Pulsed Microwave Discharge Research Articles

Pulsed Microwave Discharge Ignition of Propane-Air Mixture with Different Excess Air Ratios

Pulsed Microwave Discharge Ignition of Propane-Air Mixture with Different Excess Air Ratios

Spatio-temporal analysis of power deposition and vibrational excitation in pulsed N2 microwave discharges from 1D fluid modelling and experiments

Vibrational excitation of N2 beyond thermodynamic equilibrium enhances the reactivity of this molecule and the production of radicals. Experimentally measured temporal and spatial profiles of gas and vibrational temperature show that strong vibrational non-equilibrium is found in a pulsed microwave discharges at moderate pressure (25 mbar) in pure N2 outside the plasma core and as an effect of power pulsing. A one dimensional radial time-resolved self-consistent fluid model has been developed to study the mechanism of formation of vibrationally excited N2. In addition to the temperature maps, time-resolved measurements of spontaneous optical emission, electron density and electron temperature are used to validate the model and the choice of input power density. The model reveals two regions in the plasma: a core where chemistry is dominated by power deposition and where vibrational excitation starts within the first ∼10 µs and an outer region reliant on radial transport, where vibrational excitation is activated slowly during the whole length of the pulse (200 µs). The two regions are separated by a sharp gradient in the estimated deposited power density, which is revealed to be wider than the emission intensity profile used to estimate the plasma size. The low concentration of excited species outside the core prevents the gas from heating and the reduced quenching rates prevent the destruction of vibrationally excited N2, thereby maintaining the observed high non-equilibrium.

Numerical model of a pulsed subcritical streamer microwave discharge for problems of plasma ignition of fuel mixtures in the gas phase

An approximate model for estimating plasma heating and conversion caused by a subcritical streamer microwave discharge has been considered and verified. Ignition occurs in an environment with a pressure of 13 kPa, a temperature of 150 K, there is an external air flow at a speed of up to 500 m/s, an initiator antenna and a flat mirror are used to focus electromagnetic radiation, a stoichiometric mixture of propane-air and pure propane is supplied through the cavity in the antenna. Radiation power is 3 kW. The model is three-stage and semi-empirical. The plasma region and its conductivity are set based on experimental statistics; this is a key feature that reduces the consumption of computing resources. The finite element method is used. At the first stage, the Boltzmann equation for the electron gas in the medium is solved in the zero-dimensional formulation for the given parameters of the external electric field. The distribution functions of the electronic energy are obtained as well as the functions of the reaction coefficients. At the second stage, the Helmholtz equations are solved to obtain the distribution of electromagnetic fields near the antenna-initiator, taking into account the given conducting region “plasma”. Based on the obtained distributions of the electric field, the Joule heating powers and the values of the reaction coefficients are calculated. At the third stage, the Navier-Stokes and transfer equations of various types of particles for a compressible medium are solved; taking into account combustion processes for given local heating and plasma reactions. The results are compared with the data of a physical experiment. The distributions of temperature, composition of the medium, velocity of the medium are considered for given local heating power and additional reactions in the plasma region. A stoichiometric mixture of propane with air and pure propane supplied through the antenna are ignited by plasma under the conditions under consideration: the mixture burns in a small area, propane is oxidized in a thin layer of mixing with air. The temperature fields and composition of the medium are compared with photographs of the flame from the experiment. The numerical study shows that under all the conditions considered the model gives results close to the experiment, but there is an overestimation of the power required for ignition by up to two times. The study of the processes of ignition of gaseous mixtures by a subcritical microwave discharge is of interest for the design of propulsion systems with increased reliability, with the possibility of using hardly flammable mixtures. The proposed model gives approximate estimates, while the requirements for resources and time are significantly reduced compared to classical models.

Investigation of N(4S) kinetics during the transients of a strongly emissive pulsed ECR plasma using ns-TALIF

A nanosecond-two-photon absorption laser-induced fluorescence (ns-TALIF) technique was employed to investigate the transients of a strongly emissive, pulsed microwave discharge. We analysed the conditions that have to be fulfilled in order to use the TALIF intensities measured with the laser central frequency tuned to the absorption peak instead of the fully integrated laser excitation spectrum absorption for measuring N-atom densities. We demonstrated the validity of the method under our conditions and applied it to the straightforward monitoring of N-atom densities during the transients of a pulsed ECR plasma. In particular, we demonstrated the existence of an unexpected increase in N(4S) atom density of about 10% at the early stage of the transition from high- to low-power phase. Using a self-consistent, quasi-homogenous plasma model encompassing a detailed state-to-state kinetics, a thorough analysis of the N-atom and N2(B) kinetics was carried out and enabled this enhancement to be attributed to surface de-excitation of N(2D) and N(2P) atoms. The rise of the N(4S) atom density can serve as an indicator of the concentration of N(2D) and N(2P) atoms in the discharge.

Nitrogen fixation in pulsed microwave discharge studied by infrared absorption combined with modelling

A pulsed microwave surfaguide discharge operating at 2.45 GHz was used for the conversion of molecular nitrogen into valuable compounds in several gas mixtures: N2:O2, N2:O2:CO2 and N2:CO2. The ro-vibrational absorption bands of the molecular species were monitored by a Fourier transform infrared apparatus in the post-discharge region in order to evaluate the relative number density of species, specifically NO production. The effects of specific energy input, pulse frequency, gas flow fraction, gas admixture and gas flow rate were studied for better understanding and optimization of the NO production yield and the corresponding energy cost (EC). By both the experiment and modelling, a highest NO yield is obtained at N2:O2 (1:1) gas ratio in N2:O2 mixture. The NO yield reveals a small growth followed by saturation when pulse repetition frequency increases. The energy efficiency start decreasing after the energy input reaches about 5 eV/molec, whereas the NO yield rises steadily at the same time. The lowest EC of about 8 MJ mol−1 corresponding to the yield and the energy efficiency of about 7% and 1% are found, respectively, in an optimum discharge condition in our case.

Insights into CO2 conversion in pulsed microwave plasma using optical spectroscopy

Low pressure surfaguide microwave discharge working in pulsed regime and used for CO2 conversion is characterized by optical spectroscopy methods aiming at better understanding of the conversion process. The rotational, vibrational, and electron excitation in plasma were studied. The translational (Tgas) and vibrational (Tvibr) temperatures are found elevated in the discharge active zone. At low pressure (<1 Torr) discharge demonstrates non-equilibrium with Tgas∼ 700 K (0.06 eV) and electron temperature (Te) of nearly 2 eV. Non-equilibrium state gradually disappears as the gas pressure increases, resulting in a growth of Tgas and decrease of Te. Tvibr of nitrogen also slowly decreases towards the equilibrium condition. The CO2 conversion and energy efficiencies have been examined as a function of the discharge pulse parameters, such as the plasma pulse duration, repetition rate and duty ratio, clarifying additional possibilities for optimization of conversion in the pulsed microwave discharges.

Kinetic simulation of the transition from a pulse-modulation microwave discharge to a continuous plasma

The generation of a very strong peak current in the first period (PCFP) in a pulse-modulated microwave discharge has been discussed in previous studies. In this paper we focus on the transition process from a pulsed discharge to a fully continuous one driven by the pulsed microwave power source by means of a kinetic model. The computational results show that by increasing the duty cycle or voltage modulation rate (VMR), the discharge eventually becomes fully continuous and PCFP can no longer be observed. In the transition process, the distributions of the electric field, electron energy probability function (EEPF) and plasma density are discussed according to the simulation data, showing different discharge structures. The simulations indicate that many high-energy electrons with electron energy larger than 20 eV and low-energy electrons with electron energy less than 3 eV could be generated in a pulsed microwave discharge, together with a reversal electric field formed in the anode sheath when PCFP occurs. However, only medium-energy electrons could be observed in a fully continuous discharge. Therefore, by investigating the transition process the pulse-modulated microwave discharges can be further optimized for plasma applications at atmospheric pressure.

Combustion Dynamics of a Propane–Air Gas Mixture when It Is Ignited by a Streamer Microwave Discharge

The results of experimental studies of combustion of propane–air gaseous mixture when it was ignited by a microwave discharge have been described. The mixture with different propane content fills a sealed radio-transparent tube placed along the axis of a focused linearly polarized quasi-optical microwave beam at atmospheric pressure. Multi-point ignition of the mixture is carried out near one end of the tube by a pulsed microwave discharge with a surface-developed streamer structure. The growth of gas pressure in the tube as propane burned was recorded in the experiments. The microwave pulse energy being invested in high-temperature discharge plasma has been estimated in them. The minimum percentage of propane in the mixture at which the microwave discharge ignites it has been determined in experiments. The time dependence of the pressure increase in the tube as the propane burns determines the combustion process dynamics.

Dynamics of Focused Pulsed Microwave Discharge in Air

The formation dynamics of the parameters of a microwave discharge in air localized at electric field maxima created by a specially developed focusing system is investigated using numerical simulations within an extended hydrodynamic model. All main parameters of the microwave discharge plasma are determined. The results on neutral gas heating demonstrate good agreement with the earlier experimental results.

Experimental study of pulsed microwave discharges at pressures ranging over five orders of magnitude

Microwave discharge igniter (MDI) is a discharge system developed to initiate combustion in automotive engines. The MDI uses a sequence of N = 700 microwave (2.45 GHz) pulses 100 ns in duration separated by 1 μs. The initial breakdown is provided by the first microwave pulse, 5 μs in duration. The aim of pulsing the microwave signal is to keep an optimal combination of parameters when, even at elevated pressures, (i) the discharge propagates over the largest possible volume; (ii) the plasma is non-equilibrium. Properties of plasma produced by MDI igniter in non-combustible gas mixtures at ambient gas temperature and gas pressure in the range between 0.2 mbar and 8 bar were studied experimentally. Discharge spatial structure was analyzed with the help of time-resolved ICCD imaging. Near-UV optical emission spectra taken in different pulses provided the information about rotational and vibrational temperatures. The electric field was estimated on the basis of ratio of emission of the second positive and the first negative systems of molecular nitrogen.

Динамика горения пропан-воздушной газовой смеси при ее поджиге стримерным сверхвысокочастотным разрядом

AbstractThe results of experimental studies of combustion of propane–air gaseous mixture when it was ignited by a microwave discharge have been described. The mixture with different propane content fills a sealed radio-transparent tube placed along the axis of a focused linearly polarized quasi-optical microwave beam at atmospheric pressure. Multi-point ignition of the mixture is carried out near one end of the tube by a pulsed microwave discharge with a surface-developed streamer structure. The growth of gas pressure in the tube as propane burned was recorded in the experiments. The microwave pulse energy being invested in high-temperature discharge plasma has been estimated in them. The minimum percentage of propane in the mixture at which the microwave discharge ignites it has been determined in experiments. The time dependence of the pressure increase in the tube as the propane burns determines the combustion process dynamics.

Atmospheric pressure water-vapor plasma in an air-shielded environment by water flow

Atmospheric pressure plasma sources in air-shielded environment by water flow were developed for a plasma process utilizing water-derived reactive species without air contamination in the atmosphere. An atmospheric pressure plasma jet with He gas was enclosed by a cylindrical water flow preventing air contamination to the plasma without using a gas flow shield, vessel or pump. Suppression of air contamination into the plasma was confirmed by optical emission spectroscopy. Furthermore, a microwave-excited water-vapor plasma source in a space shielded by a cone-shaped water flow was developed using a coaxial waveguide with a ring-shaped slot to use a high-density hydroxyl radical without air contamination at atmospheric pressure. A water-vapor plasma without air was successfully realized by pulsed-microwave discharges. Production of a hydroxyl radical was confirmed from the optical emission from the plasma. As an application example of the water-vapor plasma in the air-shield environment, atmospheric pressure photoresist removal treatment was demonstrated and the removal rate as high as 2.2 μm min−1 was obtained.

Surface Functionalization of Polyethylene Granules by Treatment with Low-Pressure Air Plasma.

Polyethylene granules of diameter 2 mm were treated with a low-pressure weakly ionized air plasma created in a metallic chamber by a pulsed microwave discharge of pulse duration 180 μs and duty cycle 70%. Optical emission spectroscopy showed rich bands of neutral nitrogen molecules and weak O-atom transitions, but the emission from N atoms was below the detection limit. The density of O atoms in the plasma above the samples was measured with a cobalt catalytic probe and exhibited a broad peak at the pressure of 80 Pa, where it was about 2.3 × 1021 m−3. The samples were characterized by X-ray photoelectron spectroscopy. Survey spectra showed oxygen on the surface, while the nitrogen concentration remained below the detection limit for all conditions. The high-resolution C1s peaks revealed formation of various functional groups rather independently from treatment parameters. The results were explained by extensive dissociation of oxygen molecules in the gaseous plasma and negligible flux of N atoms on the polymer surface.

Confluence or independence of microwave plasma bullets in atmospheric argon plasma jet plumes

Plasma bullet is the formation and propagation of a guided ionization wave (streamer), normally generated in atmospheric pressure plasma jet (APPJ). In most cases, only an ionization front produces in a dielectric tube. The present study shows that two or three ionization fronts can be generated in a single quartz tube by using a microwave coaxial resonator. The argon APPJ plumes with a maximum length of 170 mm can be driven by continuous microwaves or microwave pulses. When the input power is higher than 90 W, two or three ionization fronts propagate independently at first; thereafter, they confluence to form a central plasma jet plume. On the other hand, the plasma bullets move independently as the lower input power is applied. For pulsed microwave discharges, the discharge images captured by a fast camera show the ionization process in detail. Another interesting finding is that the strongest lightening plasma jet plumes always appear at the shrinking phase. Both the discharge images and electromagnetic simulations suggest that the confluence or independent propagation of plasma bullets is resonantly excited by the local enhanced electric fields, in terms of wave modes of traveling surface plasmon polaritons.

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.

Synthesis of oxide and nitride ceramics in high-power gyrotron discharge

Synthesis of oxides, nitrides, and oxynitrides of silicon and aluminium by a pulsed microwave discharge in the mixtures of metal and dielectric powders is described. The microwave pulses were generated by high-power gyrotron (frequency 75 GHz, power up to 550 kW, pulse duration from 0.1 to 15ms). SiO2 + β-Si3N4 (1:1 by molar) and α-Al2O3 + AlN (2:1 by molar) mixtures with Mg (1 and 5wt%) were treated in air with microwave pulses with power of 250÷400 kW and duration of 2÷8 ms. It was found that the discharge cannot be initiated for both mixtures in absence of Mg at any pulse power and duration. When 1% of Mg was added, the discharge was observed for both mixtures under 8 ms pulses of 400 kW; however, the amounts of materials produced were not enough for analysis. With 5% of Mg the discharge was observed for both mixtures under 8 ms pulses of 350 kW, and products of the plasma-chemical processes in the Al2O3 + AlN mixture were analyzed.

Plasma-catalytic conversion of CO2 and CO2/H2O in a surface-wave sustained microwave discharge

The conversion of CO2 and CO2/H2O mixtures on a TiO2 supported NiO catalyst in a pulsed surface-wave sustained microwave discharge has been investigated. The influence of the catalyst preparation method (conventional calcination (in air or Ar) vs. Ar plasma-assisted decomposition) on the CO2 conversion and its energy efficiency has been studied. The results demonstrate that the Ar plasma-treated catalyst is more active compared to the conventional calcined one. The plasma-treated catalyst increases the CO2 conversion and its energy efficiency almost by a factor of two, compared to the plasma only assisted CO2 dissociation, while the conventional calcined catalysts affect the CO2 conversion rather insignificantly. The conversion of CO2 is found to be about 45% at 70Torr in pure CO2 with Ar plasma-treated catalyst, having an energy efficiency of 56%. In the case of CO2/H2O mixture, the CO2 conversion efficiency reaches 42% (energy efficiency is 52%) at 60Torr. The catalyst characterization shows that Ar plasma treatment may result in a higher density of oxygen vacancies and a comparatively uniform distribution of NiO on the TiO2 surface. The dissociative electron attachment of CO2 at the catalyst surface enhanced by the oxygen vacancies and plasma electrons may explain the increase of conversion and energy efficiencies in this case. A mechanism of plasma–catalytic conversion of CO2 at the catalyst surface in CO2/H2O mixture is proposed.

Kinetic study on non-thermal volumetric plasma decay in the early afterglow of air discharge generated by a short pulse microwave or laser

This paper reports a kinetic study on non-thermal plasma decay in the early afterglow of air discharge generated by short pulse microwave or laser. A global self-consistent model is based on the particle balance of complex plasma chemistry, electron energy equation, and gas thermal balance equation. Electron-ion Coulomb collision is included in the steady state Boltzmann equation solver to accurately describe the electron mobility and other transport coefficients. The model is used to simulate the afterglow of microsecond to nanosecond pulse microwave discharge in N2, O2, and air, as well as femtosecond laser filament discharge in dry and humid air. The simulated results for electron density decay are in quantitative agreement with the available measured ones. The evolution of plasma decay under an external electric field is also investigated, and the effect of gas heating is considered. The underlying mechanism of plasma density decay is unveiled through the above kinetic modeling.

Experimental study of plasma decay in pulsed microwave discharges of H2, CH4 and their mixtures

The results of the experimental investigations of the plasma decay in X-band pulsed microwave discharges produced by surface waves in hydrogen and hydrogen–methane mixtures under pressures of 0.5–5 Torr are presented. The electron density in the decaying plasma was measured by the resonator method using a high-Q quasioptical resonator. These investigations demonstrated a significant influence of ion conversion processes on the composition of ions in a hydrogen–methane plasma. Such conversion processes result in a change in ionization-recombination balance, which, in turn, influences the parameters of the discharge plasma. Ion recharging processes quickly convert the ‘primary’ ions in hydrogen into ions. It was found that the addition of even a few percent of CH4 to hydrogen, after a sequence of ion-molecular reactions, leads to the formation of complex ions Cx. Recombination coefficients and the dominant type of ions in hydrogen and hydrogen–methane plasma are reported.

Plasma assisted catalytic decomposition of CO2

In this research, conversion of carbon dioxide in the presence of TiO2 supported NiO catalyst has been investigated in a pulsed surface-wave sustained microwave discharge. The catalyst is prepared by combination of impregnation and plasma-treatment methods. The decomposition of TiO2 supported nickel nitrate was done by plasma, induced in three different gases (O2, Ar, CO2). NiO/TiO2 catalysts were formed. The supplied gas has a prominent effect on the chemical and physical properties of the prepared catalysts. The catalyst prepared with an Ar plasma increases almost by a factor of 2 the CO2 conversion and energy efficiencies, while the O2 or CO2 plasma prepared catalysts show little effect on the CO2 conversion in comparison with catalyst-free plasma assisted dissociation. The results from the catalyst characterization by different techniques demonstrated that Ar plasma pre-treatment resulted in formation of oxygen vacancies. Dissociative electron attachment of CO2 at the catalyst surface enhanced by oxygen vacancies and plasma electrons could explain the improved conversion and energy efficiency. A constant catalytic activity was observed indicating continuous catalyst regeneration under reactor conditions. A mechanism of plasma-catalysis synergy is proposed.