Electromagnetic vibrator: An initiator of air breakdown in the subcritical field of a quasi-optical microwave beam

The subject of investigation is the resonance properties of cylindrical electromagnetic vibrators with different diameters and spherically rounded ends acting as initiators of air breakdown in the subcritical field of a quasi-optical microwave beam. The vibrator is placed both in the field of a traveling electromagnetic wave and at the antinode of the field above a screen inserted in the microwave beam. The shortening of the half-wave vibrator that governs its ability to initiate air breakdown, the field at the top-pole of this vibrator, and a tolerance on its resonance length are found from experimental data.     

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.     

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.     

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.     

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)     

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.     

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.     

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.     

Generation of dual pulses of the runaway electron beam current during the subnanosecond breakdown of atomic and molecular gases

With a diaphragm placed behind the anode foil, dual runaway electron beams have been provided in helium, hydrogen, nitrogen, and air under a pressure of several torrs to several dozen torrs and a high-voltage pulse amplitude of about 250 kV. These beams consist of two pulses with commensurable amplitudes with a time interval between them of several dozen picoseconds to several hundred picoseconds. It has been shown that the breakdown of the interelectrode gap at pressures from several torrs to several dozen torrs may occur in different regimes and dual pulses of the electron beam current are registered when the initial current through the gap is below 1 kA. It has been found that a supershort avalanche electron beam that consists of one pulse is generated when the delay of breakdown equals several hundred picoseconds. It has been shown that, when the gas pressure reaches several hundred Torr, including atmospheric pressure, the runaway electrons are detected behind the foil after the termination of the supershort avalanche electron beam pulse.     

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.     

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.     

Generation of runaway electrons in a nonuniform electric field by applying nanosecond voltage pulses with a frequency of 100–1000 Hz

Generation of runaway electrons and X-ray radiation in helium and air under the action of a pulsed-periodic discharge in a nonuniform electric field is studied. Positive and negative voltage pulses with a repetition rate of up to 1 kHz, a duration on the order of 1 ns, and an incident wave amplitude of 12.5 kV are applied to a needle-plane gap. For both polarities of the main voltage pulse and a helium pressure from several Torr to several tens of Torr, the arrival of negative reflected voltage pulses at the gap is shown to be accompanied by an electron beam generation. X-ray radiation is detected in a wide range of pressure, including under normal pressure of helium and air.     

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...     

宽气压范围空气中微波击穿电场的计算公式

为了简便快捷地计算微波击穿电场,依据电子扩散模型的基本理论,结合气体放电的基本参量,应用特征扩散长度的概念,给出了适合于规则结构微波部件的击穿电场的计算方法。为避免各种气体参数的不确定性对计算准确度的影响,对等效直流电场与特征扩散长度之间的实验关系进行了拟合,并根据等效直流电场的定义,得出了一个适用于较高气压范围的击穿电场计算表达式。为了将该计算表达式扩展到更低的气压范围,综合考虑了电子扩散模型和基于二次电子发射现象的真空微放电机理,引入了一个合理形式的等效扩散长度,进一步给出了适合于更广气压范围的微波击穿电场的计算表达式,计算结果更符合A.D.Macdonald的实验结果。     

高功率微波输出窗内侧击穿动力学的PIC/MCC模拟研究

高功率微波在受控热核聚变加热、微波高梯度加速器、高功率雷达、定向能武器、超级干扰机及冲击雷达等方面有着重要的应用.本文针对高功率微波输出窗内侧氩气放电击穿过程,建立了二次电子倍增和气体电离的一维空间分布、三维速度分布（1D3V）模型,并开发了相应的PIC/MC程序代码.研究了气压、微波频率、微波振幅对放电击穿的影响.结果表明：在真空情况下,介质窗放电击穿只存在二次电子倍增过程;在低气压和稍高气压时,二次电子倍增和气体电离共存;在极高气压时,气体电离占主导.给出了不同气压下电子、离子的密度和静电场的空间分布.此外还观察到,在500 mTorr时,随着微波振幅或微波频率的变化,气体电离出现的时刻和电离产生的等离子体峰值位置有较大差异,尤其是当微波频率（GHz）在数值上是微波振幅（MV/m）的2倍时,气体电离出现的较早.     

波导内场增强法测量微波气体击穿

 提出并研究了波导内场增强法测量微波气体击穿的实验方案,在密闭波导内加金属针实现电场增强,通过调节微波源辐射功率及两针距离,使微波传输达到气体击穿的条件。介绍了实验原理及诊断方法,进行了频率2.86 GHz、脉宽180 ns的微波在0.1 MPa空气中的击穿实验,分析了实验中得到的典型波形。结果表明实验现象与设计吻合,验证了该方案研究微波气体击穿的可行性。     

Electron band structure detection by high-order optical harmonic generation in solids

As the threshold field strength for the breakdown in air significantly exceeds the maximum measured thundercloud strength 3 kV/cm/atm, the problem of lightning initiation remains unclear. According to the popular idea, lightning can be initiated from streamer discharges developed in the enhanced electric field in a vicinity of hydrometeors. To test the idea, we carry out numerical simulations of positive streamer development around charged water drops at air pressure typical at thundercloud altitudes and at different background fields, drop sizes and charges. With real drop sizes and charges, the electric field required for the streamer formation is stronger than the measured fields; therefore, second mechanism is required to amplify the local field.