X波段长脉冲高功率微波产生的实验研究

 获得长脉冲高功率微波（HPM）输出是HPM源技术追求的重要目标之一。从物理机理上分析了影响慢波结构HPM器件实现长脉冲HPM输出的因素,并利用长脉冲脉冲功率源和过模慢波结构HPM器件,开展了X波段长脉冲HPM产生实验。实验中,采用介质-铜阴极,并在慢波结构表面镀Cr,在导引磁场约0.7 T、二极管电压约400 kV、电流约10 kA、束流脉宽200 ns的条件下,获得了功率500 MW、脉宽约100 ns、主模为TM₀₁的X波段长脉冲HPM输出。对实验结果的分析表明,脉冲功率源与HPM器件的阻抗不匹配,是导致HPM器件输出微波脉宽比电子束脉宽短、以及HPM器件输出微波功率效率较低的主要原因。     

高功率微波介质窗表面矩形槽抑制二次电子倍增

为深入研究高功率微波(HPM)作用下介质窗沿面击穿破坏的物理机制,探索提高闪络场强阈值的方法和途径,开展了介质窗表面矩形刻槽抑制电子倍增的理论与试验研究。首先根据动力学方程建立了介质窗表面电子倍增模型并分析了介质窗槽内电子运动轨迹,考虑了矩形槽结构对表面微波电场的影响,理论分析表明在闪络击穿的起始和发展阶段矩形槽可有效抑制电子倍增。在S波段(2.86 GHz,脉宽1μs)下开展了介质窗表面矩形刻槽的击穿破坏试验,试验结果发现表面矩形刻槽可大幅度提高微波传输功率,在槽深(1.0mm)一定时不同的刻槽宽度(0.5 mm和1.0 mm)对应的微波功率抑制范围不同。采用PIC-MC仿真模拟槽内倍增电子的时空演化,仿真结果很好地验证了试验现象。     

X波段馈源输出窗高功率微波击穿实验装置

为了开展高功率微波(HPM)馈源输出窗介质击穿实验研究,设计了一种组合型X波段高功率微波(HPM)喇叭馈源击穿实验装置。装置采用变张角喇叭与可移动介质输出窗组合的结构,通过调节变张角喇叭口面与输出窗间的距离,使得介质输出窗内表面电场强度可调。数值模拟结果表明:在满足馈源喇叭驻波比小于1.15,E面和H面基本等化的情况下,当调节变张角喇叭口面与介质输出窗距离在0~400 mm范围内变化时,HPM馈源输出窗上的电场强度变化为32.6~87.0 kV.cm-1,满足了在真空度3×10-3Pa、脉冲宽度20 ns条件下,HPM介质击穿对电场强度变化的要求。根据数值模拟结果,设计加工了HPM介质击穿实验装置,并成功地应用于GW级HPM馈源输出窗介质击穿实验研究。     

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。     

高功率微波作用下介质窗表面电子运动2维仿真

建立了真空中高功率微波作用下介质窗表面电子运动2维仿真模型,充分考虑了微波电磁场及介质表面静电场等影响因素。通过对不同电子出射初始角度和微波场参数(电场幅值、频率及电子出射时电场相位)对电子运动状态影响的仿真分析,得到了二次电子倍增过程中电子在复合场下的运动轨迹、电子重新返回介质表面的撞击能量及返回时间等状态参数,获得了电子运动状态参数随电子出射角度和微波场参数的变化规律。研究发现:电子出射角度对其运动状态有显著影响,电子存在运动轨迹最大的某一出射角度,该角度下电子拥有最大的撞击能量;微波电场幅值的增加将使电子撞击能量增加,返回时间减小,微波电场相位的变化使电子的撞击能量和返回时间呈周期振荡,这从本质上解释了电子数量在二次电子倍增过程中以微波频率两倍周期振荡的原因;随着微波频率的增加电子将由简单的类抛物线运动转变为复杂的振荡运动。     

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。     

基于漏波波导的X波段高功率微波天线

 基于漏波波导行波天线辐射理论,设计了一种X波段基于漏波波导的高功率微波（HPM）天线。采用微扰法和横向谐振法对天线的辐射特性进行分析,结合数值模拟优化给出了一种基于漏波波导的X波段HPM天线的设计方案。数值模拟表明:该天线在9.6 GHz时增益为26.2 dBi,口径效率大于70%,反射系数小于-20 dB。通过理论分析与数值模拟得到该天线的功率容量大于200 MW,在最大增益点上对ns量级短脉冲的远场响应波形不存在畸变,验证了该天线在HPM条件下使用的可行性。     

S波段相对论速调管振荡器的实验研究

 在20 GW加速器平台上开展了S波段相对论速调管振荡器（RKO）的单次和重复频率束流调制和微波辐射的实验研究。采用无箔空心阴极和0.9 T的恒流源磁场引出束压1 MV、束流13 kA、脉宽40 ns的环形电子束驱动RKO,该电子束经过3个紧密耦合的扩展互作用腔再经过一段漂移管的群聚后,产生了7.8 kA/20 ns的基波调制束流,该调制束流激励三轴输出腔,单次运行输出了3.5 GW的微波辐射,束波转换效率29%,脉宽20 ns;脉冲重复频率20 Hz运行时,输出微波功率3.4 GW,束波转换效率26%。该振荡器具有起振时间快、输出频谱较纯和结构紧凑等优点。     

X波段相对论返波管谐振反射器

基于TPG2000强流电子束加速器和带谐振反射器的相对论返波管振荡器,开展了X波段高功率微波产生实验研究,获得了功率约2.5 GW,脉宽约20 ns的微波输出。理论分析及模拟了不同倒角大小对谐振反射器的表面电场及截止性能的影响,并对不同倒角开展了实验研究。结果表明,对谐振反射器倒角可增加输出微波脉冲宽度,且随着倒角增加,微波脉宽增加,效率略有降低。在谐振反射器倒角5 mm情况下,利用电压900 kV,电流9 kA的强流电子束,实验获得了功率约2.5 GW、脉宽大于25 ns的微波输出。     

110 GHz微波输出窗内表面次级电子倍增特性的电磁粒子模拟

在输出窗内表面上,次级电子倍增是限制高功率微波功率容量的主要因素之一,因而开展相关研究具有重要的意义.在微波频率为110 GHz下,本文通过一维空间分布和三维速度分布的电磁粒子模型对次级电子倍增过程及其引起的损失功率进行了数值模拟.重点研究了介质表面处的微波电场和介质材料种类对损失功率的影响.模拟结果表明,在次级电子倍增达到稳态之后,尽管电子数密度高于临界的截止数密度,但是微波电场没有发生明显的改变.这是因为在很高的静电场下,电子主要聚集在介质表面附近若干微米的区域,远小于相应的趋肤深度.倍增稳态时的电子数密度随着微波电场升高而增加,然而损失功率与表面处的微波功率之比增加得较为缓慢.在倍增达到稳态之后,由于蓝宝石表面附近的电子数密度最高,石英晶体表面附近的次之,熔融石英表面附近的数密度最低,所以相应的损失功率依次减小.为验证模型的准确性,将倍增阈值的模拟值与实验数据进行了对比,并讨论了两者之间的差异.     

过模返波管的正反馈机制

作为一个典型的高功率微波振荡器,过模返波管(backward wave oscillator,BWO)的束波互作用过程复杂,束流负载效应影响明显,但是作为振荡器本身,其本质就是一个正反馈电路,电子从阴极发射后,穿过谐振反射腔和慢波结构(slow-wave structure,SWS),在SWS区电子动能转化为微波能,其中的一部分微波反馈到谐振反射腔,实现对电子束的调制,其他微波通过后面输出端口向外辐射.本文根据这种正反馈机制,建立器件工作模式等效电路和束波互作用的自洽过程,从理论上给出正反馈机制对器件模式控制、起振电流等参数的影响,并模拟研究了这种反馈机制对模式控制的影响,由此设计了一个能够在(1 MV,20 kA)电子束条件下克服模式竞争的过模BWO,其微波输出功率为7.9 GW,频率为8.68 GHz,相应的效率为39.5%.     

Measurement of high-power microwave pulse under intense electromagnetic noise

KALI-1000 pulse power system has been used to generate single pulse nanosecond duration high-power microwaves (HPM) from a virtual cathode oscillator (VIRCATOR) device. HPM power measurements were carried out using a transmitting-receiving system in the presence of intense high frequency (a few MHz) electromagnetic noise. Initially, the diode detector output signal could not be recorded due to the high noise level persisting in the ambiance. It was found that the HPM pulse can be successfully detected using wide band antenna, RF cable and diode detector set-up in the presence of significant electromagnetic noise. Estimated microwave peak power was ∼59.8 dBm (∼1 kW) at 7 m distance from the VIRCATOR window. Peak amplitude of the HPM signal varies on shot-to-shot basis. Duration of the HPM pulse (FWHM) also varies from 52 ns to 94 ns for different shots.     

加工工艺对微波窗材料介电性能的影响

聚四氟乙烯、聚乙烯、有机玻璃等介质材料因其良好的透波性能和优异的机械性能被广泛用作高功率微波的输出窗口,但关于窗口加工处理工艺对材料介电性能的影响却少有研究。利用高压电桥及高阻计对不同介质材料不同工艺下的基本介电参数进行了测量,利用静电感应法对不同介质材料表面0.8~0.9 eV浅能级的电子陷阱密度分布进行了测试,利用波导口开展了S波段800 MW、百ns微波脉冲作用下的介质击穿实验,考察了材料表面粗糙度、划痕对介质窗表面击穿的影响。结果表明:烘烤工艺在一定程度上降低了材料的耐击穿性能,增加了介质损耗;平行于电场方向的划痕形成了材料表面击穿的通道,而该通道的存在又进一步加剧了击穿的发生。     

Study of superradiance based on a low-voltagebackward wave oscillator

According to the small size requirement for wide-band high-power microwave radiation, a superradiance backward wave oscillator (BWO) is proposed to generate such high-power microwave radiation with a low voltage (～20 kV) pulse power supply and low guiding magnet field (～0.1 T). In order to get a high-efficiency C-band superradiance BWO with a low beam voltage and a low guiding magnet field, the mechanism of superradiance in a BWO is explored in particle-in-cell simulation. With the oversized structure, the simulation shows that a microwave power of 405 kW with a frequency of 5.6 GHz and a spectrum width of 500 MHz can be obtained with a voltage of 23 kV and magnetic field of 0.1 T.     

Scaling studies and time-resolved microwave measurements on arelativistic backward-wave oscillator

A relativistic backward-wave oscillator (BWO) operating at a frequency near 8 GHz has been built. The parameters of the 60-ns electron beam driving this microwave source are varied over the ranges 0.8-1.5 MV and 2-10 kA. Several different annular cathodes for launching the electron beam are tried, varying the outer radius and shape. The axial magnetic field guiding the beam through the BWO is varied between 0.6 and 3 T. The power transfer downstream to an output waveguide is investigated as a function of the shape of the transition from the BWO to the waveguide. The scaling of the output power and frequency with these variations is discussed. Time-resolved measurements of 2-ns-long segments of the microwave output are shown. In observations of the microwave signal, it is found that the frequency shifts as the output power envelope passes through a sharp dip. It is proposed that this shift corresponds to a change in the longitudinal operating mode of the BWO     

Experimental investigation of backward wave oscillator with low magnetic field

A backward wave oscillator (BWO) is introduced in the paper. On the accelerator of Simus-700,it is experimentally investigated. Under the condition that the electron energy is 740 keV, the beam current is 7 kA and the guiding magnetic field is at 0.68 T, the performance of 1.15 GW microwave output power at 9.1 GHz microwave frequency with 22 ns pulse width and 22% conversion efficiency are reached.     

Efficiency enhancement in high power backward-wave oscillators

High power microwave (HPM) sources based on the backward-wave oscillator (BWO) have been investigated for the past two decades primarily because of their potential for very high efficiency (15 to 40%) operation. Several different effects have been proposed to explain this high efficiency compared to conventional BWOs. One of the major contributors to the high efficiency of the plasma-filled Pasotron HPM BWO source is the presence of optimally phased end reflections. The Pasotron uses a long-pulse (⩾100 μs) plasma-cathode electron-gun and plasma filled slow-wave structure to produce microwave pulses in the range of 1 to 10 MW without the use of externally produced magnetic fields. The efficiency of the Pasotron can be enhanced by up to a factor of two when the device is configured as a standing-wave oscillator in which properly phased reflections from the downstream collector end of the finite length SWS constructively interfere with the fundamental backward-wave modes and improve the coupling of the beam to the circuit. Operation in this configuration increases the efficiency up to 30% but causes the frequency to vary in discrete steps and the output power to change strongly with beam parameters and oscillation frequency     

高功率微波介质窗表面电子倍增二维粒子模拟

基于第一性原理的粒子模拟方法,对高功率微波器件中介质窗表面电子实际形成和发展的变化情况进行了研究。使用VORPAL粒子模拟软件,建立一个简单的TEM波垂直入射介质窗表面的二维模型,采用Vaughan二次电子发射模型,利用蒙特卡罗碰撞方法处理电子与背景气体之间的弹性碰撞、激发碰撞和电离碰撞,获得了介质窗表面电子倍增的图像。模拟结果表明,介质窗表面电子数量在一定的时间内达到饱和状态,其振荡频率是入射射频电场频率的两倍。改变初始发射种子电子的数量、入射射频电场的幅值以及背景气体的压强等关键性参数,可得到不同条件下介质窗表面电子数量的变化规律。     

Efficiency enhancement of high power vacuum BWO's using nonuniformslow wave structures

The Sinus-6, a high-power relativistic repetitively-pulsed electron beam accelerator, is used to drive various slow wave structures in a BWO configuration in vacuum. Peak output power of about 550 MW at 9.45 GHz was radiated in an 8-ns pulse. We describe experiments which study the relative efficiencies of microwave generation from a two-stage nonuniform amplitude slow wave structure and its variations without an initial stage. Experimental results are compared with 2.5 D particle-in-cell computer simulations. Our results suggest that prebunching the electron beam in the initial section of the nonuniform BWO results in increased microwave generation efficiency, Furthermore, simulations reveal that, in addition to the backward propagating surface harmonic of the TM₀₁ mode, backward and forward propagating volume harmonics with phase velocity twice that of the surface harmonic play an important role in high-power microwave generation and radiation