Oscillator

A relativistic electron beam (500 keV, 200 A, 10 ns) generated magnetically tunable microwave radiation in a frequency range of 9-13 GHz when it is injected into an X-band rectangular waveguide immersed in a uniform axial magnetic field (4-10 kG). The mechanism of the microwave radiation was identified as the gyrotron backward wave interaction. The output power of the radiated microwave increased exponentially with the increase of the cavity length.     

Nonlinear Analysis of A Slotted Third-Harmonic Gyro-TWT Amplifier

The distribution of RF field of the slotted cylindrincal waveguide is discussed and the self-consistent nonlinear theory of the beam-wave interaction is presented in this paper.The behavior of the 95-GHz slotted third-harmonic gyrotron traveling-wave amplifier (gyro-TWT) with a uniform section is simulated by a Runge-Kutta algorithm code for an axis-encircling electron beam with velocity spread. Some important relations in the process of the beam-wave interaction are abtained and analyzed.     

Experimental investigation of a broadband dielectric-loadedgyro-TWT amplifier

The bandwidth of a gyrotron travelling wave amplifier has been broadened by incorporating a dielectric-loaded interaction waveguide to reduce the circuit's dispersion. This proof-of-principle experiment was designed for the X-band frequency range and operates in the fundamental mode of a rectangular waveguide loaded with dielectric Macor. The amplifier is zero-drive stable and demonstrates a peak output power of 55 kW, 11% efficiency, 27 dB saturated gain with an unprecedented untapered gyro-TWT constant-drive bandwidth of 11% and a saturated bandwidth of 14%. Its performance can be further enhanced by reducing the beam's axial velocity spread as shown by previous simulation studies     

Theory and simulation of an 85 GHz quasioptical gyroklystron experiment

The linear theory used to design a two-resonator 85 GHz quasioptical gyroklystron with a nonuniform magnetic field is presented. It is shown that a tapered magnetic field in the prebunching resonator has a relatively small effect on the electron bunching parameter. The effect of velocity spread of the electron beam can be minimized by adjusting the magnetic field strength in the two resonators. Measured amplifier performance is in good agreement with calculations from the nonlinear multimode simulation code. Gyrophase bunching of the electrons is preserved over the long drift region (30 radiation wavelengths) even though no attempt has been made to minimize the velocity spread of the beam.     

Design of a Large Orbit Gyrotron with a Permanent Magnet System

The paper presents results of numerical analysis and outlines the computer-aided design of a novel high-harmonic gyrotron with a beam of electrons gyrating along axis-encircling trajectories. The electron beam is formed by a novel electron-optical system (EOS) based on an electron gun of diode type with thermionic cathode and gradual reversal of the magnetic field. The results of numerical simulations predict satisfactory performance of the EOS and appropriate beam quality parameters. The tube design allows one to install different cavities optimized for excitation of TE_4,1 mode at the fourth harmonic of the cyclotron frequency or TE_3,1 mode at the third one. The target parameters of the device are: frequency about 112 GHz; output power near 1 kW and efficiency of several percent.