渐变螺距螺旋线慢波系统高频特性的仿真分析

 为实现渐变螺距螺旋线慢波结构高频参数的高精度估计，基于MAFIA仿真平台，研究了螺距变化对超宽带螺旋线慢波系统(4~18 GHz)的色散、互作用阻抗与衰减常数的影响，获得了各高频参数随螺距变化的规律。研究表明：相速几乎随螺距增大而线性变大，互作用阻抗与螺距是非线性的关系，衰减常数随螺距增大而非线性地减小；由于在维持电子注与电磁波速度同步的限制下，螺距变化的幅度不可能很大，因此可近似按线性关系来处理互作用阻抗及衰减常数与螺距之间的关系。由此提出了一种可精确计算渐变螺距螺旋线慢波结构高频参数的方法－线性插值法     

螺旋线厚度对螺旋线慢波结构高频特性的影响

在螺旋坐标系下采用近似场匹配法研究了厚度较厚的螺旋线慢波结构的高频特性,得到了其色散方程,并数值计算了螺旋线厚度对慢波系统的相速的影响。结果表明:随螺旋线厚度的增加,慢波结构的色散曲线变得平坦,色散变弱,带宽增加。与软件仿真结果及Hook的螺旋线模型对比表明:采用近似场匹配法的计算结果与CST仿真吻合得更好,比Hook模型具有更高的准确性,从而证明所采用理论方法的有效性。     

带状注行波管双矩形螺旋线慢波结构高频特性北大核心CSCD

针对新型螺旋线慢波结构——双矩形螺旋线慢波结构,即在金属屏蔽框内平行加载两个具有矩形横截面形状的自由螺旋线慢波结构,利用三维电磁仿真软件对其高频特性(色散特性和耦合阻抗)进行模拟研究。结果表明:在相同位置处,与单矩形螺旋线慢波结构相比,双矩形螺旋线慢波结构色散特性变化很小,却有着更高的耦合阻抗;同时,在包含这些位置的空间部分,可采用带状电子注与该慢波结构进行注波互作用,使输出功率进一步得到提高。     

螺旋线行波管慢波结构设计及注波互作用模拟

通过模拟计算,分析螺旋线内径和螺距变化对色散和耦合阻抗的影响,优化慢波结构,初步设计了Ku波段螺旋线行波管慢波结构。模拟行波管输入输出结构,得到输入端反射系数小于-19 dB,电压驻波比小于1.24。电子聚焦系统采用周期永磁聚焦,磁场周期为8.5 mm,计算得到磁场峰值为0.17 T。为提高注波互作用效率,采用具有动态速度渐变特性的慢波结构,使得电子注与高频场有足够的互作用时间,从而保证电子不断地将能量交给高频场。运用三维PIC粒子模拟软件分析行波管的注波互作用,得到在12.5~16 GHz频率范围内输出功率大于88.7 W,电子效率大于14.8%,增益大于34.6 dB。     

衰减器对螺旋线慢波结构高频特性影响的理论研究

以加载衰减器的螺旋线慢波结构作为研究对象，采用螺旋导电面模型，用真空层模拟螺旋带的厚度，用均匀介质层等效分立的介质夹持杆，并考虑到各横向区域横向传播常数的不同，得到了任意次模式的色散方程和耦合阻抗的表达式. 在此基础之上，分析了衰减器对主模和-1次模式的衰减常数、相位常数和耦合阻抗的影响.所得结果对设计衰减器具有理论指导意义，为螺旋线慢波系统高频特性的改善以及反射振荡和返波振荡的抑制提供了理论依据. 关键词： 螺旋线 行波管 衰减 色散     

圆波导梳状慢波结构的耦合阻抗

 对目前慢波结构的模拟计算中存在的问题进行了深入分析，并提出了新的计算耦合阻抗的方法。研究了圆波导梳状慢波结构基波的耦合阻抗,其值可以在较宽频带内大于1 W。该结构内孔径为波长的1/4~1/3，是同一频段螺旋线慢波结构的5倍左右，能容纳比螺旋线大25倍的电流，能够承受比螺旋线大25倍的平均功率。互作用计算表明采用这种慢波结构的行波管在Ka波段可获得的最高功率达170 kW。     

翼片加载螺旋线慢波结构的螺旋带模型

 翼片加载螺旋线慢波结构广泛应用于大功率、宽频带行波管中。建立了翼片加载螺旋线慢波结构的螺旋带模型，得到了实用的色散方程、耦合阻抗和衰减常数的表达式。利用导出的方程对实际行波管的螺旋慢波结构进行计算，并与测量结果和导电面模型进行了比较，计算结果与测量结果具有良好的一致性。分析了不同管壳半径对色散特性的影响。     

低增益kW级mm波螺旋线行波管高频结构设计

根据固态器件的发展和返波振荡抑制的特点,针对kW级mm波螺旋线行波管设计了低增益高频结构,通过切断、螺距渐变和添加衰减器的方式对返波振荡进行了抑制。通过微波仿真软件对其色散特性、耦合阻抗特性进行了模拟计算;通过三维大信号注波互作用计算软件进行了注波互作用计算与优化,并使用粒子模拟(PIC)软件进行了计算。结果显示,在频带29~31GHz范围内,输出功率达到1002 W,增益大于21dB,电子效率大于16.8%,返波振荡得到抑制。     

金属柱平板慢波系统高频特性研究

对于离散的金属柱结构构建的周期性平板慢波系统,本文利用3维FDTD方法结合HFSS仿真软件深入分析了该慢波系统的高频特性.研究了金属柱高度、周期长度对色散特性的影响,计算了耦合阻抗,并与传统光栅慢波系统特性进行了对比分析.分析表明金属柱慢波系统既有与传统光栅慢波系统相似的高频特性,又具有独自的特点,位于离散的金属柱周期间隙中的电子注互作用耦合阻抗具有对称性;金属柱结构用作真空电子器件的高频系统可增加发生有效互作用的电子注厚度,降低起振电流密度,提高器件效率.本文的分析结果为设计低电流密度工作的多电子注短毫 关键词： 金属柱平板慢波系统 3维FDTD算法 色散特性 耦合阻抗     

螺旋线型三导体脉冲形成线快波抵消方法

针对螺旋线型三导体脉冲形成线的快波振荡问题,提出一种阻抗分两段阶梯变化的螺旋线结构,分析了此类螺旋线快波反射与慢波反射相互抵消快波的条件,给出两段变阻抗螺旋线慢波系数之间的关系式以及匝数密度之比计算方法,通过仿真计算证明该方法能有效消除快波反射对螺旋线输出脉冲的影响。     

Nonlinear time-dependent simulation of helix traveling wave tubes

A one-dimensional nonlinear time-dependent theory for helix traveling wave tubes is studied. A generalized electromagnetic field is applied to the expression of the radio frequency field. To simulate the variations of the high frequency structure, such as the pitch taper and the effect of harmonics, the spatial average over a wavelength is substituted by a time average over a wave period in the equation of the radio frequency field. Under this assumption, the space charge field of the electron beam can be treated by a space charge wave model along with the space charge coefficient. The effects of the radio frequency and the space charge fields on the electrons are presented by the equations of the electron energy and the electron phase. The time-dependent simulation is compared with the frequency-domain simulation for a helix TWT, which validates the availability of this theory.     

Tape Analysis of an Inhomogeneously-Loaded Helical Slow-Wave Structure for Broad-Band Traveling-Wave Tubes

Theoretical simulation of the dispersion and interaction impedance characteristics of an inhomogeneously-loaded helical slow-wave structure is validated. The structure is supported by double-curve-shaped rods which are smoothed out into a number of dielectric tubes with their respective effective permittivity values. The effects of the helix thickness are taken into account by considering a free-space gap equal to the difference between the mean helix radius and the outer helix radius. Moreover, the helix tape model is used instead of the simpler sheath-helix model. The theoretical predictions are compared with those of MAFIA simulation. The dispersion error is found to be within 3–6 percent and the impedance characteristic is in great agreement with that of MAFIA simulation. At last, for the sake of comparison, the cold-test characteristics under sheath-helix model are also provided.     

Radio-Frequency Characteristics of a Printed Rectangular Helix Slow-Wave Structure

A new type of printed rectangular helix slow- wave structure （SWS） is investigated using the field-matching method and the electromagnetic integral equations at the boundaries. The radio-frequency characteristics including the dispersion equation and the coupling impedance for transverse antisymmetric （odd） modes of this structure are analysed. The numerical results agree well with the results obtained by the EM simulation software HFSS. It is shown that the dispersion of the rectangular helix circuit is weakened, the phase velocity is reduced after filling the dielectric materials in the rectangular helix SWS. As a planar slow-wave structure, this structure has potential applications in compact TWTs.     

基于金属线阵列嵌入的低频宽带电路模拟吸波体设计

由于等效电阻谐振条件限制, 传统的单层电路模拟吸波体(CA)结构在低频段不能形成多个谐振点. 为了突破这个限制, 本文提出了一种金属线阵列嵌入的单层CA结构. 该结构在低频段形成了双谐振峰吸收, 拓展了吸波频带. 采用准静态模型, 分析金属线阵列嵌入的单层CA结构内的电磁波的散射. 分析结果表明, 金属线阵列的嵌入使结构的介质层在低频激发等离子体谐振. 在该谐振频率点, 结构形成额外吸收峰, 该吸收峰和单层CA结构原有的吸收峰一起实现了双峰吸收. 实验和FDTD数值计算结果相符合, 结果表明该结构在不增加厚度的前提下, 扩展了低频段的吸波频段. 关键词： 金属线阵列 宽带吸波 电路模拟吸波体 电阻型频率选择表面     

Phenomenological model and experimental study of DNA absorption spectra in THz range

The dielectric permittivity tensor is analyzed qualitatively for the DNA nematic liquid crystal in the THz frequency range. The analysis is performed within the self-consistent phonon approximation based on the PBD model. It is found that DNA depolarization and absorption spectra depend on such parameters as molecular length and helix period. Specifically it is shown that a localization of the absorption lines in the frequency range is determined by the DNA helix structure whereas the DNA length is responsible for a separation between the absorption lines. It is also shown that the gyrotropic properties of the considered liquid crystal depend on a relation between the molecular length and its helix period. The molecules with even number of half helix periods demonstrate the strongest gyrotrophy whereas the molecules with odd number of half helix periods have the weakest gyrotrophy. The model is used to suggest the experimental methodology for determining the DNA length and helix structure based on measurements of the depolarization and absorption spectra in aqueous solutions of biopolimers. Methods of applied THz spectroscopy for determination of the conformational state of molecules in solution is announced. Some preliminary experimental results are presented, including details of measuring the absorption spectra of dry DNA sample and aqueous solutions of biopolimers.     

Modeling of a high frequency ultrasonic transducer using periodic structures

Solidly mounted integrated transducers with a Bragg cell inserted between the piezoelectric film and the substrate are investigated for high frequency ultrasonic applications. A numerically stable recursive one dimensional transmission/reflection model was used to analyze the behavior of the periodic structure. This theoretical analysis includes the study of the influence of the acoustic properties of the constitutive layer, the effect of the number of cells and their arrangement. A 35 MHz integrated transducer consisting in a PZT ceramic laid down on a Au/PZT Bragg cell deposited on a porous substrate was fabricated and characterized. Both theoretical and experimental results highlight the interest of using a periodic structure for high frequency ultrasonic applications.     

Light scattering by DNA structure due to phase transition

Modelling and numerical calculations of light scattering by the deoxyribonucleic acid (DNA) molecule were investigated using the transition matrix method at the phase transition region. In this model for a DNA molecule with helix structure, the electrical field was established and the transition matrix calculated for this field. Numerical computation of a simple cylindrical model, represented by adenine–thymine (A–T) and guanine–cytosine (G–C) formed with a lamellar-like helix, enabled important results to be obtained. These results were generalized for different molecular orientations with dielectric properties at the phase transition, which occurred due to the electric field changing. Finally, these results are associated with improved models for previous geometries.     

高效双螺旋点扩展函数相位片的设计与实验研究

双螺旋点扩展函数具有随离焦连续旋转变化的特性, 结合单分子定位方法可用于厚样品三维超分辨成像及分子定位追踪研究. 但双螺旋点扩展函数不足之处是光能利用率低, 对于光子数受限的荧光显微成像而言其应用受限. 本文通过对双螺旋点扩展函数在空域、频域和拉盖尔-高斯模式面等三个不同域中进行约束优化. 模拟结果表明, 优化后的双螺旋点扩展函数的光能效率提高了30多倍. 同时, 基于最优设计方案制备了相位片, 并实验验证了该设计的正确性. 与文献报道相比较, 本文结果在成像深度和光能利用率方面都有所改善. 关键词： 双螺旋点扩展函数 超分辨成像 轴向定位 优化算法