【作者】 杨杰；

【导师】 舒挺； 樊玉伟；

【作者基本信息】 国防科学技术大学 ， 物理电子学， 2013， 博士

【摘要】 碳纤维天鹅绒阴极具有放气量少和耐烧蚀的特点，是常规天鹅绒阴极的一种重要改进材料，能够更好地满足高功率微波和强激光泵浦的实际应用需求。本论文从碳纤维天鹅绒材料制备、阴极等离子体光学诊断平台建立、碳纤维天鹅绒阴极工作性能研究及其应用等方面开展了系统深入的研究。相关工作对于拓展阴极基础研究、深入认识强流电子束二极管基础物理问题，以及改善天鹅绒阴极应用性能具有重要的理论及现实意义。本论文的研究内容主要包括以下方面：1.碳纤维天鹅绒材料制备根据碳纤维电导率高的特点，理论分析了良导体纤维在静电场中的动力学特性；采用碳纤维表面改性的方法，解决了碳纤维短切丝在气体环境中的团聚问题；采用在植绒装置中填充N2/SF6混合气体的方法，解决了碳纤维短切丝引起的植绒电极短路问题；制备了碳纤维天鹅绒材料，长度为2.0±0.5mm的碳纤维短切丝均匀分布于基底表面，覆盖率达5%。2.强流电子束阴极等离子体光学诊断平台建立搭建了作为阴极等离子体光学诊断平台基础的脉冲功率调制器，其输出电压百kV级、脉冲宽度110ns、二极管电流kA级；采用在脉冲功率调制器主开关之后提取高速相机触发信号的方法，避免了主开关百纳秒甚至微秒级动作抖动对高速相机纳秒级同步触发的影响；研制了信号处理器，在5ns时间内获得了抖动小于1.5ns的高速相机同步触发时间基准；采用高压电脉冲延时辅以二极管等离子体发光信号延时的方案，实现了高速相机的纳秒级同步触发；综合采用屏蔽和滤波等电磁兼容措施保证了高速相机在距离脉冲功率调制器5m范围内安全稳定工作；开展了阴极等离子体光学高速照相实验，图像时间分辨率为10ns，空间分辨率为0.167mm/pixel。3.碳纤维天鹅绒阴极的工作性能研究(1)研究了阴极边缘效应。采用屏蔽环抑制了阴极边缘效应和电子束径向扩张，在此基础上确定了用于阴极性能研究的二极管结构。(2)对比研究了常规天鹅绒和碳纤维天鹅绒阴极的等离子体特性。碳纤维天鹅绒阴极的阈值场强为7±1kV/cm，约为常规天鹅绒阴极阈值场强的1/3；碳纤维天鹅绒阴极有利于改善二极管长时间的宏观电稳定性；提出了一种基于光学高速照相和数字图像处理技术的方法，据此计算得到了阴极等离子体二维膨胀速度；在阴极起动阶段，碳纤维天鹅绒阴极等离子体可以在更短的时间内覆盖整个阴极表面；在电流平顶阶段，碳纤维天鹅绒阴极等离子体的轴向膨胀速度从2.3cm/μs下降到1.3cm/μs，相比之下，常规天鹅绒阴极等离子体轴向膨胀速度从7.3cm/μs下降到5.3cm/μs；采用碳纤维天鹅绒阴极避免了电流脉冲末期的波形畸变。(3)对比研究了阴极和阳极材料对二极管放气的影响。当阴极电流密度小于230A/cm2时，碳纤维天鹅绒阴极减少了二极管放气量；阳极材料表面经过钝化和阳极化处理后形成了致密的薄膜，减少了二极管放气量。(4)对比研究了常规天鹅绒和碳纤维天鹅绒阴极的重复频率工作稳定性。在重复频率20Hz条件下，当电流密度小于100A/cm2时，碳纤维天鹅绒阴极的不稳定度约为常规天鹅绒阴极的1/2；在电流密度为60~70A/cm2条件下，当重复频率相同时，碳纤维天鹅绒阴极工作更为稳定。(5)研究了阳极溅射对阴极的烧蚀。根据阳极表面入射电子束功率的时间变化和空间分布特性，修正了Mesyats关于阳极表面热过程研究的模型，据此得到的理论结果可以在更长的时间和更宽的空间范围内分析脉冲电子束辐照条件下阳极表面的热过程；当入射电子束功率达108W/cm2时，铝、不锈钢以及黄铜等阳极材料表面严重烧蚀；阳极溅射会破坏常规天鹅绒阴极表面形貌，造成纤维的碳化和脱落，导致二极管宏观电稳定性变差。4.碳纤维天鹅绒材料应用(1)开展了碳纤维天鹅绒阴极在MILO中的应用。在电子束参数为600kV、50kA的实验条件下，与常规天鹅绒阴极相比，使用碳纤维天鹅绒阴极不仅减少了MILO输出微波的倍频成分，而且提高了微波的峰值功率。(2)开展了碳纤维天鹅绒材料在微波隐身和杂散光抑制领域的应用。研究了碳纤维天鹅绒的微波隐身性能，覆盖率为5%的碳纤维天鹅绒对9.2~11.8GHz微波的反射率为-4.8~-5.0dB；研究了碳纤维天鹅绒对紫外―近红外区杂散光的抑制效应，覆盖率为2%的碳纤维天鹅绒对紫外―近红外光的反射率均小于1.4%。更多还原

【Abstract】 It has been shown that, in comparison with conventional velvet, carbon fiber velvetcathode is more practical in such applications as high-power microwave generation andpumping of gaseous lasers for low out-gassing and good ablation resistance. In thisdissertation, the related aspects of carbon fiber velvet have been studied systematicallyand profoundly, such as material fabrication, establishment of the platform for opticaldiagnostics of cathode plasma, property investigations and applications. These effortsare of great theoretical and realistic significance for both basic and applied research ofthe carbon fiber velvet. The detailed contents include the followings:1. Fabrication of the carbon fiber velvetThe dynamics of the conductive fibers in electrostatic field were studiedtheoretically. The method based on the surface modification was proposed for thecarbon fiber dispersion in circumstances of gas. The N2/SF6gas mixture was filled intothe electrostatic flocking box to avoid the short circuit between electrodes. The velvetconsists of numerous2.0±0.5-mm-long carbon fibers aligned perpendicularly to thesubstrate. The fiber packing fraction is about5%.2. Establishment of the platform for optical diagnostics of cathode plasmaAs the basis of the platform, the pulse power modulator could provide an electricalpulse with voltage of several hundred kV and pulse width of110ns. The diode currentup to kA level could be extracted. The trigger signal for the camera was picked up afterthe main switch of the modulator, which could avoid the disadvantageous influence ofthe jitter time (ns~μs) caused by the breakdown of the gas gap. Then the samplednegative electrical pulse was converted into a TTL signal via a processor within5ns.The jitter time was less than1.5ns. And this was taken as the synchronization time base.The synchronization scheme relying mainly on electrical pulse delay supplemented bylight delay was employed to make sure that the camera could work synchronously withlight production and transportation from the cathode plasma within the time scale ofnanosecond. The shielding and filtering methods were used to make sure that thecamera could work properly and steadily within5meters of the modulator. At last, thecathode plasma imaing experiments were carried out on this platform. The temporal andspatial resolution was10ns and0.167mm/pixel, respectively.3. Research of the carbon fiber velvet properties(1) Research of the cathode edge effect. It was shown that the cathode edge effectcould be eliminated by using the screening ring. Based on this, the diode structure wassettled.(2) Comparison of the cathode plasma characteristics. It was shown that the carbonfiber velvet turned on at the electric field of7±1kV/cm, which was nearly one third of that of the conventional velvet. The long-term electrical stability of the diode could beimproved by using carbon fiber velvet cathode. The2-D expansion velocity of thecathode plasma was investigated by utilizing the digital image processing methods.During the rise time of the current pulse the plasma could cover the entire surface of thecarbon fiber velvet cathode in a shorter time. The value of the axial velocitycomponents of the carbon velvet cathode plasma decreased from2.3cm/μs to1.3cm/μsduring the current flattop. By contrast, the value of the axial velocity components of theconventional velvet cathode plasma decreased from7.3cm/μs to5.3cm/μs. Moreover,the waveform distortion in the latter part of the current could be avoided by using thecarbon fiber velvet cathode.(3) Comparison of the effects of the cathode and anode materials on the outgassingof the diode. When the current density was less than230A/cm2, the amounts of gasesgenerated in the diode could be reduced by using the carbon fiber velvet cathode. Thedense passive and anodized coating could be created on the anode surface. And thiscould reduce the amounts of gases generated in the diode.(4) Comparison of the pulse-to-pulse instability in the rep-rate mode. When thecurrent density was less than100A/cm2, the instability was about half as many as that ofthe conventional velvet at20Hz rep-rate. When the current density was60~70A/cm2,the carbon velvet cathode was more robust at the same rep-rate.(5) Research of the anode splash on the erosion of the velvet cathode. Based on thetemporal variations and spatial distributions of power density of the electron flow, theprevious model put forward by Mesyats was modified. The thermal regime of the anodewas studied theoretically. It was shown that, for the power density of108W/cm2,conditions for intense erosion of the anode material were created. The anode splashdestroyed the surface morphology of the velvet, resulting in the carbonization anddetachment of the fibers. Moreover, this decreased the electrical stability of the diode.4. Application of the carbon fiber velvet(1) Application of the carbon fiber velvet in MILO. The conventional and carbonfiber velvet cathode were studied utilizing a stainless steel MILO. It was shown that notonly the higher harmonic of the output microwave was suppressed but also the peakpower was increased when the carbon fiber velvet was used in MILO with diode voltageof600kV and beam current of50kA.(2) Application of the carbon fiber velvet in microwave stealth and stray-lightsuppression. It was shown that the reflectance values of the carbon fiber velvet withfiber packing fraction of5%was-4.8~-5.0dB in the frequency range of9.2~11.8GHz.The surface of the carbon fiber velvet with fiber packing fraction of2%yieldedreflectance values of less than1.4%for ultraviolet, visible, and near-infrared light.更多还原