A. P. Ershov

Microwave and Direct-Current Discharges in High-Speed Flow: Fundamentals and Application to Ignition

DOI: 10.2514/1.24803 The main parameters and properties of an electrodeless freely localized microwave discharge, a surface microwave discharge, transversal in relation to the gas flow direct-current and pulse-periodic electrode discharges, andacombined microwavedirect-current dischargeareexperimentally investigated.It isshown thatalltypesof the discharges result in a reliable ignition of hydrocarbon fuel. Combustion of a propane–air supersonic stream is realized under condition of the combined surface microwave and direct-current discharge. To find out the influence ofdifferent channels ofenergy transfer onignition ofcombustible mixtures in asupersonic flow,the kinetic modelof ignition of hydrocarbons–air mixtures, and taking into account the influence of electric field on processes of dissociation molecules and creation of the active radicals, excited and charged (electrons, positive and negative ions) particlesunderconditionsofnonequilibriumplasmaofthegasdischargewasdeveloped.Mathematicalmodelinghas revealed a strong influence of the reduced electric field on induction period. The microwave discharges may find applications in different fields of supersonic plasma aerodynamics and in development of new-generation plasma sources for plasma chemistry, nano- and microelectronics purposes (plasma treatment of surfaces, etching, and film deposition), and so on.

Mechanisms of microwave surface discharge propagation

A microwave discharge propagating over the surface of a dielectric antenna is studied. It is experimentally shown that the velocity of discharge propagation over the surface is maximal early in microwave pulse application and grows with the applied power. The breakdown wave defines the velocity of the discharge at its early stages (t = 1–3 μs). Ambipolar diffusion governs the discharge propagation at the stage of its evolution (t= 3–100 μs), and, finally, slow surface combustion is possible only at the stationary stage of the discharge (t > 100 μs). The electric field is localized in a thin (∼1 mm) surface layer. High values of the reduced electric field, E/n = 100–500 Td, provide efficient energy deposition to the plasma, i.e., favor the rapid heating of the gas and the efficient generation of charged particles. This makes the discharge promising for hypersonic aerodynamics.

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;

Comparative study of inductively coupled and microwave BF3 plasmas for microelectronic technology applications

A comparison of ICP and microwave plasma sources was carried out under the same discharge conditions, in the same discharge chamber and using the same diagnostics method. Investigations were fulfilled in a wide range of external discharge parameters (at pressures 0.5 - 20 mTorr and for powers deposited in the plasma 400 - 1500 W) in boron trifluoride and in argon discharges. A variety of plasma parameters (Te, ne, n+, EEDF) and their radial profiles at a 2 cm distance above a wafer holder were determined by using single Langmuir probe technique. Analysis of measurements has shown that the charged particles concentrations in ICP plasma are higher than are obtainable in microwave discharge, for deposited power 1.2kW the ICP source produced ion number density ~1012 cm-3. The required plasma uniformity can be maintained in ICP plasma over a more wide range of external discharge parameters then in microwave plasma. The use of microwave plasma source gives a bi-Maxwellian type EEDF, whereas the EEDF of ICP plasma is close to the single Maxwellian distribution with electron temperature higher then the temperature of cold electrons in microwave discharge. BF3 plasma is electronegative, with a degree of electronegativity ~0.3-0.5 for both plasma sources.

Probe Diagnostics of Gas Discharges in Supersonic Airflows

A new case of probe diagnostics of plasma density under the condition typical for plasma aerodynamic applications (high voltage and electric e eld, high nonequilibrium between components, high gas pressures, and a considerable compression effect ) has been examined. The experiments have been carried out in dc and pulsed periodic discharges in supersonic aire ows with Mach number M = 2 at the static ambient pressure range 10 ‐200 torr. A personal-computer controlled double probe with an optogalvanic isolation based on an optocoupler and a e ber optical waveguide have been developed and tested in the gas discharges. A new theory of probe diagnostics of plasmas with a very high electric e eld in a supersonic aire ow is developed based on a computer simulation of the interaction of the probe and the plasma e ow. A combined effect of the e ow compression and the electric e eld perturbation on the plasma density near the probe is revealed. This effect can limit the area of applicability of the probe diagnostics. Measures to prevent this negative effect are suggested. The results of probe diagnostics of the plasma density are in good agreement with those obtained using the microwave interferometer and the spectral method.

The probe diagnostics of free-burning arc in the atmosphere

A procedure of ac probe measurements is suggested for spatially unstable and unsteady-state atmospheric-pressure plasma. The procedure is evaluation tested by way of measuring the distribution of concentrations of charged particles in the interelectrode space of extended arc with graphite electrodes, which burns in free atmosphere of air. This arc is distinguished by an atypical geometry of the discharge channel, significant instability of its position in space, and appreciable current and voltage fluctuations.

Microwave Discharges: Fundamentals and Applications

[Abstract] The main parameters and properties of an electrodeless freely localized microwave discharge and a surface microwave discharge are experimentally investigated . It is shown, that plasma of a surface microwave discharge at low air pressure is especially non -equilibrium. Electron energy distribution function essentially differs from equilibrium function and strongly enriche s by fast electrons. It is shown, that the microwave discharges result in a reliable ignition of hydrocarbon fuel. The micro wave discharges may find applications in different fields of a supersonic plasma aerodynamics and in development of new -generation plasma sources for plasma chemistry, nano - and micro electronics purposes (plasma treat ment of surfaces, etching, and film de position), and so on.

Microwave discharge on external surface of quartz plate

A new way of creation of the stable and constantly reproduced at various experiments surface microwave discharge near a flat dielectric antenna was developed and experimentally investigated at a wide range of air pressure p=10-3 ÷103 torr. The breakdown characteristics determining a threshold of a surface microwave discharge appearance were studied. Longitudinal size and velocity of spreading of a surface microwave discharge in air were determined at a wide range of gas pressure, microwave power and pulse duration. The spatial-temporary evolution of electron density, gas and vibrational temperatures were investigated. It is shown that the surface microwave discharge can be used as a plasma source for microelectronics and material processing.

DISCHARGE AND FLAME PLASMAS PROBE DIAGNOSTICS IN SUPERSONIC AIR-PROPANE FLOWS

An application of electrical probes for measurements of electric discharge plasma density in supersonic air and air-propane mixture flows and corresponding flame plasmas have been investigated. The electric discharges igniting supersonic air-propane flow have been DC and pulsed ones, and pulsed plasma jets injected into a supersonic flow. Both designs of probe measurement circuits and methods of probe characteristics processing in case of single and double probe have been developed.