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高功率微波窗口击穿及馈源技术

Study of Dielectric Window Breakdown and Feed Technqies for High Power Microwave Systems

【作者】 秋实

【导师】 焦永昌;

【作者基本信息】 西安电子科技大学 , 电磁场与微波技术, 2010, 博士

【摘要】 馈源的功率容量已经成为HPM (High Power Microwave, HPM)技术发展的一个重要的限制,开展提高馈源功率容量的技术研究,对于HPM实用化具有很强的现实意义。馈源的功率容量在很大程度上取决于馈源的结构形式和介质窗口材料的击穿特性,本文重点以几种常用的窗口材料为实验材料,通过理论分析和实验验证相结合的方法,研究介质窗击穿特性,考察介质窗表面特性及其对介质击穿过程的影响,探讨HPM模式转换器、旋转关节及馈源的设计技术。主要工作包括:采用X射线光电子能谱(X-ray Photoelectron Spectroscopy, XPS)、光学显微镜观察、红外光谱等几种技术手段对介质材料表面组分进行了分析,发现介质表面态中元素组分和材料肌体内部的元素组分比例以及原子构造均有着较大的差异:表面态中除了材料本身的组分以外,还存在O、N、K等元素,认为这些元素的存在,是介质表面击穿阀值明显下降的原因之一。对微波场下介质窗口的击穿现象进行了理论探索和实验研究,阐述了微波作用下介质发生击穿的一般物理过程,讨论了一次电子产生、二次电子倍增过程及其影响因素;研究了介质窗加工处理工艺对窗口表面态、介电特性的影响,认为长时间(24小时)的烘烤对介质损耗和体电阻均有较大影响,测量了击穿过程中介质表面的带电现象,分析了介质表面电子陷阱密度分布对介质表面电荷沉积的影响;通过观察击穿前后试件的表面,发现在击穿的初期,杂质参与了击穿的早期过程,是击穿的薄弱点,介质表面光洁度越高其抗击穿电压值越大,平行于电场的划痕明显地降低了击穿阈值并加剧了介质表面的击穿。研制了用于观察微波场下介质表面电致发光现象的日盲型紫外探测仪和X射线探测设备,建立了利用光电诊断手段实验研究介质窗口击穿的方法,开展了百千瓦级波导口介质击穿实验,获得了介质窗的击穿判据,对击穿点移动现象进行了光学诊断并进行了理论解释,认为击穿产生的等离子体是引起窗口透射波过早截止的原因;用功函数的概念解释了波导口介质窗“三相点”的起电现象。建立了用于研究高功率(GW级)、窄脉冲(纳秒级)微波作用下介质击穿现象的实验装置,开展了介质击穿实验,获得了几种介质窗材料的击穿阈值;得到了介质内部的树枝状击穿通道图像并对其成因进行了分析。测量了HPM源及介质窗放电过程中的紫外线和X射线,并基于能带理论提出了HPM作用下介质窗口击穿时一次电子的产生机制;认为HPM作用下介质窗的起电现象主要是源于HPM源产生的X射线对介质窗的轰击以及发生在介质窗表面的局部微放电;HPM源产生的X射线和紫外线是介质窗表面吸附气体解吸和离化的主要原因之一。通过对介质窗击穿后的表面形态微观分析,提出了HPM介质窗损坏的两种形式:一种是由于微放电形成的火花闪烁(Sparking)引起的介质窗表面“烤焦”,另一种是由于击穿引起的介质窗损坏。设计制作了用于某武器实验样机的E面/H面辐射方向图等化的馈源,并对该馈源的介质窗口寿命进行了高功率考核,结果表明在注入功率大于1GW、脉冲宽度20ns时,馈源窗口的寿命大于6×104个脉冲。提出并研制了三镜波束波导馈电的双反射面天线、TMol圆波导弯头和组合式旋转关节,实验结果表明功率容量大于1GW,较好地解决了高功率微波传输线的功率容量问题。

【Abstract】 The power capacity of feed has been an important limitation for the development of high-power microwave (HPM) technology, and it is very meaningful to enhance the power capacity of feed for the practicality of HPM. The power capacity of feed is greatly dependent on the structure form and breakdown characteristics of dielectric window materials. In this dissertation, with several commonly used window materials, the breakdown characteristics of dielectric window and the surface characteristics of dielectric window and theirs effects on the dielectric breakdown process are studied through comprehensive theoretical analysis and experimental verification. Moreover, the HPM mode convertor, rotary joint, and the design technology of the feed are discussed. The work in this paper includes:X-ray photoelectron spectroscopy (XPS), optical microscopy, and infrared spectrum were employed to analyze surface composition. It was found that there is significant difference in chemical composition and atomic structure between material surface and body. At surface, besides the certain composition of dielectric material, such elements as O, N, and K were also found, which are believed to be a major reason for the dramatic decline in surface breakdown threshold.The phenomena of dielectric window breakdown under microwave field are investigated theoretically and experimentally. The general physical process of dielectric breakdown under microwave field is described, and the generation of primary electron, the multipactor process of secondary electron and their effect factors are discussed. The effect of maching and treatment technics of dielectric window on window surface state and dielectric characteristics are also studied. It was found that a long time (24 hours) roasting affects the dielectric loss and body resistance largely. The electrification on the dielectric surface during breakdown process was measured, and the effect of electron trap density distribution on dielectric surface charge accumulation was analyzed. By observing specimen surface before and after breakdown, it was found that at the initial stage, impurities participates breakdown and becomes the weakest point in the breakdown process, the breakdown threshold of dielectric material increases with the improvement of dielectric material surface finish, and surface scratches parallel to the electric field direction reduces the breakdown threshold obviously and deteriorates the breakdown of dielectric surface.A solar blind ultraviolet detector and an X-ray detector were developed for observing surface electroluminescent phenomenon under microwave field. The experimental method of investigating dielectric window breakdown with photoelectric diagnostic tools was established, and the breakdown experiments of 100 kW-class waveguide end dielectric were carried out. Breakdown criterion of dielectric window was obtained, the phenomenon of breakdown point shift was diagnosed with optics and explained theoretically, and it was believed that plasma is the reason for the premature cutoff of transmission wave through the window. The electricity phenomenon at the "triple-point" of waveguide end dielectric window can be explained by adopting the concept of work function,The experimental device investigating the dielectric breakdown caused by high power (GW-class) and narrow pulse (ns-class) microwave was set up, and the experiments were conducted. The breakdown thretholds for several dielectric window materials were achieved, the images of tree-like breakdown channel within the dielectric were obtained, and the reason for this was analyzed. Ultraviolet and X-ray emitted by HPM sources and dielectric window discharging were detected, and the mechanism of primary electron generation during dielectric window breakdown under HPM was brought forward based on band theory. The electricity phenomenon of dielectric window under HPM mainly results from the HPM-sourced X-ray bombardment on dielectric window as Well as local micro-discharge happened at dielectric window surface. It has been proved that HPM-sourced X-ray and ultraviolet are the major reason for desorption and ionization of the gas absorbed at the dielectric window surface. Based on the surface micromorphology analysis after breakdown of dielectric window, the damage of HPM dielectric window can be categorized into two different types. One is caused by micro-discharge sparkling’scorching’the dielectric window surface. The other is caused by the breakdown of dielectric window itself.The feed with equal E and H patterns that was used for an experiment prototype was designed and manufactured, and a high power test of the dielectric window life was carried out. The results showed that the feed window life is more than 6×104 pulses under incident microwave with power larger than 1 GW and pulsewidth of 20 ns. A dual-reflector antenna fed by the three-reflector beam waveguide, a TM01 circular waveguide elbow, and a knockdown rotary joint were proposed and developed, which meet the power capacity requirement of HPM transmission line successfully.

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