110 GHz太赫兹波大气击穿阈值及最大传输能量密度

随着110 GHz高功率太赫兹波功率容量的提升,其引起的大气击穿问题越来越受到重视。将若干等效电离参数表达式引入到电子雪崩密度方程中,计算了不同压强下的大气击穿阈值。结果表明,由Ali等效电离参数得到的110 GHz击穿阈值与实验数据符合得很好。在此基础上,利用Ali等效电离参数对逃逸传输能量密度与太赫兹波振幅的关系进行了分析。结果表明,当太赫兹波振幅小于击穿阈值时,逃逸传输能量密度随功率密度的增加线性增加;当太赫兹波振幅大于击穿阈值时,逃逸传输能量密度随功率密度先减小后增大。     

Microwave breakdown in slots

The properties of microwave-induced breakdown of air in narrow metallic slots are investigated, both theoretically and experimentally, with emphasis on factors important for protection against transmission of incident high-power microwave radiation. The key factors investigated are breakdown power threshold, breakdown time, peak-leakage power, and total transmitted energy, as functions of incident pulse shape and power density. The theoretical investigation includes estimates of the electric field intensification in narrow slots and basic breakdown plasma modeling. New results important for application to the high-power microwave field, such as the influence of pulse shape on breakdown time and peak-leakage power, are presented. The experimental investigation comprises a set of slot breakdown experiments at atmospheric pressure, which are analyzed to extract key parameters, such as transmission cross section, breakdown time, peak leakage power, and transmitted energy. The experimental data is compared and shown to be in good agreement with results obtained in the theoretical investigation.     

FDTD方法分析高功率微波对大气的电离与击穿

 应用麦克斯韦方程和电子流体方程,利用时域有限差分方法(FDTD)计算模拟了高功率微波(HPM)对大气的电离与击穿;该方法用瞬时电场代替等效电场,时刻更新大气电离击穿过程中的电离频率和碰撞频率,消除了近似解析法未考虑大气电离击穿过程中电场幅度衰减而引起的误差,计算得到击穿阈值大小随海拔高度的变化趋势与文献所得的变化趋势相吻合,其值略大于近似解析解;并通过仿真计算分析了HPM脉冲幅值、脉宽以及海拔高度等参数对大气击穿的影响。     

A New Method for Measuring Breakdown and Maintenance Fields in Microwave Discharges

A new experimental approach for comparative measurements of breakdown and maintenance electric fields in microwave discharges is described. The method is based on the analysis of the RF pulses transmitted to a matched load, in absence and in presence of the discharge, by the applicator employed for coupling the RF power to the gas under investigation. Results on breakdown and maintenance field in microwave discharges in nitrogen, at a frequency of 2.45 GHz, for flow and non-flow conditions, in the pressure range 0.5 to 6 torr, are reported.     

大气压微波等离子体射流装置放电特性研究

基于同轴传输线结构设计了两种不同喷嘴结构的大气压微波等离子体射流(MW-APPJ)装置,其工作频率2.45 GHz,工作气体为氩气,分别研究了两种不同喷嘴结构对等离子体放电特性产生的影响。仿真结果表明,MW-APPJ在气体喷嘴处会产生高强度的电场,经过优化结构,实现在频率2.45 GHz下,喷嘴处的场强满足氩气电离的击穿场强阈值要求。同时,利用多物理场耦合仿真软件对装置的气流分布进行了稳态模拟,并通过实验对比分析了两种喷嘴结构下大气压氩等离子体射流的基本特性。实验结果表明,不同的喷嘴结构会影响等离子体装置的反射系数随输入功率的变化规律,但并不影响等离子体射流长度随输入功率的变化规律和反射功率随进气流量的变化规律;同时,在大气压下,稳态微波等离子体射流呈现出类金属性,等离子体中的电子只能在很薄的区域中吸收微波能量,因而造成微波的反射功率较大。     

Microwave Breakdown of Water Vapor

Continuous wave breakdown electric field strengths of water vapor have been measured by a resonant cavity technique at 2.45 GHz, in the pressure range from 20 mtorr to 22 torr. Two cavities were used, one having a characteristic diffusion length of 0.39 cm and the other of 1.0 cm. The results, including a minimum breakdown threshold of 165 V/cm, occurring at a pressure of 1.4 torr, are typical of a combined diffusion and attachment-controlled gas breakdown. These data are required for an investigation of a microwave-aided freeze-drying process.     

带阻性负载细导线对电磁脉冲响应的有限差分算法

在原始的FDTD细线算法基础上,把带负载细导线模型分成导线、电阻和吸收三个部分,分别用不同的偏微分方程描述,使其可以处理两端带有纯电阻性负载细线电磁脉冲散射问题,进而得到电阻负载上消耗的总能量及对应的功率消耗。用该方法计算得到的结果与文献结果进行了比较,证明了该方法的有效性。最后用此方法对一种典型情况进行了计算,并对结果进行了分析。该方法是对FDTD方法中细线算法的补充和提高,经过修改也可以用于非阻性负载的情况。     

时域有限差分法在超高速通信系统中带隙型光子晶体光纤瞬态三维截面上特性研究

利用时域有限差分法针对满足带耦合腔PCFS条件的Maxwell方程组进行电磁场三维矢量变换,来建立关于入射光在光子晶体光纤传播物理数值模型。通过数值仿真表明:在波导色散作用下,当空气间隙增大,其TE波在直角坐标系中三维矢量数值大小与网格步长递增呈正比变化,且瞬态截面分裂速度减缓且数目递减、体积变大;反之,当空气孔间隙减小,其TM波三维矢量随着传输网格步长递增,瞬态截面分裂呈细丝状且数目递增、体积变小。同时,随着角频率w0增大,磁场x,y方向中心强度随时间网格呈对称式变化且峰值强度逐渐递增;同时在L+Nd=150(N=0,1,2;l0)处存在拐点;在电场z方向,振幅抖动愈加频繁且峰值连续上涨,相位峰值逐渐递减且脉宽呈压缩变化。     

Thermal ionization instability of an air discharge plasma in a microwave field

Results are presented from experimental studies of the initial stage of an air discharge initiated in a linearly polarized quasi-optical microwave beam. The discharge was excited at an air pressure at which the electron-neutral collision frequency in the discharge plasma was considerably higher than the circular frequency of the electromagnetic field and at a microwave field amplitude close to the threshold field for air breakdown. The experiments revealed relatively bright plasma channels stretched along the microwave electric field. The development rate of these channels and their characteristic transverse dimensions are estimated. A comparison of the experimental data and theoretical estimates indicates that the channels observed arise due to the onset of thermal ionization instability in the microwave discharge plasma.     

典型放电气体的击穿场强阈值研究

为了研究等离子体产生时的气体击穿特性,利用低气压条件下气体击穿场强阈值模型,分析了He、Ne、Ar、Kr、Xe和Hg蒸汽等6种典型放电气体的击穿阈值随入射波频率、电子温度、气体压强以及气体温度的变化规律。结果表明:气体击穿阈值随气体压强的增大而减小,随气体温度、电子温度和入射脉冲频率的增大而增大。气体压强和入射频率对击穿阈值的影响大于气体温度和电子温度,在所考虑的范围内,气体压强对击穿场强的影响约为100 V/m,入射脉冲频率对击穿场强的影响为50~300 V/m,气体温度和电子温度对击穿场强的影响为20~30 V/m。当考虑气体压强、气体温度以及电子温度等因素的影响时,各种气体的击穿场强阈值产生的变化规律相类似;但考虑入射频率的影响时,不同气体的击穿场强阈值差异很大。在所考虑的典型放电气体中,Xe具有最低的击穿场强阈值,He的击穿阈值最大。     

微观缺陷对材料损伤阈值的影响分析

为了研究材料表面高频缺陷对损伤阈值的影响规律,针对典型表面圆锥凸起状表面缺陷建立了三维分析模型,使用有限元算法计算了两种不同波长激光辐照下,表面缺陷周围的电场变化规律。基于激光诱导材料产生损伤的场效应理论,对缺陷存在条件下材料的损伤阈值进行了定量计算。结果表明,缺陷的存在会对电场产生明显的放大效应,这将直接导致损伤阈值的下降;损伤阈值与缺陷高度成反比关系,与入射波长成正比关系;缺陷导致的损伤阈值下降幅度会随脉宽增加先减小后增大,当入射波长分别为351nm和1064nm时,缺陷导致的损伤阈值下降幅度分别在脉宽为1.80ps和15.80ps处最小。     

微波大气击穿阈值的理论研究

本文通过对使用有效场强(或均方根场强)得到的微波大气击穿阈值表达式进行讨论, 指出其推导中所做的假设及这些假设应用到微波大气击穿过程中存在的问题. 然后分别使用解析理论和数值模拟对微波大气击穿过程中的有效电子温度变化过程和击穿阈值进行研究, 并将其与直流电场进行比较. 分析发现在高气压下, 电子能量转移频率高, 有效电子温度随电场大幅振荡, 由于电离频率随有效电子温度的增长率大于电子能量损失随有效电子温度的增长率, 因此在高气压时, 微波大气击穿阈值低于使用有效场强的击穿阈值. 通过大量分析, 给出了理论推导和数值模拟得到的微波大气击穿阈值拟合表达式.     

On the influence of the voltage pulse rise time and cathode geometry on the generation of a supershort avalanche electron beam

The influence of the voltage pulse rise time on the amplitude of a runaway electron beam and X-ray generation in air and nitrogen under atmospheric pressure is studied experimentally and theoretically. Generalization of the whistle criterion for the case of a nonuniform field is suggested. It is shown that the maximal energy of beam electrons and the beam current amplitude grow when the voltage pulse rise time decreases. It is found that the amplitude of the runaway electron current reaches a maximum at a certain curvature of the cathode. The maximal energy of electrons increases when the radius of curvature of the cathode exceeds the value at which the beam current amplitude is the highest. If the field is nonuniform, its critical value at which many electrons run away is more than an order of magnitude lower than in the uniform field.     

Enhancement of the electric field threshold for microwave breakdown by crossed magnetic field

The enhancement of breakdown threshold is of benefit to the high power microwave transmission. We propose a magnetic field in the transverse direction to the electric field to enhance the breakdown threshold. A theory of electric field threshold with crossed magnetic field for short pulse is developed, and verified by particle-in-cell/Monte Carlo collision (PIC/MCC) simulations. The result shows that the crossed magnetic field can enhance the breakdown threshold significantly.     

Ps four-wave mixing measurements of electric field in a ns pulse discharge in a hydrogen diffusion flame

Time-resolved electric field in ns pulse discharge plasmas generated in room air and in an atmospheric pressure hydrogen diffusion flame has been measured by ps four-wave mixing, for plane-to-plane electrode geometry. Electric field is put on the absolute scale using the Laplacian field measured before breakdown. The results show that peak electric field during breakdown in the flame, approximately 40?kV/cm, is significantly lower compared to that in room air, 75?kV/cm, due to higher temperature of combustion products. In both cases, peak electric field is higher compared to DC breakdown field. Both in air and in the flame, the electric field follows the applied voltage before breakdown and decreases rapidly after breakdown, due to charge separation and plasma self-shielding. The electric field in air is compared with the predictions of an analytic model of ns pulse breakdown, showing good agreement between the predicted and the measured breakdown field. The model also predicts earlier breakdown as well as breakdown voltage reduction as the temperature is increased, in qualitative agreement with the experimental data. The use of the present ps four-wave mixing diagnostics for measurements of electric fields below ～20?kV/cm in atmospheric pressure flames is challenging, due to low signal-to-noise. The sensitivity of the present diagnostics is controlled by the high temperature and low N₂ fraction in the combustion product mixture, as well as by the limited bandwidth of the Stokes beam generated by the stimulated Raman cell, which provides access only to several rotational levels of nitrogen molecules. The present diagnostics will have much better sensitivity in high-pressure flames, since the four-wave mixing signal scales as the squared number density of nitrogen.     

温度相关的微波频率下氯化钠水溶液介电特性

针对NaCl水溶液(0.001–0.5 mol/L)介电特性, 实验调查了频率(200–6.25 GHz), 温度(293–353 K)、 浓度相关复介电常数. 结果表明: 频率增大的过程中虚部呈逐渐减小的趋势, 高温使离子扰动增大, 破坏了溶液内部水分子四面体结构和氢键构象而使介电常数实部减小. 与纯水相比, 溶液的损耗角正切在高温353 K低频区下降明显. 同时发现2.45和5.8 GHz的复介电常数随温度变化的温度窗效应, 温度窗效应导致微波加热时耗散功率的振荡变化, 温度分布不均匀现象在实验中得以证实. 关键词： 微波 复介电常数 介电特性 高温探头     

Resonant interaction of an electromagnetic wave with an inhomogeneous plasma slab

Resonant interaction at oblique incidence of an electromagnetic wave on an inhomogeneous plasma slab is studied. The time evolution of this interaction is solved numerically from two-fluid equations, adiabatic equation for electron pressure and from Maxwell equations. It is shown that the electromagnetic energy of an incident wave is transformed both into the heat energy and into the energy of plasma oscillations in the direction of density gradient. The distribution of the transformed energy between the heat energy and the energy of plasma oscillations is strongly dependent on the plasma temperature. The ratio of heat energy to the energy of plasma oscillations is growing with growing temperature. The plasma oscillations are generated by magnetic induction of the penetrating wave. In a cold plasma they are generated especially in the overdense region and their frequency is equal to local plasma frequency. The electric field in the direction of plasma gradient has a form of a wave packet whose envelope reaches a maximum at resonance. The characteristic wavelength in the wave packet decreases and the amplitude of the packet increases with the time.     

发光等离子体对6GHz高功率微波的防护性能研究

为研究发光等离子体对高功率微波的防护性能,建立了一维条件下等离子体与高功率微波相互作用的物理模型,并采用数值仿真得到了不同条件下的微波透射效果,分析了发光等离子体对高功率微波的防护性能。随后,实验研究了双层柱状等离子体阵列对6 GHz高功率微波脉冲的透射效果,实验结果与仿真结果相符,说明高功率微波的入射使等离子体产生了非线性效应。实验结果还表明,TE极化时的防护效果要优于TM极化时的防护效果;等离子体击穿场强阈值随电场作用空间的增大而减小;TE极化时等离子体对高功率微波脉冲的屏蔽效能最高可达13 d B,且随入射功率的增大而进一步增大。     

Generation of single-cycle THz transients with high electric-field amplitudes

Single-cycle terahertz (THz) transients in the frequency range 0.3-7 THz with electric-field amplitudes of more than 400 kV/cm are generated by four-wave mixing of the fundamental and the second harmonic of 25 fs pulses from a Ti:sapphire amplifier in ionized air. These transients are fully characterized by electro-optic sampling with ZnTe and GaP crystals. One can tune the center frequency of the THz transients by varying the length of the incident pulse. The electric-field amplitude increases linearly with the incident pulse energy.     

介质表面附近微波大气击穿的理论研究

将麦克斯韦方程组和简化等离子体方程耦合求解, 对介质表面附近大气击穿形成等离子体的过程进行了理论研究. 分别使用一维、二维模型对等离子体的形成过程及等离子体对电磁波的反射、吸收过程进行了模拟研究. 一维计算结果发现在n_e = 0, j = 0两种边界条件下, 虽然形成的等离子体密度分布相差较大, 但二者得到的微波反射、吸收、透射波形彼此相差不大. 初始电子数密度厚度为20 mm的条件下, 得到界面附近的等离子体密度大于5 mm厚度的情况. 二维计算结果发现, 由于TE10模在波导中心位置处的微波电场最强, 电子碰撞电离首先在中心位置处形成等离子体, 当等离子体密度达到一定值(临界密度附近)时, 波导中心介质表面处微波场强减小, 等离子体区域沿着介质表面向两侧移动. TE10模在波导边缘处微波电场强度小于击穿阈值, 因此等离子体区域不可能移动到波导边缘附近.