Microwave breakdown of air initiated by a short electromagnetic vibrator

The electrical breakdown of air (under a pressure from several tens of Torr to several hundreds of Torr) is initiated in a linearly polarized quasi-optical microwave beam using a rounded-end cylindrical metallic electromagnetic vibrator placed parallel to the electric component of the microwave field. The vibrator is much shorter than the wavelength of the field. The field strength at the top of the vibrator as a function of its length is determined. An empiric expression relating the field strength at the top with the vibrator length-to-diameter ratio is found for the vibrator length range studied. The practicability of locally measuring the field in a quasioptical microwave beam is substantiated. The idea is to determine the maximal air pressure at which electrical breakdown of air initiated by a short vibrator placed at a given point of the beam takes place.     

Deeply undercritical microwave discharge excited by the field of a quasi-optical electromagnetic beam in a supersonic air jet

Electric discharge in a supersonic air jet is studied. It is ignited in a linearly polarized quasi-optical microwave electromagnetic beam the initial field intensity of which is much lower than the breakdown level. Electric breakdown is initiated by a tubular electromagnetic vibrator, one end of which has spikes and is covered by a quartz tube. Atmospheric air enters into a low-pressure working chamber through the inner channel of the vibrator. As a result, an immersed supersonic air jet forms in the chamber at the outlet from the quartz tube. A microwave discharge ignited in this jet is “attached” to aft spikes of the vibrator. The energy deposit into the discharge plasma and the effective area of energy interaction between the discharge and excited microwave field are estimated from the temperature and stagnation pressure distributions in the wake of the discharge.     

Electric discharge in air in a deeply subcritical field of a quasi-optical microwave beam

We describe the results of experiments on initiation of an electric discharge in air in a quasi-optical microwave beam by an electromagnetic vibrator fixed above the screen. This method for initiating the electrical breakdown makes it possible to obtain a discharge at a level of the electric field component in the microwave, which two orders of magnitude lower than the minimal critical field of the electrodeless breakdown of air. In our experiments, the threshold value of the air pressure determining the low- and high-temperature forms of the microwave discharge are determined depending on the field level.     

Induced field of an electromagnetic vibrator above a conducting screen placed in a microwave beam

The induced field of a cylindrical electromagnetic vibrator with spherically rounded ends is calculated. The vibrator is above a flat screen placed in a linearly polarized quasi-optical microwave beam. The plane of the screen is perpendicular to the Poynting vector of the radiation. The axis of the vibrator is aligned with the vector of the exciting field electrical component. In calculation, the length of the vibrator and the vibrator-screen distance were varied, while its diameter and the initial field were kept constant. It is found that the induced field of the vibrator with a length close to the half-wavelength of the field is maximal at the ends of the vibrator and the field strength resonantly depends on the length of the vibrator. The shortening of the “half-wavelength” vibrator that provides a maximal induced field is determined. The result of numerical simulation is to an extent intriguing. It is revealed that the induced field of a resonance half-wavelength vibrator rises considerably when the vibrator-screen distance becomes shorter than the quarter-wavelength of the field. The Q factor of an equivalent electromagnetic oscillating circuit characterizing the vibrator also grows, and the induced field more and more concentrates between the screen and the surface of the vibrator’s ends facing the screen. Full-scale experiments qualitatively support theoretical predictions. The results allow researchers to considerably extend the application area of vibrators as initiators of breakdown in high-pressure gases to ignite microwave discharges in quasi-optical beams with a low initial field.     

Multipoint Ignition of a Gas Mixture by a Microwave Subcritical Discharge with an Extended Streamer Structure

The results of experimental studies on using an electrical discharge with an extended streamer structure in a quasioptical microwave beam in the multipoint ignition of a propane–air mixture have been reported. The pulsed microwave discharge was initiated at the interior surface of a quartz tube that was filled with the mentioned flammable mixture and introduced into a microwave beam with a subbreakdown initial field. Gas breakdown was initiated by an electromagnetic vibrator. The dependence of the type of discharge on the microwave field strength was examined, the lower concentration threshold of ignition of the propane–air mixture by the studied discharge was determined, and the dynamics of combustion of the flammable mixture with local and multipoint ignition were compared.     

Initiation of Gas Electrical Breakdown in the Field of a Microwave Traveling over the Metal Surface

Experimental data on electrical breakdown of air in a linearly polarized traveling microwave with a deeply subcritical field are presented. Breakdown is initiated by a linear electromagnetic vibrator or a set of vibrators. They are placed over a metal surface the plane of which contains the wavevector of the microwave and its electrical component. The axes of the vibrators are parallel to this component, and their distances to the metal surface are shorter than the quarter-wavelength of the field. In experiments with the single vibrator, this distance is varied. When the set of vibrators is used, they were placed one after another along the wavevector of the field.     

Detachment of electrons from atmospheric oxygen molecules in a high electric field

The results of experiments on the electrical breakdown of air in a quasi-optical microwave beam with a deeply subcritical initial field have been reported. Breakdown has been initiated by a cylindrical vibrator with spherically rounded ends and a vibrator with a tapered end. The experimental data suggest that the amount of initial breakdown-initiating electrons in a breakdown area can be provided by the detachment of electrons from atmospheric oxygen molecules. The effect of detachment depends on the field strength.     

Effective area of energy interactions between the plasma of a deeply subcritical microwave discharge and its initiating electromagnetic field

Experiments on initiating electrical air discharge in an airtight radiotransparent volume have been described. The discharge is initiated by a quasi-optical linearly polarized microwave beam with a deeply subcritical field by means of an electromagnetic vibrator mounted above a screen. The results make it possible to calculate the effective area of energy interaction between the plasma of the discharge and its initiating microwave field. It has been shown that this area considerably exceeds the cross-sectional area of the discharge.     

Ionization-overheating instability threshold in the electrodeless microwave discharge plasma

Experiments with a pulsed freely localized electrodeless microwave discharge in air under a pressure corresponding to the ascending branch of the pressure dependence of the breakdown field are described. The discharge is initiated in a focused quasi-optical electromagnetic beam. The minimal threshold values of the electromagnetic field (for which a brighter plasma channel extended along the field is formed in the initially spatially homogeneous diffuse discharge plasma) are determined by varying air pressure for several fixed values of this field in the focal region of the beam. In accordance with the prevailing theory, this phenomenon is interpreted as the result of evolution of ionization-overheating instability in the microwave discharge plasma.     

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.     

Electrical discharge excited by the deeply undercritical field of a microwave beam in a high-speed jet of air and air-propane mixture

An electrical gas discharge initiated by a tubular linear electromagnetic vibrator is studied. The discharge is excited by the deeply undercritical linearly polarized field of a quasi-optical microwave beam. Experiments are conducted at a flow velocity of several hundreds of meters per second. The discharge region is photographed, and the stagnation temperature of the flow in the wake of the discharge is measured. It is shown that a deeply undercritical microwave discharge may arise in air even at such high flow velocities. Moreover, it is found that the discharge ignites and stabilizes the discharge region in a “lean” air-propane mixture. In such an experimental design, propane completely burns out when the flow velocity is smaller than some threshold value. When the flow velocity is high and the Mach number of the flow approaches unity, throttling effects and “thermal blocking” of the jet are observed.     

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.     

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。     

Pulsed microwave discharge in atmospheric air in the focus of a two-mirror resonator

Free-localized pulsed microwave discharge in atmospheric air in the focus of an open two-mirror high-Q resonator excited by linearly polarized electromagnetic radiation with a wavelength of 4.3 cm is described. This discharge is analogous to the previously studied streamer resonance microwave discharge ignited under similar conditions but with an electromagnetic radiation wavelength of 8.9 cm. Starting from a certain overcritical electric field, the discharge plasma channel has a high-temperature core.     

Features of the development of pulsed microwave discharges in various gases in a quasioptical beam

Photographs of pulsed microwave discharges initiated by a metallic sphere placed at the focus of a quasioptical electromagnetic beam with linear polarization of the field in air, sulfur hexafluoride, hydrogen, and helium under a pressure of several hundred Torr are presented. The common and distinctive features of the discharges in these gases are noted. Zh. Tekh. Fiz. 68, 33–36 (April 1998)     

高功率微波脉冲大气击穿概率研究

高功率微波大气击穿实验中,入射功率在大气击穿阈值附近,即使外界条件相同,大气击穿可能发生也可能不发生。针对这一问题,基于大气击穿机理,将大气击穿分为首个电子出现在击穿区域和高功率微波电场导致雪崩击穿两个过程。针对第一个过程,建立了改进的电子连续性方程,引入平均电子产生率分析大气击穿发生前电子出现的概率问题;针对第二个过程,建立了高功率微波大气雪崩击穿概率模型。综合两个过程,建立了高功率微波大气击穿概率模型,仿真了不同压强条件下大气击穿的概率,并与相关实验数据进行了比对,仿真结论与实验数据吻合较好。     

Range of existence of self-sustained, subcritical microwave streamer discharges

Data are presented from a study of subcritical microwave streamer discharges in air in an electromagnetic beam. It is shown that, depending on the degree of subcriticality, this kind of discharge can have two forms: a self-sustained discharge and a discharge that is attached to the initiating trigger. The range of subcriticality for the initial field is determined as a function of air pressure within which the self-sustained developed discharge form exists. Zh. Tekh. Fiz. 69, 19–24 (November 1999)     

Microwave Breakdown of Air Initiated by an Electromagnetic Vibrator Placed on a Dielectric Surface

Technical Physics - The feasibility of air breakdown initiation by a quasi-optical subcritical-field microwave beam has been studied experimentally. Breakdown has been initiated by a linear...     

Surface streamer microwave discharge

An atmospheric-pressure pulsed microwave discharge on the surface of an insulating plate and between two insulating fllms is considered. The discharge is initiated in a quasi-optical microwave beam away from the beam-forming elements. The TEM field of the beam is less than the critical breakdown value. The initiator is a metallic dipole placed either directly on the plate surface facing away from the radiation source or between the films. The discharge has the form of branching streamer channels adjacent to the surface and filling up the cross section of the microwave beam with a field level higher than the value separating the subcritical and highly subcritical forms of the microwave streamer discharge.     

Breakdown in air in a rising microwave field

Results are presented from studies of a high-pressure electrodeless breakdown in air at the focus of a standing wave in a high-Q quasi-optical two-mirror resonator pumped by single microwave pulses. In the experiment, the breakdown occurred at the front of the pulse of the resonator field. The breakdown field substantially exceeded the critical level and, under fixed conditions, showed a scatter from pulse to pulse. It is shown that the experimentally found excess in the threshold breakdown field over the critical level is due to the fact that the resonator field increases as a discharge plasmoid forms during breakdown and that the appearance of an electron initiating breakdown in a gas is a random event.