A. F. Aleksandrov

Freely localized microwave discharge in a supersonic gas flow

A discharge produced by a focused microwave beam in a supersonic gas flow has been investigated experimentally. It is shown that the degree of ionization and the gas temperature in the discharge are fairly high and that the main properties of the discharge plasma are only slightly affected by the supersonic air flow. Discharges produced by focused microwave beams can find application in supersonic plasma aerodynamics.

Ignition of hydrocarbon films under the conditions of surface microwave discharge

In the present work, rapid plasma-stimulated ignition of liquid hydrocarbons was carried out in still air under conditions of surface microwave discharge. It was shown that, depending on the microwave input power, the breakdown time changed in a range from 5 to 30μs, ignition occurred on an antenna in the area of the surface microwave discharge at a temperature not exceeding 1000 K, and the speed of the front boundary propagation of the intense ignition region near the antenna was 300 m s−1.

Self-consistent model of an inductive rf plasma source in an external magnetic field

A theory is developed that makes it possible to calculate RF power absorption in an inductive plasma source. Conditions are determined under which most of the power is deposited in the plasma. It is shown that these conditions correspond to the excitation of spatial waves (an oblique Langmuir wave and a helicon wave). A simple self-consistent model of a plasma source is proposed that describes all of the experimentally observed distinctive properties of plasma sources well.

Gasdynamic perturbations under conditions of surface microwave discharge in air

An investigation is performed of gasdynamic perturbations arising in the neighborhood of a dielectric antenna, on the external surface of which a microwave discharge is initiated at high pressures of air, where the electron-molecule collision frequency is much higher than the circular frequency of electromagnetic field. The time dependence of gas temperature is obtained for different values of power input, and it is demonstrated that, in the stage of formation of discharge at atmospheric pressure of air, the gas is heated at a rate of 30–70 K/μs. The electron concentration in plasma channels at atmospheric pressure does not exceed 1015–1016 cm−3. A study is made of the evolution of shock waves arising under conditions of surface micro-wave discharge at different pressures of surrounding gas, different durations of stimulation, and different power inputs to the discharge. It is demonstrated that the shock wave velocity in the vicinity of the antenna reaches a value of 1 km/s.

Properties of a low-pressure inductive RF discharge I: Experiment

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.

Surface microwave discharge in high-speed air-hydrocarbon flows

The influence of nonequilibrium plasma of the surface microwave discharge on the ignition of supersonic (M = 2) propane-air flow, as well as alcohol, benzene, and kerosene for sub- and supersonic air flow is studied.

Electromagnetic waves in cylindrical plasma-dielectric structures

AbstractsThe theory of electromagnetic waves in cylindrical structures (waveguides) with plasma and plasma-dielectric fillings is presented in a systematic form. The case of isotropic cold electron plasma with sharp transverse plasma/vacuum, plasma/dielectric, plasma/plasma, and plasma/metal interfaces is considered. Particular attention is given to surface plasma waves and their interaction with bulk electromagnetic modes. The coupling of E and B waveguide modes that is due to the transverse inhomogeneity is investigated. The important question of the applicability of the potential (electrostatic) approximation in the theory of plasma waves and the role of nonpotential effects are discussed.

Air ionization in a near-critical electric field

A set of experimental dependences of the air ionization effective rate on the electric field strength is presented. The concept of the critical breakdown field is discussed. It is indicated that experimental data are quantitatively inconsistent with analytical results based on this concept. This inconsistency is eliminated if the ionization balance takes into account not only dissociative adhesion of electrons to oxygen molecules but also their detachment from the molecules that gained a charge during the charge exchange process. Based on the results obtained, a new physical interpretation of the critical field is suggested. A formula for the effective rate of air ionization in near-critical fields is derived.

Combined M W-DC Discharge in a High Speed Propane -Butane -Air Stream

The mechanism of the gas -phase oxidation of various combustible gases, including hydrocarbons and hydrogen, has been thoroughly studied, with the emphasis on their ignition mechanism. The great majority of publications in this field have dealt with factors determining the induction per iod preceding the ignition event. In recent decades, there has also been much literature discussing the possibility of effectively controlling combustion processes by various physical means. Use of the gas discharge is one of such ways , promoting an intens ification of chain combustion of hydrocarbons. However, the ignition kinetics is not completely understood even for the rather simple model system hydrogen -oxygen under low -temperature gas -discharge plasma conditions, which are established at large values of the reduced electric field. Therefore, for a deeper insight in the physicochemical processes occurring in the low -temperature plasma initiation of the ignition of a combustible gas, the experimental study of the effect of a gas discharge on the ignition event should be fulfilled and accompanied by mathematical modeling. The study of the ignition and combustion of hydrogen -containing mixtures under low -temperature plasma conditions is of importance from various standpoints: it is necessary to carry out b oth fundamental research in the mechanism and kinetics of atom -molecule reactions in a strong electric field and an analysis of a variety of applied problems, including the optimization of plasma chemical processes. One important problem is to develop the physical principles of the burning of high speed flow of combustible gases. Under such conditions it is necessary to ensure a rapid space ignition of the high -velocity hydrocarbon flow. To do this, it is necessary to minimize the induction period. In our l aboratory, we initiate d ignition with dc discharges (either longitudinal or transverse to the flow), periodic pulsed discharges, and freely localized and surface microwave discharges. Initially, the effect of low temperature plasma on the combustion kineti cs of a gaseous fuel was experimentally studied for a propane -butane air flow with a Mach number of M=2. Our experimental setup consists of a cylindrical vacuum chamber with an inner diameter of 1 m and a length of 3 m, a high -pressure air receiver, a high -pressure propane -butane receiver, a system for mixing the propane -butane mixture with air, a system for producing a high speed propane -butane -air flow, an aerodynamic channel, a discharge section, different plasma generators, a pulsed high -voltage power s upply, a synchronization system, and a diagnostic system. The air flow rate can be varied between 25 and 100 g/s; the propane -butane flow rate, between 1 and 8 g/s. The basic part of this setup is the vacuum chamber, which serves to produce a high speed fl ow and is a reservoir for the exhaust gases and combustion products. The vacuum systems allows operation in a wide pressure range of p=10 -3 -10 3 torr . We used some types of a gas discharge for ignition: freely localized microwave discharge , surface microwav e discharge, direct current discharge, and pulsed transverse electrode discharge. The ignition of the high speed stream was detected as a glow in the aerodynamic channel downstream of the discharge section. No glow was observed when a gas discharge was gen erated in an air flow, when it was generated in a high speed propane -butane -air flow but its parameters (pulse duration, discharge current, electric field strength in the plasma, and the electric power deposited in the discharge) were inappropriate for ign ition, or when the mixture was far from stoichiometric. Induction time was simultaneously derived from different measurements: (1) the minimum pulse duration resulting in a glowing flame in the aerodynamic channel downstream of the discharge section; (2) the time taken by the intensity of the molecular band of the excited CH * radical (the (0;0) band due to the A 2 ��X 2 � transition), with an edge wavelength of �=431.5 nm, to achieve the maximum growth rate;

The Effective Ionization of Air and Oxygen in a Near-Critical Electric Field at High Pressures

The effective ionization of air and oxygen at early stages of electric discharge development at medium and high pressures is analyzed. Ionization in an applied field with the participation of background electrons, electron sticking to and detachment from oxygen atoms and molecules, and recharging and conversion of negative ions is considered. The dependence of ion-molecular processes on external field is taken into account. The effective ionization value is shown to be different in constant and microwave fields. The effective air ionization values obtained in a microwave field are in agreement with the experimental data. It is shown that background electrons determine the possibility of effective ionization, whereas conversion processes determine the existence of a threshold E/N value, where E is the electric field strength and N is the density of neutral particles.