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.     

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.     

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.     

The electromagnetic field in the plasma jet of a microwave plasmatron

A coaxial microwave plasmatron operating at a frequency of 10 GHz is investigated. The microwave field distribution in the plasma jet of the plasmatron is studied using a vibrating string as a small perturbation source. The phase structure of the microwave field inside the plasma is found to differ from that on the outer side of the plasma jet boundary. A slow surface electromagnetic wave propagating along the plasma jet is observed.     

Domains of existence of various types of microwave discharge in quasi-optical electromagnetic beams

High-and medium-pressure discharges excited by quasi-optical microwave beams in air are classified using experimental data. Diffuse, streamer, overcritical, undercritical, and deeply undercritical discharges are distinguished. The domains of existence of each type of discharge are determined as functions of the air pressure and field strength for two particular electromagnetic wavelengths.     

On the 1920 Å continuous spectrum of the supersonic argon jet excited by an electron beam

The T₀/?(T₀) dependences (T₀ is the intensity of the 1920 Å continuum at its maximum, ? is the concentration of argon atoms at the intersection of an electron beam and a jet, T₀ is the temperature of argon at the nozzle) for the 1920 Å continuum radiated by a supersonic argon jet excited by an electron beam were investigated at various pressures of argon at the entry to a nozzle. The experiments performed suggested a possible generation mechanism of the 1920 Å continuum emitted by an argon jet.     

Corona discharge as a temperature probe of atmospheric air microwave plasma jet

We developed and tested a new method for temperature measurements of near-LTE air plasmas at atmospheric pressure. This method is specifically suitable for plasmas at relatively low gas temperature (800–1700 K) with no appropriate radiation for direct spectroscopic temperature measurements. Corona discharge producing cold non-equilibrium plasma is employed as a source of excitation and is placed into the microwave plasma jet. The gas temperature of the microwave plasma jet is determined as the rotational temperature of N₂? produced in the corona discharge. The corona probe temperature measurement was tested by the use of a thermocouple. We found a fairly good agreement between the two methods after correcting the thermocouple measured temperatures for radiative losses. The corona probe method can be generally applied to determine the temperature of the near-LTE plasmas and contrary to the thermocouple it can be used for higher plasma temperatures and is not affected by radiative losses and problems of interaction with the microwave plasma and electromagnetic fields.     

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.     

Microwave streamer discharge in a supersonic air flow

Data are presented from experiments on the ignition of a pulsed, triggered microwave streamer discharge at the focus of a cm-band TEM wave in an immersed supersonic air jet. It is shown experimentally that for velocities of the air in the jet up to 500 m/s, the structure of the discharge remains qualitatively unchanged and retains its streamer character. The finite size of the transverse cross section of the jet determines some features of the discharge. Zh. Tekh. Fiz. 69, 14–18 (November 1999)     

Numerical modeling of a XeCl laser excited by microwave discharge

12 , 169 (1987). Reasonably good agreements in the peak output power and laser efficiency have been achieved. Model calculations also predict that an efficiency as high as 2.7% can be obtained once the conditions of the above-mentioned experiments have been optimized. From the consideration that the skin depth effectively limits the absorption length of the microwave pumping and hence the excitable volume, it is concluded that high input power densities (>2 MW/cm³) and higher gas pressures (between 3 and 10 atm) are the preferable conditions to achieve higher efficiency. Preliminary calculations on CCl₄ containing XeCl gas mixtures show that improvement in laser efficiency by several folds may be achieved as a result of the higher intrinsic efficiency of excimer formation. Received: 23 September 1996 / Revised version: 25 March 1997     

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.     

Longitudinal DC electric discharge in a supersonic air flow

A longitudinal dc electric discharge in a submerged high-pressure supersonic air jet is described. Photographs of the discharge are provided. The experimental voltage across the discharge gap and the discharge current are given for two resistances of the resistor that limits the discharge current over a certain range of the discharge channel length along the air flow. The current-voltage discharge characteristic is provided at a constant discharge length. The main discharge characteristics are obtained from a comparison of the experimental and theoretical results calculated on the basis of the simplest model.     

Regular set of gas discharges on the surface of a dielectric in a quasi-optical microwave beam

Various ways of producing a regular set of plasma objects in the field of a quasi-optical microwave beam using passive vibrators are considered. These objects provide the basis for a periodic vortex structure on the surface of an immersed dielectric body. The properties of a deeply subcritical surface discharge in the flow, as well as its structure and spatial and energy characteristics, are described.     

On the transverse distribution of a resonance field excited by a Gaussian electromagnetic beam on the critical surface of a radially nonuniform plasma sphere

The transverse distribution of the resonance field excited by a Gaussian electromagnetic field on the critical surface of a radially nonuniform plasma sphere is studied. Analytical expressions are obtained for this distribution. It is shown that when a laser beam is focused in front of or behind a spherical target, identical values of the integrated resonance coefficient can correspond to substantially different (in width) distributions of the resonance field over the spherical critical surface. Zh. Tekh. Fiz. 67, 125–128 (October 1997)     

脉冲激励混响室测试区域电磁场均匀性

研究了电磁脉冲激励混响室内电磁波传播规律,通过仿真建模和实验两方面对脉冲激励混响室测试区域内电磁场均匀性进行了研究。提出了混响室时域均匀性的概念及其计算方法,并利用此方法计算了仿真和实验结果。结果表明：脉冲激励混响室内电磁环境无论在时域上还是频域上,均满足国际标准规定的均匀性要求;在一定空间内,混响室内也可以形成统计平均的脉冲场电磁环境;在混响室内进行电磁脉冲场的电磁敏感度测试是可行的。     

Electron acceleration by gaussian electromagnetic beam in a stationary magnetic field

Relativistic electron motion in the electromagnetic Gaussian beam that propagates along a stationary magnetic field is studied. It is shown that, if the cyclotron resonance conditions are initially satisfied, electrons can be efficiently accelerated over a relatively small interval at a slightly lower rate than in a plane accelerating wave.     

Computation of the normal injection of a hydrogen jet into a supersonic air flow

The interaction of a planar supersonic air flow with the hydrogen jet injected perpendicularly across the slot from the duct walls is studied numerically. An algorithm is constructed for solving the Favre-averaged Navier — Stokes equations for the flow of a thermally perfect multicomponent gas based on the ENO scheme. The influence of the jet Mach number and the ratio of the jet and flow pressures on the shockwave structure of the flow and the jet penetration depth is shown.