Three-dimensional simulation method of multipactor in microwave components for high-power space application

Based on the particle-in-cell technology and the secondary electron emission theory, a three-dimensional simulation method for multipactor is presented in this paper. By combining the finite difference time domain method and the panicle tracing method, such an algorithm is self-consistent and accurate since the interaction between electromagnetic fields and particles is properly modeled. In the time domain aspect, the generation of multipactor can be easily visualized, which makes it possible to gain a deeper insight into the physical mechanism of this effect. In addition to the classic secondary electron emission model, the measured practical secondary electron yield is used, which increases the accuracy of the algorithm. In order to validate the method, the impedance transformer and ridge waveguide filter are studied. By analyzing the evolution of the secondaries obtained by our method, multipactor thresholds of these components are estimated, which show good agreement with the experimental results. Furthermore, the most sensitive positions where multipactor occurs are determined from the phase focusing phenomenon, which is very meaningful for multipactor analysis and design.

An improved secondary electrons energy spectrum model and its application in multipactor discharge

Secondary electron emission(SEE)of metal and dielectric materials plays a key role in multipactor discharge,which is a bottle neck problem for high-power satelliate components.Measurements of both the secondary electron yield(SEY)and the secondary electron energy spectrum(SES)are performed on metal samples for an accurate description of the real SEE phenomenon.In order to simplify the fitting process and improve the simulation efficiency,an improved model is proposed for the description of secondary electrons(SE)emitted from the material surface,including true,elastic,and inelastic SE.Embedding the novel SES model into the electromagnetic particle-in-cell method,the electronic resonant multipactor in microwave components is simulated successfully and hence the discharge threshold is predicted.Simulation results of the SES variation in the improved model demonstrate that the multipactor threshold is strongly dependent on SES.In addition,the mutipactor simulation results agree quite well with the experiment for the practical microwave component,while the numerical model of SEY and SES fits well with the sample data taken from the microwave component.

PIC-MCC Simulation for HPM Multipactor Discharge on Dielectric Surface in Vacuum

In order to understand the physical mechanism of multipactor discharge on dielectric window surface under high power microwave （HPM） excitation in vacuum, an electron movement simulation model based on the particle-in-cell （PIC） Monte Carlo （MC） is built in this paper. The influences of microwave electromagnetic field and electrostatic field from dielectric surface charging are simultaneously considered in this model. During the simulation, the emission velocity and angle distribution of secondary electrons from the dielectric surface are taken into account. The movement trajectories of electron clusters under complex field excitation are obtained. The influences of emergence angle and microwave electromagnetic parameters on the electron movement are analyzed. It is found that the emergence angle of electrons from the surface has significant effect on its movement, and both the impact energy and return time of electrons oscillate periodically with the phase of microwave field. The number of secondary electrons and induced electrostatic field from multipactoring are also investigated. The results reveal that both values oscillate periodically at twice the microwave frequency, which is due to the electron impact energy oscillating with microwave period. A schematic diagram is proposed to explain the periodical oscillation phenomena.

A hybrid mode of one- and two-surface multipactor on grooved dielectric surface

A hybrid mode of one- and two-surface multipactor on the grooved dielectric surface is studied in detail using both an analytical approach and two-dimensional particle-in-cell（2D PIC） simulations. When the groove width L 〈 eE_0/（4πm_ef^2）,there are one-surface multipactor and one-order two-surface multipactor on the grooved dielectric surface, and only one slope of the groove has the multipactor anytime. When L 〉 eE0/（4πme f^2）, both slopes may have the multipactors. The electron surface density of the multipactor discharge has a sharp increase at the length L = eE_0/（4πm_ef^2）.

Effects of the insulated magnetic field and oblique incidence of electrons on the multipactor in MILO

The theoretical analysis and actual performance of the single-surface multipactor discharge model in the presence of a magnetic field are conducted through simulations. The effects of the magnitude of the insulated magnetic field and the oblique incidence of electrons on the multipactor are analysed. The results show that the multipactor susceptibility region shrinks gradually as the magnetic field increases when the electron cyclotron frequency is close to the RF frequency of the electric field. As a result, the evolution of the multipactor discharge will reach saturation earlier and become saturated at a higher level than the case when the magnetic field is absent, but the change of evolution and saturation as the insulated magnetic field increases is not obvious.

Hybrid domain multipactor prediction algorithm and its CUDA parallel implementation

Based on the finite element method(FEM)in the frequency domain and particle-in-cell approach in the time domain,a hybrid domain multipactor threshold prediction algorithm is proposed in this paper.The proposed algorithm has the advantages of the frequency domain and the time domain algorithms at the same time in terms of high computational accuracy and considerable computational efficiency.In addition,the compute unified device architecture(CUDA)acceleration technique also can be employed to further enhance its simulation efficiency.Numerical examples are carried out to demonstrate the effectiveness of the proposed algorithm.The results indicate that the multipactor threshold can be accurately predicted and the computational efficiency can be improved.

考虑空间电荷自洽作用和绝缘磁场的单边multipactor模型

在考虑了波束相互作用腔中的射频场、聚焦磁场和空间电荷产生的静电场及空间电子束形成自洽相互作用的基础上,针对典型的高功率微波器件——磁绝缘线振荡器建立了单边次级电子倍增模型,统计分析了整个倍增过程中次级电子数目及器件腔壁吸收碰撞电子能量随时间的变化规律,给出了不同聚焦磁场值和电子碰撞角时次级电子的倍增情况.

星载微波部件微放电阈值的改进多粒子蒙特卡罗计算方法

星载微波部件的微放电效应是导致航天器谐振类设备失谐、噪声电平抬高、输出功率下降,甚至影响通信信道乃至整个微波传输系统彻底失效的瓶颈问题之一。在设计阶段对星载微波部件微放电效应进行精准的评估是减少地面反复试验,避免延误研制周期的重要手段。为了进一步改善现有蒙特卡洛方法的计算准确度问题,文中提出了一种精度更高的计算星载微波部件微放电阈值的蒙特卡罗方法,该方法对参与微放电过程的初始电子进行了动态调整,采用四阶龙格-库塔法推进微波部件中电子的运动轨迹,基于Furman模型描述电子与微波表面相互作用的二次电子发射过程,按照碰撞电子产生的实际二次电子个数及对应能量参与碰撞时刻后的微放电过程,该多粒子-多碰撞过程更加客观、准确地表征了微放电效应发生的物理过程。以平板传输线和同轴传输线为例,文中所提出的方法相对于已有的蒙特卡罗方法计算精度显著提升,同时计算效率优于商用CST的粒子模拟结果。

Secondary electron yield of air-exposed ALD-Al_(2)O_(3) coating on Ag-plated aluminum alloy

Secondary electron yield(SEY)of air-exposed metals tends to be increased because of air-formed oxide,hydrocarbon,and other contaminants.This enhances the possibility of secondary electron multipacting in high-power microwave systems,resulting in undesirable occurrence of discharge damage.Al_(2)O_(3) coatings have been utilized as passive and protective layers on device packages to provide good environmental stability.We employed atomic layer deposition(ALD)to produce a series of uniform Al_(2)O_(3) coatings with appropriate thickness on Ag-plated aluminum alloy.The secondary electron emission characteristics and their variations during air exposure were observed.The escape depth of secondary electron needs to exceed the coating thickness to some extent in order to demonstrate SEY of metallic substrates.Based on experimental and calculated results,the maximum SEY of Ag-plated aluminum alloy had been maintained at 2.45 over 90 days of exposure without obvious degradation by applying 1 nm Al_(2)O_(3) coatings.In comparison,the peak SEY of untreated Ag-plated aluminum alloy grew from an initial 2.33 to 2.53,exceeding that of the 1 nm Al_(2)O_(3) sample.The ultra-thin ALDAl_(2)O_(3) coating substantially enhanced the SEY stability of metal materials,with good implications for the environmental dependability of spacecraft microwave components.

Effect of desorbed gas on microwave breakdown on vacuum side of dielectric window

The gas desorbed from the dielectric surface has a great influence on the characteristics of microwave breakdown on the vacuum side of the dielectric window. In this paper, the dielectric surface breakdown is described by using the electromagnetic particle-in-cell-Monte Carlo collision(PIC-MCC) model. The process of desorption of gas and its influence on the breakdown characteristics are studied. The simulation results show that, due to the accumulation of desorbed gas, the pressure near the dielectric surface increases in time, and the breakdown mechanism transitions from secondary electron multipactor to collision ionization. More and more electrons generated by collision ionization drift to the dielectric surface, so that the amplitude of self-organized normal electric field increases in time and sometimes points to the dielectric surface. Nevertheless, the number of secondary electrons emitted in each microwave cycle is approximately equal to the number of primary electrons. In the early and middle stages of breakdown, the attenuation of the microwave electric field near the dielectric surface is very small. However, the collision ionization causes a sharp increase in the number density of electrons,and the microwave electric field decays rapidly in the later stage of breakdown. Compared with the electromagnetic PIC-MCC simulation results, the mean energy and number of electrons obtained by the electrostatic PIC-MCC model are overestimated in the later stage of breakdown because it does not take into account the attenuation of microwave electric field. The pressure of the desorbed gas predicted by the electromagnetic PIC-MCC model is close to the measured value,when the number of gas atoms desorbed by an incident electron is taken as 0.4.

C波段小型化高功率微波输出窗的设计

为了满足高功率微波系统对微波输出窗高功率容量和紧凑化的应用需求,以传统盒型窗的设计理论为基础,通过优化窗体结构和添加过渡段等手段,设计了一种C波段小型化高功率微波输出窗。通过增大窗体表面积、改变矩形波导-圆波导过渡段的连接方式可提高功率容量并缩小微波输出窗的纵向尺寸;采用“I”型的窗体结构可有效抑制三相点(真空-介质-金属)附近的次级电子倍增效应对输出窗性能的影响。在电磁仿真的基础上采用粒子模拟(Particle-in-Cell)的方法研究了微波输出窗三相点附近的次级电子倍增效应,从微观角度进一步证实了“I”型窗体结构可使三相点位置发生移动,减小三相点发射的电子在窗片表面产生次级电子倍增效应的概率,降低微波输出窗的击穿风险。设计结果表明,微波输出窗在中心频点处的主模反射系数低于0.01,传输效率高于99.9%,功率容量可达47.9 MW。

星载大功率复杂微波部件微放电效应数值模拟

随着航天器有效载荷技术向高功率、小型化持续发展,复杂结构微波部件微放电数值模拟与阈值分析成为影响微放电分析的基础瓶颈问题。基于电磁时域有限差分计算方法与粒子模拟技术,结合二次电子发射模拟,提出了微放电电磁粒子联合仿真方法,数值模型中考虑了真实电子间的库仑力以及电子运动产生的电荷和电流变化对电磁场的影响,解决了复杂结构微波部件微放电三维数值模拟技术难题。实现了在统一的三维空间网格与时间步进行电磁场值演变计算、电子运动状态变化推进计算与二次电子产额与能量分布计算,基于得到的二次电子数目随时间变化趋势实现了微放电阈值预判,通过微放电电子随时间演化获得了微放电过程具体物理图像及放电位置,并与实际器件微放电实验进行了对比验证。结果表明,所提出的三维电磁粒子数值模拟方法可对大功率微波部件微放电效应的物理过程与具体放电位置进行三维描述,预测的阈值与微放电实验测量值吻合良好,误差小于1.2dB,验证了该方法的有效性与准确性,对于深入研究微放电效应微观物理机制、提高大功率微波部件微放电设计与分析水平具有重要意义。

基于频域电磁场的微波器件微放电阈值快速粒子模拟

提出了一种利用频域电磁场快速计算微波器件微放电阈值的粒子模拟方法.首先通过CST微波工作室频域求解器获得微波器件中频域电磁场分布,在微放电过程模拟时将其转换到时域,再采用Boris算法求解电磁场中的电子运动,然后判断电子是否与三角面片边界相交,进行二次电子发射处理.变化输入功率,经过系列粒子模拟后,根据电子数目随时间的变化曲线确定微放电阈值.采用该方法分别对平行平板和同轴传输线微波器件的微放电阈值进行模拟计算,并与CST粒子工作室的模拟结果进行对比.结果表明,两者获得的阈值基本一致,但本方法的计算效率提高了1—2个数量级.

刻槽绝缘子真空表面闪络特性分析

基于一台长脉冲低阻抗强流相对论电子束加速器,对聚四氟乙烯、高分子量聚乙烯和聚碳酸酯在真空百ns高压脉冲下的绝缘特性进行了测量,重点研究了周期刻槽结构对材料绝缘强度的提升能力。实验表明:刻槽结构的引入可以有效地提高介质材料的闪络时延;聚碳酸酯是3种材料中比较优越的一个,刻槽处理可以使其闪络延时获得90%左右的提升。扫描电镜分析的结果也表明材料的表面特性是影响介质样品闪络时延的一个重要因素。

真空高功率微波介质窗表面击穿破坏现象的研究进展

综述了国内外真空中高功率微波(HPM)下介质窗表面击穿问题的研究现状和进展。在介质窗表面击穿实验研究方面,介绍了国外最具代表性的研究成果,给出了介质窗材料表面及内部的破坏发展规律,并提出相应的理论模型。在理论仿真方面,重点介绍了国外在运用蒙特卡罗(Monte Carlo)程序和PIC模型对认识HPM下介质窗表面倍增放电机理上做出的突出贡献,给出了HPM下介质窗表面电子在不同影响因素下的运行状态,并提出了一个理论模型,从本质上解释了倍增电子数目和表面静电场以微波频率的2倍振荡的原因。介绍了目前几种可有效抑制介质窗表面微波击穿的技术手段。

基于临界电子密度的多载波微放电全局阈值分析

多载波微放电即发生在宽带、大功率真空无源微波部件中的二次电子倍增放电现象,是影响空间和加速器应用中无源微波部件长期可靠性的主要隐患.多载波微放电全局阈值功率的预测对于工作在真空环境中的微波部件至关重要,但迄今尚无有效方法进行上述阈值的准确分析.本文将微放电发生过程中二次电子分布区域等效为等离子体,通过在理论上建立微波部件的电磁特性和电子密度间的对应关系,提出了一种基于测试系统可检测水平的多载波微放电全局阈值功率分析方法.为了能够通过蒙特卡罗优化方法得到全局阈值,进一步基于电子加速的类半正弦等效,提出了微放电演化过程中电子数涨落的快速计算方法.基于以上两种方法得到的针对实际微波部件的全局阈值分析结果与实验结果相符合.不同于传统基于多载波信号功率分析的经验方法,本文基于临界电子密度判断依据和电子数涨落快速计算,为多载波微放电全局阈值的准确预测提供了一种高效的分析方法.

高功率微波沿面闪络击穿机制粒子模拟

针对高功率微波介质沿面闪络击穿物理过程,首先建立了理论模型,包括:动力学方程、粒子模拟算法、二次电子发射,以及电子与气体分子蒙特卡罗碰撞模型、电子碰撞介质表面退吸附气体分子机制;其次,基于理论模型,编制了1D3VPIC-MCC程序,分别针对真空二次电子倍增、高气压体电离击穿和低气压面电离击穿过程,运用该程序仔细研究了电子和离子随时间演化关系、电子运动轨迹、电子及离子密度分布、空间电荷场时空分布、电子平均能量、碰撞电子平均能量、碰撞电子数目随时间演化关系、电子能量分布函数、平均二次电子发射率以及能量转换关系。研究结果表明:真空二次电子倍增引发的介质表面沉积功率只能达到入射微波功率1%左右的水平,不足以击穿;气体碰撞电离主导的高气压体电离击穿,是由低能电子(eV量级)数目指数增长到一定程度导致的,形成位置远离介质表面,形成时间为μs量级;低气压下的介质沿面闪络击穿,是在二次电子倍增和气体碰撞电离共同作用下,由于数目持续增长的高能电子(keV量级)碰撞介质沿面导致沉积功率激增而引发的,形成位置贴近介质沿面,形成时间在ns量级。

基于ZnO阵列的银表面二次电子发射抑制技术

随着卫星有效载荷的射频功率越来越大,传统的微放电抑制方法已经无法满足大功率卫星有效载荷的需求。降低大功率射频部件内表面的二次电子发射系数是抑制微放电效应的重要方法之一,通过在金属银表面构造纳米量级ZnO阵列,实现了纳米尺度银陷阱结构的制备,研究了晶种制备方式、锌盐浓度对ZnO阵列生长的影响。结果表明,采用紫外照射法制备晶种获得的ZnO阵列在样片表面分布均匀,提高锌盐浓度可改善ZnO阵列的分布均匀性。分析了ZnO阵列排列密度对银膜构筑的影响,发现在低密度的ZnO阵列上更加容易镀覆金属银。因此,获得了镀银表面基于ZnO阵列的陷阱结构制备的工艺技术,实现金属银表面二次电子发射系数最大值降低36.3%。

介质面刻槽抑制二次电子倍增蒙特卡罗模拟

利用蒙特卡罗方法,针对介质表面刻槽抑制二次电子倍增的实验现象,进行了数值模拟研究。给出了二次电子倍增动力学方程、刻槽边界条件、二次电子初始能量与角度分布以及发射率分布关系;讨论了槽深、槽宽对二次电子倍增的抑制效果,以及同一刻槽结构对不同微波场强度和频率的二次电子倍增抑制能力;分析了双边二次电子倍增区域。数值研究结果表明:增加槽深、缩短槽宽可以抑制二次电子倍增;同一刻槽结构,更易于抑制高频场、场强较低或较高下的二次电子倍增;刻槽尺寸的选择还应避开双边二次电子倍增区间。将数值模拟结果与相关实验现象进行了对比,吻合得较好。

微放电试验中种子电子加载方法比较

为改进微放电试验的有效性,针对微放电试验中种子电子的加载方法进行了研究,介绍了辐射源、紫外光源、电子枪三种加载方法,并说明和比较了上述方法的优缺点和适用范围。接着重点介绍了两种辐射源加载种子电子的方法:β衰变和γ跃迁,并对两种方法加载种子电子的特性进行了定量分析。所得结果表明,基于β衰变的^(90)Sr及同时进行β衰变和γ跃迁的^(137)Cs均可产生能够穿透毫米量级铝质微波部件壁厚的不同数量的种子电子,适合用于微放电试验中的种子电子加载。