GAIN AND BANDWIDTH ANALYSIS OF A VANE-LOADED GYRO-TWT

Theoretical investigation of the peak-gain and 3-dB bandwidth of the vane-loaded gyro-traveling wave tube (gyro-TWT) amplifier in the small-orbit TE₀₁ waveguide mode configuration at 35 GHz has been presented. The vane-loaded gyro-TWT enjoys higher gain and bandwidth compared to that of the smooth-wall device. In the analysis, the azimuthal harmonic effects generated due to the angular periodicity of vanes in the wedge-shaped metal vane-loaded cylindrical waveguide interaction structure have been taken into account in the cold (beam-absent) dispersion relation only.     

Analysis of an azimuthally periodic vane-loaded cylindrical waveguide for a gyro-travelling-wave tube

A cylindrical waveguide, provided with wedge-shaped metal vanes projecting radially inward from the wall of the guide, excited in the transverse electric (TE) mode, was analysed. The analysis was carried out considering the angular harmonics generated by the angular positioning of the vanes. A set of equations was generated in the Fourier amplitudes of field constants. The condition for nontrivial solutions for the field constants gave the dispersion relation of the structure. From the expression of power flow down the structure, its interaction impedance was also estimated. The shape of the dispersion characteristics and the value of the cutoff frequency as well as the interaction impedance characteristics of the waveguide were found to depend on the vane parameters their number as well as their radial and angular dimensions. The optimum vane parameters were obtained corresponding to the minimum variation of the slope of the ω-β dispersion plot, such parameters being useful from the standpoint of the bandwidth of a gyro-travelling-wave tube (gyro-TWT) using a vane-loaded cylindrical waveguide as the interaction structure. The dispersion and impedance characteristics, which were found typically for the TE₀₁ mode as defined for the structure, taking four vanes, were more sensitive to the number and angular width of the vanes than to their radial depth. The value of the interaction impedance, calculated at the potential beam position, was found to be higher for a loaded waveguide than for an unloaded one, and it depended on the frequency of operation relative to the cutoff. The interaction impedance also depended on the position of the beam relative to the waveguide wall where it was estimated, and hence the optimum beam position corresponding to the maximum interaction impedance was found. The theory was validated against the dispersion characteristics reported elsewhere typically for four-vane magnetron-like structures excited in the 2π mode. Although the present study was restricted to ‘cold’ analysis of the structure in the absence of the electron beam, it could provide important feedback for analysing a gyro-TWT, using a vane-loaded cylindrical waveguide, and hence for predicting the structure parameters for the wide-band performance of the device.     

Effects of Beam and Magnetic Field Parameters on Highly Competing TE01 and TE21 Modes of Vane Loaded Gyro-TWT

In this paper, the dispersion relation for a vane-loaded cylindrical waveguide, considering the azimuthal harmonic effects arising from the azimuthal periodicity of the vanes, excited in transverse electric (TE) modes and the gain for a gyro-TWT have been presented. The effects of beam and magnetic field parameters on two highly competing modes have been studied.     

Effects of circuit loss and space charge in gyro-travelling-wave tubes

A modified dispersion relation of a gyro-TWT using a cylindrical waveguide excited in aT E _mn -mode is derived incorporating the effect of the transverse space charge forces. The relation is subsequently studied considering the circuit losses and interpreted for the conventional TWT-type gain equation. The second harmonic operation using theT E ₀₂ cylindrical waveguide mode resonated with the second beam-harmonic mode is found to be more sensitive to the Pierce's loss and space-charge parameters for a gyro-TWT than the fundamental operation using theT E ₀₁ cylindrical waveguide mode resonated with the first beam-harmonic mode. It is established that these parameters substantially influence the device gain and must be considered for the prediction of its accurate value. Furthermore, these parameters when properly controlled can also improve the device bandwidth.     

Ku波段螺旋波纹波导回旋行波管色散特性研究

为抑制回旋行波管的自激振荡和增加回旋行波管带宽,俄罗斯G.Denisov等人提出一种新型回旋行波管结构——螺旋波纹波导回旋行波管。通过螺旋波纹波导的特殊结构使通过波导的两种模式发生耦合,耦合出一种新的工作模式,从而改变色散特性,达到抑制自激振荡和增加带宽的目的。通过螺旋波纹波导的色散方程,分析其色散曲线,从而分析螺旋波纹波导作为回旋行波管高频系统的优势。     

A Stable 0.2-THz Coaxial-Waveguide Gyrotron Traveling-Wave-Tube Amplifier with Distributed Losses

For high-power operation, a THz gyrotron traveling-wave-tube (gyro-TWT) amplifier must operate in a high-order waveguide mode to enlarge the transverse dimension of an interaction waveguide. However, a gyro-TWT amplifier operating in a high-order waveguide mode is susceptible to spurious oscillations. To improve the device stability, in this study, we investigate the possibility of using a coaxial waveguide with distributed losses as the interaction structure. For the same required attenuation, all threatening oscillating modes can be suppressed using different combinations of losses of inner and outer cylinders. This provides flexibility in designing distributed losses when considering the ohmic loading of the interaction structure. We predict that the 0.2-THz gyro-TWT can stably produce a peak power of 14 kW with an efficiency of 23 %, a 3-dB bandwidth of 3.5 GHz, and a saturated gain of 50 dB for a 20-kV 3-A electron beam with a 5 % velocity spread and 1.0 velocity ratio.     

Linear Analysis of a Cyclotron Autoresonance Maser (CARM) Operating in a Transverse Magnetic Mode

In the fast-wave devices like gyrotron, gyro-peniotron and cyclotron autoresonance maser (CARM) that generate millimeter and sub-millimeter waves, the transverse dimensions of the resonator and the output cylindrical waveguide become small. In order to prevent loss of electrons and thermal loading of the rf structure, the electron beam must be kept relatively far from the walls. The latter requirement demands smaller transverse dimensions of the helical electron beam as well. In this paper linear formulation of a CARM operating in a general transverse-magnetic (TM) mode is derived, and a detailed analysis of the influences of the parameters is presented for the TM_1,1 mode CARM. It is found that, compared to the TE_1,1 mode which is often employed in gyrotron traveling wave tube (gyro-TWT) and CARM experiments, the TM_1,1 mode has a greater eigen value and consequently leads to a greater waveguide radius for a given cutoff wave number, and also, allows the electron beam to be settled close to the waveguide axis to have a small transverse dimension. Results show that a TM-mode CARM can reach high power and ultrahigh gain, just as a TE-mode CARM or a TE-mode gyro-TWT does.     

Stability analysis of TE01 gyrotron travelling wave amplifiers

The fundamental harmonic TE₀₁ gyrotron travelling wave amplifier (gyro-TWT) is a high-power, broadband, millimetre-wave amplifier with a low ohmic dissipation and a large guiding centre radius. However, spurious oscillations may reduce the amplification of the gyro-TWT. The stability of severed and distributed-loss gyro-TWTs is analysed, using a self-consistent simulation code. A nonlinear analysis of typical oscillations, including absolute instability, gyrotron backward oscillation (gyro-BWO) and reflective oscillation, is presented. Simulation results indicate that attenuating severs in the severed gyro-TWT do not suppress spurious oscillations, and that increasing the wall losses to suppress gyro-BWO in the distributed-loss gyro-TWT degrades the efficiency of the gyro-TWT amplifier. A multi-stage distributed-loss gyro-TWT design is developed to stabilize the amplification. The lossy and severed sections of the multi-stage gyro-TWT seem to increase effectively the start-oscillation currents of absolute instability and gyro-BWO, respectively. The multi-stage gyro-TWT is predicted to yield a peak output power of 155?kW at 32.9?GHz with an efficiency of 15%, a saturated gain of 45?dB and a bandwidth of 2.2?GHz for a 100?kV, 10?A electron beam with an axial velocity spread Δv_z /v_z = 5%.     

W-Band Second-Harmonic Gyrotron Traveling Wave Amplifier with Distributed-Loss and Severed Structures

The second harmonic TE₀₂ gyrotron traveling wave amplifier (gyro-TWT) is a high-power, broadband, millimeter-wave amplifier with a low applied magnetic field. Mode-selective interaction circuits were applied to suppressing spurious oscillations. However, the mode-selective interaction circuit may perturb the operating mode in the gyro-TWT. A multi-stage gyro-TWT design with distributed-loss and severed structures is proposed to stabilize the amplification. This study presents a nonlinear analysis of typical oscillations, including absolute instability, gyrotron backward oscillation (gyro-BWO) and reflective oscillation. The lossy and severed sections of the multi-stage gyro-TWT seem to increase effectively the start-oscillation currents of the absolute instability, gyro-BWO, and reflection oscillation. The multi-stage gyro-TWT is predicted to yield a peak output power of 215 kW at 89.9 GHz with an efficiency of 14.3 %, a saturated gain of 60 dB and a bandwidth of 1.7 GHz for a 100 kV, 15 A electron beam with an axial velocity spread Δν _z/ν _z = 5%.     

周期损耗介质加载波导与均匀圆波导间的模式映射

损耗介质环与金属环间隔加载的周期损耗介质波导能够有效地控制各种模式的衰减特性,这对于抑制毫米波回旋行波管放大器的绝对不稳定性和提高其性能具有重要作用.针对应用于Ka波段、TE01模的回旋行波管放大器的周期损耗介质加载圆波导,系统地分析了该周期系统与均匀系统间的模式映射关系.研究表明,当介质层厚度一定时,均匀介质加载波导中的高阶模式可以映射为光滑波导中的低阶模式,且相互映射的模式在中空区域的场型一致.周期系统中的模式表现出复合模式的分布.在一个周期中,介质段和金属段的模式分别映射为均匀介质波导和金属波导中的模式.明确周期介质加载波导系统与均匀波导系统间的模式映射关系是分析发生在这种复杂的互作用回路中的回旋电子脉塞注波互作用的前提,对简化其物理模型具有指导作用.     

Exploration of a double-tapered disc-loaded circular waveguide for a wideband gyro-TWT

The analysis of a disc-loaded circular waveguide interaction structure of a gyro-traveling-wave-tube (gyro-TWT) considering standing and propagating mode harmonics in the disc-occupied and disc-free regions, respectively, gave the beam-absent dispersion relation of the waveguide. The axial phase propagation constant predicted by the dispersion relation was substituted into the gyro-TWT gain-equation, the latter obtainable from the beam-present dispersion relation of the device. A method of double-tapering the structure dimensions was proposed that consists in tapering the disc-hole radius to distribute the midband frequency of amplification over a wide range of frequencies, and simultaneously tapering the waveguide-wall radius to compensate for gain reduction at band edges due to disc-hole radius tapering. The method has demonstrated wide device bandwidths at relatively large gain values.     

Nonlinear Simulation of a Cyclotron Autoresonance Maser (CARM) Operating in a Transverse Magnetic Mode

In the gyrotron operation, the transverse-magnetic (TM) mode is excluded because the TM mode instability vanishes when the vacuum waveguide mode and the beam mode are at grazing incidence. However, situation changes in a cyclotron autoresonance maser (CARM) interaction. In this paper nonlinear formulation of a TM-mode CARM is derived, and detailed simulations are presented for the TM_1,1-mode CARM. Simulation results show that a TM_1,1-mode CARM can reach high power of megawatts and ultrahigh gain of more than 70 dB, as a TE_1,1-mode gyrotron traveling wave tube (gyro-TWT) and TE_1,1-mode CARM did in the reported experiments.     

分布损耗回旋行波管线性分析与稳定性研究

研究了分布损耗材料导电特性的下降能够增强回旋行波管注-波互作用段对电磁波的衰减,提高了绝对不稳定性振荡的起振电流和竞争模式回旋返波振荡的起振长度;但同时减小了回旋行波管的线性增长率及其轴向功率和高频场增益.给出了石墨乳涂敷圆波导结构回旋行波管工作参数,采用电子束电流,I_0=10A、注-波互作用段长度L=10cm时的设计结果,在兼顾功率和增益的条件下,能够有效抑制不稳定性,保证其稳定工作.     

具有高稳定性的模式选择电路回旋行波管放大器

该文基于线性理论和自洽非线性理论,对具有模式选择特性的损耗陶瓷和金属环交替加载的Ka波段TE01模回旋行波管放大器进行理论建模和稳定性分析。分析表明损耗陶瓷加载波导具有模式选择特性,其传输特性类似于光滑圆波导;稳定性分析揭示了互作用系统自激振荡的内反馈机制。将理论分析与美国海军实验室的实验工作进行了比较,证明了该文理论模型的可靠性。该文的理论模型对回旋行波管放大器的设计具有一定的指导意义。     

螺旋槽波导特性的计算与模拟研究

计算了螺旋槽波导的色散特性和群速度,用“准周期边界条件”方法和谐振法模拟计算了螺旋槽波导的色散特性,两种方法的模拟结果与计算都比较一致。此外,还模拟测试了渐变与规则螺旋槽波导连接的匹配性能、螺旋槽波导高频系统的传输特性。结果表明,螺旋槽波导是非常适用于回旋行波管的高频结构。     

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     

Experimental Study of a Ka Band Gyro-TWT with the Mode-Selective Circuits

The design and experimental study of a Ka-band gyrotron-travelling-wave tube (gyro-TWT) amplifier operating in the circular TE₀₁ mode at fundamental cyclotron harmonic are presented. A mode-selective technique was adopted in the interaction circuit of the gyro-TWT to suppress spurious modes TE₁₁ and TE₂₁. A saturated peak power of 86 kW was obtained with saturated gain of 33 dB , an efficiency of 21.3%, in operating voltage 62 kV, electron current 6.5 A and -3 dB bandwidth of 2 GHz (~6%) .     

Geometrical Effects on the Performance of Coaxial-Waveguide Gyrotron Traveling-Wave Amplifiers

This theoretical investigation examines the feasibility of improving the stability of the coaxial-waveguide gyrotron traveling-wave tube (gyro-TWT) by selecting the geometrical parameter C, i.e., the ratio of the outer radius to the inner radius. The effects of the geometrical parameter C on the start-oscillation currents of oscillation modes are analyzed to determine the optimum operating conditions. Simulation results indicate that the coaxial gyro-TWT with distributed wall losses can be stably operated at a higher beam current by optimizing C. Additionally, the saturated behaviors of the operating TE₀₁ mode are evaluated for several C values to investigate the geometrical effects on the amplification of the coaxial gyro-TWT. Moreover, performance of the fundamental harmonic coaxial gyro-TWT achieved with the optimized C value is predicted under stable operating conditions.     

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.     

近太赫兹频段线性注真空器件的研究

介绍了太赫兹频段真空电子器件的研究和开发进展,包括慢波结构理论、设计、模拟及优化,微加工和微组装技术,整管技术等。这些器件包括行波管、返波管、斜注管、止带振荡器及行波管谐波放大器等,高频结构以折叠波导慢波结构为主,在太赫兹返波管中则利用叶片加载波导慢波结构。器件技术包括微机电系统(MEMS)技术,微波等离子体化学气相沉积(MPCVD)金刚石生长、金属化和封接技术等。最后给出W波段、G波段以及340 GHz部件和器件所达到的性能。