回旋管光子带隙谐振腔冷腔电磁模式分析

利用二维三角格子金属光子带隙谐振腔代替回旋管的传统柱形谐振腔,并对腔体进行了TE波模式的计算与分析.综合考虑腔体内外的结构特征,给出了光子带隙谐振腔冷腔模式理论的研究方法,发现腔内存在单模工作的可能与条件以及非角对称情况下的电磁模式分布特征.研究结果表明,利用光子带隙谐振腔代替回旋管的传统腔体,可使回旋管在不受腔体横向尺度限制的条件下实现单模工作.这对于提高回旋管的功率容量、有效实现高次单模与高次回旋谐波耦合条件下的注-波互作用、降低工作磁场并从物理上根本改变回旋管的工作状态提供了理论依据. 关键词： 回旋管 金属光子带隙 谐振腔     

Controllable spectrum of an axial-mode gyrotron with external reflections

The feasibility of controlling the spectrum of multifrequency oscillations in a gyrotron by means of external reflections is studied. It is shown that, in self-modulation oscillation modes of operation, the radiation spectrum lines may come close to the resonance frequencies of a combined electromagnetic system including the reflector-limited part of the output waveguide. Under these conditions, the frequency separation of modes and, accordingly, the self-modulation period can be controlled by varying the position of the reflector. Theoretical results are corroborated by experiments with a 30-GHz relativistic gyrotron with an external reflector.     

同轴腔高阶模式回旋管振荡器中多模起振过程的非线性模拟

给出了工作在高阶模式下的同轴腔回旋管振荡器,在考虑竞争模式情况下的多模起振过程的数理模型和具体的数值计算方法。以此为基础编制的程序对一只工作于高阶模式TE42, 22,频率为0.22 THz的同轴腔回旋管振荡器进行了多模非线性模拟研究。结果表明:在合适的腔体结构参数并考虑腔壁上的欧姆损耗情况下,竞争模式得到有效抑制,可以稳定地工作在单一模式,输出功率达0.87 MW,效率为22.8%,腔壁上的欧姆损耗低于其所能承受的最大值。     

Detailed Consideration of Experimental Results of Gyrotron FU CW II Developed as a Radiation Source for DNP-NMR Spectroscopy

A CW gyrotron for the sensitivity enhancement of NMR spectroscopy through dynamic nuclear polarization has been designed. The gyrotron operates at the second harmonic and frequency of 394.6 GHz with the main operating mode TE_0,6. Operating conditions of other neighboring cavity modes such as TE_2,6 at frequency of 392.6 GHz and TE_2,3 at frequency of 200.7 GHz were also considered. The experimental conditions of the gyrotron at low and high voltages are simulated. The output power of 56 watts corresponds to the efficiency of 2 percent at low voltage operation and frequency of 394.6 GHz is expected.     

Properties of a Second Harmonic Gyrotron Cavity with Internal Blazed Grating

Spectral domain analysis, scattering matrix analysis and PIC simulations are used here to design a second harmonic gyrotron cavity with an internal blazed grating which favors operation at the second harmonic of the electron cyclotron frequency rather than operation at the electron cyclotron frequency itself. Based on these simulations, the cavity design is optimized. The results show that a new gyrotron with higher power and higher frequency can be achieved by incorporating such a blazed grating.     

Continuous-Wave Operation of a Frequency-Tunable 460-GHz Second-Harmonic Gyrotron for Enhanced Nuclear Magnetic Resonance

The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for 700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE(11,2) and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE(11,2,q). The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ±6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%.     

Long Pulse Operation of the THz Gyrotron with a Pulse Magnet

Long pulse operation up to 1 msec of a high frequency gyrotron with a pulse magnet has been successfully carried out in a frequency range including 1 THz. In the experiments, the timing of an electron beam pulse injection is adjusted at the top of the magnetic field pulse, where the variation of field intensity is negligible. The operation cavity modes seem to be TE_{1, 12} and TE_4,12 at the second harmonics. The corresponding frequencies are 903 GHz and 1,013 GHz, respectively. Additionally several features of radiation measurement results of the gyrotron are described and brief considerations are presented.     

Numerical Simulation and Analysis of a New Type of Complex Cavity for Gyrotron Applications

A new type of complex cavity structure is used to improve the selectivity of modes in gyrotron resonant cavity with a rational position of inter-cavity wall and the shape of coupling holes between inter- and outer-cavities. The resonant characteristics of the complex cavity operating in TE₀₂₁/TE₀₃₁ mode at Ka frequency band are simulated and analyzed with Ansoft HFSS code, which shows that mode competition can effectively be alleviated with the cavity structure and it may be useful to the formation of clustered-cavity for extending interaction frequency bandwidth.     

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     

有损金属圆波导中电磁波传输特性的研究

基于电磁模式的色散方程，研究了有损金属圆波导中电磁模式的传输问题.在考虑了损耗层厚度影响的情况下，得到了各模式传输常数的近似解析表达式，此式适用于截止频率附近.通过解析表达式和波导模式色散方程，对衰减常数和相位常数进行了大量数值计算，得到了它们随损耗层厚度、频率以及损耗层电导率的变化规律，两种方法所获得的结果有较好的一致性.研究表明，损耗强度和损耗层厚度的变化会改变波导中电磁模式的色散和简并特性，对于改善高功率回旋行波放大器中注波互作用带宽和模式竞争非常有益. 关键词： 有损圆波导 相位常数 衰减常数 色散方程     

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     

Design of a Quasi-Optical Mode Converter for a Frequency Step-Tunable Gyrotron

The quasi-optical mode converter for a frequency step-tunable gyrotron which consists of a dimpled-wall antenna (Denisov-type launcher) and a beam-forming mirror system has been optimized for 9 modes from TE_17,6 at 105 GHz to TE_23,8 at 143 GHz. The first mirror is a large quasi-elliptical focusing one; the second and third are phase-correcting mirrors with a non-quadratic shape of the surface. The results of calculations show that for these modes the Denisov-type launcher has a well-focused beam with low diffraction losses, and the radiation pattern presents an almost identical field shape for all modes considered. A multi-mode optimization of the phase-correcting mirrors with two different methods has been tested. The simulations show that the phase-correcting mirrors can be used for broadband operation in the frequency range from 105 GHz up to 143 GHz in the various design modes. This quasi-optical mode converter can achieve efficiencies of 94%-98% for converting the rotating high-order cylindrical cavity modes into the usable fundamental Gaussian mode.     

High-Power Microwave Generation from a Large-Orbit Gyrotron in Vane and Hole-and-Slot Conducting Wall Geometries

The production of high-power microwave radiation from a large-orbit gyrotron in azimuthally periodic boundary systems is studied theoretically and experimentally. Linear growth rates are calculated for the 2? modes of magnetron-like vane resonator (VR) and hole- and-slot resonator (HASR) systems using a general growth-rate formalism. The experiment involves the interaction of a 2.3-MeV, 1-2-kA, 5-ns rotating electron layer with two different periodic structures. About 500 MW is produced in Ku band with a 20-slot VR system and about 300 MW is produced in X band with a 10-slot HASR system. The relative merits of both types of systems are discussed.     

95 GHz三次谐波回旋管设计

采用线性理论和非线性理论研究了回旋管谐振腔结构、寄生模式抑制及注波互作用等问题。设计了一支工作在95 GHz的三次谐波回旋管,注波互作用结构采用标准开放式谐振单腔,工作模式为TE64, 采用电压45 kV、电流5 A、横纵速度比为1.5的小回旋电子注。在不考虑电子注速度离散及厚度的情况下,非线性理论分析表明,该回旋管可以获得14 kW功率输出,横向互作用效率约为18%,整管效率约11%。     

连续调谐太赫兹回旋管非稳定振荡状态研究

太赫兹回旋管可实现高功率输出,并具有一定的频率调谐范围,是核磁共振波谱系统理想的高功率太赫兹辐射源。设计了263 GHz,TE_5,2基波连续调谐回旋管,通过磁场调节实现频率调谐范围为1.39 GHz,利用时域多模多频自洽非线性理论对设计的连续调谐回旋管非稳定振荡状态进行了研究。结果表明,在低次纵向谐波模式工作磁场范围内,当工作电流大于起振电流时,连续调谐回旋管先进入稳定状态,高次纵向谐波模式被抑制,工作模式TE_5,2的输出功率随时间不变;当电流增大,纵向谐波模式间的竞争引起回旋管由稳定状态进入到非稳定振荡状态,工作模式TE_5,2的输出功率随时间呈振荡变化且互作用效率大大降低;随着电流的进一步增大,回旋管又回到与低电流不同的稳定状态,互作用效率进一步降低。同时发现非稳定振荡状态的起始电流随着磁场增加而增大。本研究对需工作于稳定状态的面向DNP-NMR应用的连续调谐太赫兹回旋管的研制具有一定指导意义。     

A Gyrotron with a High Q Cavity for Plasma Scattering Diagnostics

A 137-GHz gyrotron with a high Q (~6000) cavity has been built and tested in short-pulse operation. Output power of 0.1-20 kW and efficiency of up to 26 percent were observed at 65-kV, 0.03-1.4-A operation in the TE031 mode. Frequency pulling of 2-kHz/V was measured. This small frequency pulling value indicates that a long pulse or CW gyrotron with high stability (<1-MHz bandwidth) is possible. Operation at currents of 3-5 A, which is very far above threshold, resulted in the excitation of many higher axial modes.     

Resonant frequencies of a rising-sun gyrotron cavity

New resonant frequency calculations for a tunable gyrotron cavity with a magnetron-like rising-sun cross-section give results that are measurably different from those found by the traditional method, especially as the wavelengths become short enough for the radial slots to support higher-order modes. Moreover, in many instances a large fraction of the microwave field energy resides in these slot modes and as a result any gyrotron interaction is expected to be small.     

Photonic-band-gap resonator gyrotron.

We report the design and experimental demonstration of a gyrotron oscillator using a photonic-band-gap (PBG) structure to eliminate mode competition in a highly overmoded resonator. The PBG cavity supports a TE(041)-like mode at 140 GHz and is designed to have no competing modes over a minimum frequency range delta omega/omega of 30% about the design mode. Experimental operation of a PBG gyrotron at 68 kV and 5 A produced 25 kW of peak power in the design mode. No other modes were observed over the full predicted operating range about the design mode. PBG cavities show great promise for applications in vacuum electron devices in the millimeter- and submillimeter-wave bands.     

Development of 394.6 GHz CW Gyrotron (Gyrotron FU CW II) for DNP/Proton-NMR at 600 MHz

Gyrotron FU CW II with an 8 T liquid He free superconducting magnet, the second gyrotron of the THz Gyrotron FU CW Series, has been constructed and the operation test was successfully carried out. It will be used for enhancing the sensitivity of 600 MHz proton-NMR by use of Dynamic Nuclear Polarization (DNP). The designed operation mode of the gyrotron is TE_2,6 at the second harmonic. The corresponding frequency is 394.6 GHz. The real operation frequency is 394.3 GHz at TE₀₆ mode, because of fabrication error of the diameter of the cavity. The operation is in complete CW at the output power of around 30 W or higher at the TE₀₆ cavity mode. There are many other operation modes at the fundamental and the second harmonic. Typical output power of the fundamental and the second harmonic are higher than 100 W and 20 W, respectively. The highest frequency observed up to the present is 443.5 GHz at the second harmonic operation of TE_6,5 mode. The measured results are compared with the theoretical consideration.