Effect of microwave frequency on plasma formation in air breakdown at atmospheric pressure

Microwave breakdown at atmospheric pressure causes the formation of a discrete plasma structure. The onedimensional fluid model coupling Maxwell equations with plasma fluid equations is used to study the effect of the microwave frequency on the formation of air plasma. Simulation results show that, the filamentary plasma array propagating toward the microwave source is formed at different microwave frequencies. As the microwave frequency decreases, the ratio of the distance between two adjacent plasma filaments to the corresponding wavelength remains almost unchanged（on the order of 1/4）, while the plasma front propagates more slowly due to the increase in the formation time of the new plasma filament.

Effect of air breakdown on microwave pulse energy transmission

The energy transmission of the long microwave pulse for the frequency of 2.45 GHz and 5.8 GHz is studied by using the electron fluid model, where the rate coefficients are deduced from the Boltzmann equation solver named BOLSIG＋. The breakdown thresholds for different air pressures and incident pulse parameters are predicted, which show good agreement with the experimental data. Below the breakdown threshold, the transmitted pulse energy is proportional to the square of the incident electric field amplitude. When the incident electric field amplitude higher than the breakdown threshold increases,the transmitted pulse energy decreases monotonously at a high air pressure, while at a low air pressure it first decreases and then increases. We also compare the pulse energy transmission for the frequency of 2.45 GHz with the case of 5.8 GHz.

Air breakdown induced by the microwave with two mutually orthogonal and heterophase electric field components

The air breakdown is easily caused by the high-power microwave, which can have two mutually orthogonal and heterophase electric field components. For this case, the electron momentum conservation equation is employed to deduce the electric field power and effective electric field for heating electrons. Then the formula of the electric field power is introduced into the global model to simulate the air breakdown. The breakdown prediction from the global model agrees well with the experimental data. Simulation results show that the electron temperature is sensitive to the phase difference between the two electron field components, while the latter can affect obviously the growth of the electron density at low electron temperature amplitudes. The ionization of nitrogen and oxygen induces the growth of electron density, and the density loss due to the dissociative attachment and dissociative recombination is obvious only at low electron temperatures.

Dependence of plasma structure and propagation on microwave amplitude and frequency during breakdown of atmospheric pressure air

The structure and propagation of the plasma in air breakdown driven by high-power microwave have attracted great interest.This paper focuses on the microwave amplitude and frequency dependence of plasma formation at atmospheric pressure using one two-dimensional model,which is based on Maxwell’s equations coupled with plasma fluid equations.In this model,we adopt the effective electron diffusion coefficient,which can describe well the change from free diffusion in a plasma front to ambipolar diffusion in the bulk plasma.The filamentary plasma arrays observed in experiments are well reproduced in the simulations.The density and propagation speed of the plasma from the simulations are also close to the corresponding experimental data.The size of plasma filament parallel to the electric field decreases with increasing frequency,and it increases with the electric field amplitude.The distance between adjacent plasma filaments is close to one-quarter wavelength under different frequencies and amplitudes.The plasma propagation speed shows little change with the frequency,and it increases with the amplitude.The variations of plasma structure and propagation with the amplitude and frequency are due to the change in the distribution of the electric field.

Experimental study of rotating wind turbine breakdown characteristics in large scale air gaps

When a wind turbine is struck by lightning,its blades are usually rotating.The effect of blade rotation on a turbine＇s ability to trigger a lightning strike is unclear.Therefore,an arching electrode was used in a wind turbine lightning discharge test to investigate the difference in lightning triggering ability when blades are rotating and stationary.A negative polarity switching waveform of 250/2500 μs was applied to the arching electrode and the up-and-down method was used to calculate the 50%discharge voltage.Lightning discharge tests of a 1：30 scale wind turbine model with 2,4,and 6 m air gaps were performed and the discharge process was observed.The experimental results demonstrated that when a 2 m air gap was used,the breakdown voltage increased as the blade speed was increased,but when the gap length was 4 m or longer,the trend was reversed and the breakdown voltage decreased.The analysis revealed that the rotation of the blades changes the charge distribution in the blade-tip region,promotes upward leader development on the blade tip,and decreases the breakdown voltage.Thus,the blade rotation of a wind turbine increases its ability to trigger lightning strikes.

汽车空调的维护与机械故障检修思考

伴随着社会经济与科技的快速发展进步,汽车已成为人们日常生活中必备的一种交通工具,其技术创新能力也显示出不断完善的趋向。然而,同时随之而来的是对汽车机械故障的关注度不断提高。故障的产生原因众多,一般可以依据其各自的特性进行归类。因此,必须掌握汽车空调维护以及机械故障的产生原因,再结合相对应的维修方法进行修理。基于此,本文将针对上述问题展开探究分析,以促使人们更好地了解汽车空调的相关知识的维修要点。

棒电极直径和端部形状对短空气间隙交流击穿电压的影响研究

空气间隙交流击穿特性试验常采用棒棒、棒板电极作为试验电极,目前棒电极直径、棒端部形状对空气间隙交流击穿电压的影响尚不清晰。为此,用直径2 cm、1 cm的圆棒和直径1 cm的尖棒搭建棒棒、棒板电极,开展5~40 cm短空气间隙下的交流击穿试验,研究棒电极直径和端部形状对空气间隙交流击穿电压的影响。结果表明:在试验棒电极直径和端部形状范围内,空气间隙距离5~20 cm时,各棒棒或棒板电极空气间隙击穿电压相对偏差在5.93%~33%之间,随着空气间隙距离增大,棒电极直径和端部形状对棒棒、棒板空气间隙击穿电压的影响逐渐减弱;空气间隙距离大于20 cm时,各棒棒或棒板电极空气间隙击穿电压相对偏差均小于5%。研究认为,空气间隙距离超过20 cm时,棒电极直径和端部形状对棒棒、棒板空气间隙击穿电压无影响。

一起35kV干式空心串联电抗器故障原因分析

目前电力系统中户外型并联无功补偿装置的电抗器大多数采用干式空心串联电抗器,在运行过程中,电抗器常因绝缘老化、局部温升过高、过电压、谐波以及环境等因素影响,导致电抗器匝间绝缘损坏击穿、线圈被雨淋时包封表面爬电,甚至局部放电电弧烧毁现象发生。为此本文介绍了某500kV变电站35kV并联电容器组的干式空心串联电抗器故障烧毁情况,进行了现场检查和返厂试验;对烧损相和非烧损相均进行了由外向内的包封层解体分析,推断出故障原因为:由于电抗器本体运行了近十年后,电抗器本体包封层的环氧材料劣化开裂,遇雨水天气时潮气浸入包封层内部,长期运行导线绝缘薄膜加速老化,在电容器组多次投切产生过电压,造成电抗器匝间绝缘损坏,最终导致电抗器绝缘击穿、起火烧毁。针对本次事故电抗器损坏情况,提出了相应的预防和处理措施。

An AlGaN/GaN HEMT with enhanced breakdown and a near-zero breakdown voltage temperature coefficient

An AlGaN/GaN high-electron mobility transistor（HEMT） with a novel source-connected air-bridge field plate（AFP） is experimentally verified.The device features a metal field plate that jumps from the source over the gate region and lands between the gate and drain.When compared to a similar size HEMT device with a conventional field plate（CFP） structure,the AFP not only minimizes the parasitic gate to source capacitance,but also exhibits higher OFF-state breakdown voltage and one order of magnitude lower drain leakage current.In a device with a gate to drain distance of 6 μm and a gate length of 0.8 μm,three times higher forward blocking voltage of 375 V was obtained at VGS =-5 V.In contrast,a similar sized HEMT with a CFP can only achieve a breakdown voltage no higher than 125 V using this process,regardless of device dimensions.Moreover,a temperature coefficient of 0 V/K for the breakdown voltage is observed.However,devices without a field plate（no FP） and with an optimized conventional field plate（CFP） exhibit breakdown voltage temperature coefficients of-0.113 V/K and-0.065 V/K,respectively.

Fine 3D control of THz emission in air with dual femtosecond laser pre-pulses at tunnelling ionisation regime

Emission of THz radiation from air breakdown at focused ultra-short fs-laser pulses(800 nm/35 fs)was investigated for the 3D spatio-temporal control where two pre-pulses are used before the main-pulse.The laser pulse induced air breakdown forms a~120μm-long focal volume generate shockwaves which deliver a denser air into the focal region of the main pulse for enhanced generation of THz radiation at 0.1-2.5 THz spectral window.The intensity of 162 pre-and main-pulses was at the tunnelling ionisation intensities(1-3)×10 W/cm and corresponded to sub-critical(transparent)plasma formation in air.Polarisation analysis of THz radiation revealed that orientation of the air density gradients generated by pre-pulses and their time-position locations defined the ellipticity of the generated THz electrical field.The rotational component of electric current is the origin of THz radiation.

一种抑制高功率微波击穿的副反射面天线罩设计

针对应用于辐射高功率微波的双反射面天线系统设计了一种新型抑制高功率微波击穿的副反射面天线罩.该副反射面天线罩为基于复合材料的曲面球壳体结构,通过在罩内填充SF6气体,能够有效抑制副反射面附近高功率微波击穿.首先仿真分析了高功率脉冲照射下的天线副反射面附近区域空气击穿空间分布,确定天线罩的最小外廓尺寸.然后开展天线罩低损耗设计方法研究,通过采用优化算法对罩体外形曲线进行优化设计,使得电磁波与天线罩体的夹角控制在44°以内,并以此作为输入优化得到使透波系数最优的蒙皮和芯层厚度参数.此外,为避免复杂的成型工艺,将副面罩简化为泡沫环筋结构,其既有低损耗特性又兼顾了一定结构强度.最后对天线罩的电气性能进行仿真与测试验证,实测结果表明这种副面罩引入的插入损耗在0.3dB以内,透波率达93.4%以上,对反射面天线系统的辐射性能影响很小.

基于激光诱导击穿光谱的果木炭燃烧对空气成分影响的研究

果木炭是一种常见的燃料,其燃烧过程中产生的气体和烟尘气溶胶会影响环境空气质量并损害人体健康,因此对果木炭燃烧过程中空气成分进行检测与分析具有重要意义。采用激光诱导击穿光谱(LIBS)技术对果木炭燃烧时的空气、烟尘气溶胶进行检测,同时检测果木炭及其燃烧灰烬作为辅助分析。对四种样品的谱线进行标定,发现果木炭燃烧时空气中碳浓度增大,生成的气溶胶中含有Ca、Mg、K、Si等元素。果木炭和灰烬的元素组成较为相似,均含有C、Fe、Mg、Ca、Sr、K、Na和Ba等元素,果木炭光谱中C、H元素谱线强度均高于灰烬。此外,结合机器学习算法对有无果木炭燃烧时的空气进行区分,选取C、CN分子特征谱线所在的波段作为聚类分析的原始特征。主成分分析(PCA)结果表明在有无果木炭燃烧两种条件下的空气能被较好地区分,证明LIBS结合PCA技术能有效地识别果木炭的燃烧并检测果木炭燃烧造成的空气污染。进一步利用LIBS结合机器学习算法对果木炭及其燃烧灰烬进行区分,发现区分效果良好,为果木炭燃烧后的回收利用提供了参考。

一种新型组合谐振腔微波等离子体炬

微波等离子体电子密度大,电子温度高,在材料合成、有害气体消解、光谱分析等领域有着广泛的应用前景。本文提出了一种新型的组合谐振腔微波等离子体炬,利用一个微波源通过功分器后分别激励两个不同的谐振腔,微波馈入谐振腔后,在谐振腔内产生驻波,在驻波波腹处电场强度最大完成等离子体的激发,实现在较小微波输入功率、大气压下的空气击穿并形成大体积的等离子体火焰。通过对谐振腔结构尺寸优化,使谐振腔内的电场强度达到激励和维持微波等离子体的要求。

汽车空调系统的原理及故障维修

当前,企业积极研发空调系统,增强空调系统性能,便于赢得消费者对汽车产品的青睐。在科学技术迅速发展的背景下,汽车空调系统不断升级,但汽车空调系统在使用过程中也会发生故障,影响用户体验。为提高对汽车空调系统的应用水平,有必要深入研究汽车空调系统的原理,分析汽车空调系统故障维修方法,保障故障维修水平。

PCB平面三电极空气火花隙开关导通特性试验研究

针对平面三电极空气火花隙开关的稳定性、响应时间等动态性能问题,设计了阴极与阳极间隙为1.6,1.8,2.0,2.2,2.4 mm的5种PCB平面三电极空气火花隙开关,分别对开关的最小击穿电压、导通性能的变化规律和放电寿命进行了试验研究。试验结果表明:PCB平面三电极空气火花隙开关的最小击穿电压随着导通次数、阴极与阳极间隙的增加而增大;在高压电容器充电电压、触发端的脉冲电压一致的条件下,相同参数开关的响应时间随着导通次数的增加而增大;在其他条件不变的情况下,开关阴极与阳极间隙越大,开关的放电寿命越长。

一起35kV干式空心并联电抗器绝缘击穿事故原因分析及解决方案

本文中作者介绍了一起500kV变电站35kV干式空心并联电抗器在进行现场耐压试验时发生绝缘击穿故障的情况,通过电抗器事故现场检查、电气试验以及返厂解体,并结合干式空心并联电抗器的结构,对电抗器故障的原因进行了分析,确定了故障原因,提出了解决方案,可避免类似事故的发生。故障分析结果和解决方案对干式电抗器的安全运行具有普遍的的借鉴意义。

交直流复合电压下气固复合绝缘的击穿特性

在新型电力系统发展阶段下,交直流混联电网系统的格局正在形成,交直流叠加作用下电气设备的绝缘性能迎来新的挑战。在气体绝缘间隙中引入聚合物隔板,能够提高气体的绝缘性能,有利于电气设备的小型化。本文基于并联回路,搭建了气固复合绝缘间隙下交直流复合电压击穿试验平台,通过改变直流电压分量、电极半径和隔板位置,研究了复合击穿电压幅值和电压分量特性。在本文试验条件范围内,当直流电压分量在20kV至62kV时,直流电压分量对复合击穿电压的影响不显著。随着隔板与高压电极距离的减小和电极半径的增大,复合击穿电压都增大。交流电压分量与直流电压分量为负线性相关。棒-板结构下隔板的作用效果最好,复合击穿电压最大提升1.81倍。

重复频率高功率微波脉冲的大气击穿

理论研究了重复频率高功率微波脉冲在大气中的传输特性,推导了n个高功率微波脉冲之后空间某点的电子密度,数值模拟并给出了在不同微波频率、大气压强、脉冲宽度以及脉冲间隔的条件下,大气击穿时,透射过去的脉冲个数与微波功率的关系。模拟结果表明:当微波脉冲参数一定时,压强越小,透射过去的脉冲个数越少,大气越容易发生击穿;当压强一定时,脉宽越宽,微波频率越小,透射过去的脉冲个数越少,大气越容易发生击穿。

短脉冲高功率微波大气击穿研究

论述了短脉冲高功率微波在波导内和自由空间中大气击穿的实验研究，给出了不同气压条件下的击穿阈值，并对两种实验条件下的结果进行了比较，两种实验的数据统计结果基本一致，证明了采用波导内大气击穿方法来研究大气击穿问题的合理性。

大气击穿对高功率微波天线的影响

高功率微波大气传播过程中,天线附近的功率密度最大,容易发生强电离或大气击穿,由此产生“尾蚀效应”等非线性衰减,因此,传输过程中产生的大气击穿限制了高功率微波天线的最大发射功率。通过分析天线近场模型,研究了矩形口径天线和圆口径天线的近场轴向功率密度分布,得到了不同口面场分布下天线的最大归一化功率密度及其最大值所处的位置,并结合大气击穿功率密度阈值计算出锥照圆口径天线的最大发射功率约为148.47 GW。