Chemical kinetics in an atmospheric pressure helium plasma containing humidity

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H 2 O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 10 14 cm −3 for OH and 3 × 10 13 cm −3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH + by proton transfer to H 2 O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H 2 O + by proton transfer to H 2 O and by electron impact dissociation of H 2 O. It is lost by reactions with other OH molecules to form either H 2 O + O or H 2 O 2 . Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications

Formation d’un plasma complétement ionisé et combustion assistée par décharges nanosecondes

Cette thèse porte sur l’étude de la formation et de la recombinaison d’un plasma complétement ionisé généré par une impulsion électrique de quelques nanosecondes. La formation du plasma se décompose en cinq étapes : (i) ionisation partielle de l’espace inter-électrode, (ii) formation d’un filament à la cathode, (iii) formation d’un filament à l’anode, (iv) propagation des filaments et (v) fusion des filaments. Ces derniers sont caractérisés par une émission intense de N+ et d’un continuum des électrons. Nos mesures montrent que la densité d’électrons augmente de 1017 à 1019 cm-3 en moins de 0.5 ns. Cette augmentation abrupte est due à l’ionisation des états excités de N et O, comme le montre un mécanisme cinétique développé dans cette thèse. Le plasma complétement ionisé est à l’équilibre chimique et thermique. Après que l’ionisation complète est atteinte, la densité et la température des électrons décroissent en raison d’une expansion isentropique de la colonne de plasma. Les décharges Nanosecondes Répétitives Pulsées (NRP) sont ensuite utilisées en régime hors-équilibre dans deux bruleurs afin de réaliser des études fondamentales et appliquées en combustion assistée par plasma. Dans le bruleur Mini-PAC, près des électrodes, la température du plasma augmente de 1500 K en raison d’une “lente” formation d’un filament. Les décharges NRP accélèrent la réactivité de la flamme comme le montrent l’augmentation de la densité des espèces excitées dans le plasma et l’enrichissement global de la flamme en OH. Dans le bruleur SICCA, les décharges NRP réduisent l’instabilité des flammes oscillantes (méthane, n-heptane et dodécane) et étendent ainsi leur limite d’extinction pauvre.This thesis analyzes the formation and recombination of fully ionized plasmas generated by nanosecond high-voltage pulses: the thermal sparks. Their formation occurs in five stages: (i) partial ionization of the plasma channel, (ii) creation of a fully ionized filament at the cathode, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The filaments are characterized by intense N+ and continuum emission. We show experimentally that their electron number density increases from 1017 to 1019 cm-3 in less than 0.5 ns. A kinetic mechanism, including the ionization of N and O excited states, successfully describes this 0.5-ns transition. After full ionization is reached, the plasma reaches thermal and chemical equilibrium and the electron number density decreases due to isentropic expansion. Then, Nanosecond Repetitively Pulsed (NRP) discharges in the non-thermal spark regime are applied in two burners for fundamental and practical studies of Plasma-Assisted Combustion (PAC). In the mini-PAC burner, the plasma temperature increases by 1500 K near the electrode due to a “slow” thermal spark formation. The NRP discharges enhance the reactivity of the entire flame, as shown by (i) the growth of the active species in the discharge vicinity and (ii) the increase of the OH number density in the entire flame. In the SICCA burner, NRP discharges reduce the instability of oscillating flames (in methane, heptane, and dodecane), and hence extend their lean blow out limit

Formation d’un plasma complétement ionisé et combustion assistée par décharges nanosecondes

This thesis analyzes the formation and recombination of fully ionized plasmas generated by nanosecond high-voltage pulses: the thermal sparks. Their formation occurs in five stages: (i) partial ionization of the plasma channel, (ii) creation of a fully ionized filament at the cathode, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The filaments are characterized by intense N+ and continuum emission. We show experimentally that their electron number density increases from 1017 to 1019 cm-3 in less than 0.5 ns. A kinetic mechanism, including the ionization of N and O excited states, successfully describes this 0.5-ns transition. After full ionization is reached, the plasma reaches thermal and chemical equilibrium and the electron number density decreases due to isentropic expansion. Then, Nanosecond Repetitively Pulsed (NRP) discharges in the non-thermal spark regime are applied in two burners for fundamental and practical studies of Plasma-Assisted Combustion (PAC). In the mini-PAC burner, the plasma temperature increases by 1500 K near the electrode due to a “slow” thermal spark formation. The NRP discharges enhance the reactivity of the entire flame, as shown by (i) the growth of the active species in the discharge vicinity and (ii) the increase of the OH number density in the entire flame. In the SICCA burner, NRP discharges reduce the instability of oscillating flames (in methane, heptane, and dodecane), and hence extend their lean blow out limit.Cette thèse porte sur l’étude de la formation et de la recombinaison d’un plasma complétement ionisé généré par une impulsion électrique de quelques nanosecondes. La formation du plasma se décompose en cinq étapes : (i) ionisation partielle de l’espace inter-électrode, (ii) formation d’un filament à la cathode, (iii) formation d’un filament à l’anode, (iv) propagation des filaments et (v) fusion des filaments. Ces derniers sont caractérisés par une émission intense de N+ et d’un continuum des électrons. Nos mesures montrent que la densité d’électrons augmente de 1017 à 1019 cm-3 en moins de 0.5 ns. Cette augmentation abrupte est due à l’ionisation des états excités de N et O, comme le montre un mécanisme cinétique développé dans cette thèse. Le plasma complétement ionisé est à l’équilibre chimique et thermique. Après que l’ionisation complète est atteinte, la densité et la température des électrons décroissent en raison d’une expansion isentropique de la colonne de plasma. Les décharges Nanosecondes Répétitives Pulsées (NRP) sont ensuite utilisées en régime hors-équilibre dans deux bruleurs afin de réaliser des études fondamentales et appliquées en combustion assistée par plasma. Dans le bruleur Mini-PAC, près des électrodes, la température du plasma augmente de 1500 K en raison d’une “lente” formation d’un filament. Les décharges NRP accélèrent la réactivité de la flamme comme le montrent l’augmentation de la densité des espèces excitées dans le plasma et l’enrichissement global de la flamme en OH. Dans le bruleur SICCA, les décharges NRP réduisent l’instabilité des flammes oscillantes (méthane, n-heptane et dodécane) et étendent ainsi leur limite d’extinction pauvre

High-speed interband cascade laser absorption sensor for multiple temperatures in CO2 rovibrational non-equilibrium

International audienc

Ionization Mechanism in a Thermal Spark Discharge

International audienceThe formation of thermal sparks generated by nanosecond pulses is studied experimentally and numerically. The increase of the electron number density, up to 10 19 cm-3 , is measured with sub-nanosecond resolution using Stark broadening of N + , Stark broadening of Hα, continuum emission of the electron, and the intensity of N + lines. Our experimental results are then compared to 0-D kinetic simulations including the electron-impact ionization of excited electronic states of N and O. The electron temperature, in equilibrium with the gas temperature, reaches 42,000K, which agrees with our experimental findings and the literature on the thermal spark

Hydrodynamic effects induced by nanosecond sparks in ambient air

International audienc

High-speed mid-infrared laser absorption spectroscopy of CO2 for shock-induced thermal non-equilibrium studies of planetary entry

Abstract A high-speed laser absorption technique is employed to resolve spectral transitions of CO $$_2$$ 2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO $$_2$$ 2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO $$_2$$ 2 asymmetric stretch fundamental bands ( $$\Delta$$ Δ v $$_3$$ 3 = 1) near 4.19 \upmu \hbox {m} μ m . The sensor probes a wide range of rotational energies as well as two vibrational states (00 $$^0$$ 0 0 and 01 $$^1$$ 1 0). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO $$_2$$ 2 non-equilibrium with a single vibrational temperature

RF-waveform optimization for MHz-rate DFB laser absorption spectroscopy in dynamic combustion environments

International audienc

Experimental and numerical characterization of a lean premixed flame stabilized by nanosecond discharges

International audienceThis article presents a joint experimental and numerical analysis of a lean turbulent premixed methane-air flame stabilized by nanosecond repetitively pulsed discharges. In the experiments, the transient effects of the discharge on combustion are quantified by optical diagnostics to characterize their impact on flame stabilization. The flame shape is studied with OH* chemiluminescence imaging and the temperature is measured by optical emission spectroscopy. In parallel, Large Eddy Simulation (LES) of the turbulent premixed flame is conducted. Combustion chemistry is modeled by an analytically reduced mechanism whereas the plasma discharge is described by a semi-empirical model. The comparison between experiments and simulations validates the numerical methodology