Nicolas Naudé

Plasma synthetic jet actuator: electrical and optical analysis of the discharge

Active flow control is based on the development of robust actuators which are reliable, small and easy to integrate. A promising actuator referred to as plasma synthetic jet actuator produces a synthetic jet with high exhaust velocities and holds the promise of enabling high-speed flows. With this high velocity jet, it is possible to reduce fluid phenomena such as transition and turbulence, thus making it possible to increase an aircrafts performance whilst at the same time reducing its environmental impact. This high velocity jet is produced by a pulsed discharge in a microcavity. In this paper, we focus on the properties of the discharge in order to understand the functioning of the actuator. In the first part an electrical description of the discharge in presented. Afterwards, optical measurements (optical emission spectroscopy and ICCD photograph) enable the determination of temperature, volume and duration of the discharge. At the end of the paper we present an electrical model of the discharge, which can be obtained both from electrical measurements and from macroscopic properties of the discharge (temperature, volume). This electrical model can easily be included in electrical simulation software.

Influence of gas flow dynamics on discharge stability and on the uniformity of atmospheric pressure PECVD thin film

The aim of this paper is to improve the understanding of the mechanisms controlling the uniformity of thin films made by atmospheric pressure plasma enhanced chemical vapour deposition (AP-PECVD). To reach this goal, the influence of the gas flow-rate and injection design on the thin film thickness uniformity is studied through experiments and numerical simulation in the case of silica-like layers deposited from silane and nitrous oxide using a nitrogen Townsend dielectric barrier discharge. Results show that whatever the gas flow-rate, when the gas is injected uniformly parallel to the substrate, the obtained layer is always uniform along the substrate width, while when the gas is injected perpendicularly to the substrate, the width-uniformity of the layers decreases when the gas flow-rate increases. The layer non-uniformity is related to the penetration of gas recirculation into the discharge zone, which was confirmed by computational fluid dynamics. This link is corroborated experimentally by a clear improvement of the deposit uniformity when the discharge cell dimensions are modified in order to reduce the recirculation influence on the discharge. A plausible hypothesis for the layer uniformity dependence on the recirculation is the possible enhancement of powder formation and growth in the recirculation zone: when the particle size is large enough, electrons may attach on the powder inducing electron depletion at the origin of the discharge instabilities.

Absolute nitrogen atom density measurements by two-photon laser-induced fluorescence spectroscopy in atmospheric pressure dielectric barrier discharges of pure nitrogen

In this paper, two-photon absorption laser induced fluorescence spectroscopy is used to follow the nitrogen atom density in flowing dielectric barrier discharges fed with pure nitrogen and operating at atmospheric pressure. Two different dielectric barrier discharge regimes are investigated: the Townsend regime, which is homogeneous although operating at atmospheric pressure, and the more common filamentary regime. In both regimes, densities as high as 3×1014∕cm3 are detected. However, the N atoms kinetic formation depends on the discharge regime. The saturation level is reached more rapidly with a filamentary discharge. For a given discharge regime, the N atom density depends strongly on the energy dissipated in the plasma between the gas inlet and the measurement position, whether the energy is varied by varying the position of the measurements, the gas flow, or the dissipated power. Experiments performed in the postdischarge show that the N atom decay cannot be simply attributed to three-body recombina...

Determination of the electron temperature in plane-to-plane He dielectric barrier discharges at atmospheric pressure

Optical emission spectroscopy (OES) measurements coupled with a collisional-radiative model were used to characterize a plane-to-plane dielectric barrier discharge at atmospheric pressure operated in nominally pure helium. The model predicts the population densities for the n = 3 levels of He excited by electron impact processes from either ground or metastable states and takes into account excitation transfer processes between He n = 3 levels as well as all relevant radiative decays and quenching reactions. Time-resolved OES measurements indicate that line ratios from He n = 3 triplet states (for example, 587.5 nm-to-706.5 nm) and singlet states (for example, 667.8 nm-to-728.1 nm) first sharply rise as the discharge ignites and then slowly decrease as it extinguishes. Assuming that n = 3 levels are first populated only by electron impact on ground state He atoms and then only by electron impact on metastable He atoms as the discharge current and thus the metastable number density rise, triplet and singlet line ratios predicted by the model become in each opposite case solely dependent on the electron temperature T e (assuming Maxwellian electron energy distribution function). The values of T e deduced from the analysis of both ratios were relatively high early in the discharge cycle (around 1.0–1.4 eV) and then much lower near discharge extinction (around 0.15 eV). For analysis of time-integrated (or cycle-averaged) OES measurements, the electron temperatures were closer to the 0.15 eV values near the end of the discharge cycle, in good agreement with the values expected from theoretical predictions in the positive columns of He glow discharges at atmospheric pressure.

Influence of the voltage waveform during nanocomposite layer deposition by aerosol-assisted atmospheric pressure Townsend discharge

This work examines the growth dynamics of TiO2-SiO2 nanocomposite coatings in plane-to-plane Dielectric Barrier Discharges (DBDs) at atmospheric pressure operated in a Townsend regime using nebulized TiO2 colloidal suspension in hexamethyldisiloxane as the growth precursors. For low-frequency (LF) sinusoidal voltages applied to the DBD cell, with voltage amplitudes lower than the one required for discharge breakdown, Scanning Electron Microscopy of silicon substrates placed on the bottom DBD electrode reveals significant deposition of TiO2 nanoparticles (NPs) close to the discharge entrance. On the other hand, at higher frequencies (HF), the number of TiO2 NPs deposited strongly decreases due to their “trapping” in the oscillating voltage and their transport along the gas flow lines. Based on these findings, a combined LF-HF voltage waveform is proposed and used to achieve significant and spatially uniform deposition of TiO2 NPs across the whole substrate surface. For higher voltage amplitudes, in the pre...

Experimental and modelling study of organization phenomena in dielectric barrier discharges with structurally inhomogeneous wood substrates

The spatial organization of dielectric barrier discharges operating at atmospheric pressure in the presence of complex wood substrates was analysed using optical imaging, current?voltage (I?V) characteristics, and optical emission spectroscopy combined with a collisional?radiative model to extract the average electron energy. The structural inhomogeneities of selected wood species produced non-uniform light emission patterns while maintaining homogeneous-like I?V characteristics and spatially uniform average electron energy. Based on a simple electrical model of the discharge, this localization was ascribed, at least partially, to a spatial modulation of the relative dielectric permittivity on ?early? versus ?late? wood affecting the local voltage applied to the gas, and thus the local discharge current.

Organization of Dielectric Barrier Discharges in the Presence of Structurally Inhomogeneous Wood Substrates

The spatial organization of atmospheric-pressure plasmas controlled by dielectric barriers in the presence of complex wood substrates was analyzed using optical imaging and current-voltage (I-V) characteristics. While all wood species investigated yielded homogeneous-like I-V characteristics, the structural inhomogeneities of selected wood species produced nonuniform light emission patterns with less intense emission on early versus late wood sections.

Adding of Nitrogen in Helium DBD: Consequences on the Self-Organization of the Discharge

Depending on the operating conditions, different regimes can be obtained in a dielectric barrier discharge (DBD): filamentary, diffuse/homogeneous, or self-organized. For a planeto-plane DBD operated at high frequency (150 kHz) and at atmospheric pressure in helium gas, we show that the addition of a small amount of nitrogen induces a transition from the homogenous regime to a self-organized regime characterized by the appearance of several stable strips always located at the same position.

Influence of a square pulse voltage on argon-ethyl lactate discharges and their plasma-deposited coatings using time-resolved spectroscopy and surface characterization

By comparing time-resolved optical emission spectroscopy measurements and the predictions of a collisional-radiative model, the evolutions of electron temperature (Te) and number density of argon metastable atoms [n(Arm)] were determined in argon-ethyl lactate dielectric barrier discharges. The influence of a square pulse power supply on Te, n(Arm), and discharge current is evaluated and correlated with the chemistry and the topography of plasma-deposited coatings. Pulsed discharges were found to have shorter (100 ns) but stronger (1 A) current peaks and higher electron temperatures (0.7 eV) than when using a 35 kHz sinusoidal power supply (2 μs, 30 mA, 0.3 eV). The n(Arm) values seemed to be rather stable around 1011 cm−3 with a sinus power supply. In contrast, with a pulse power supply with long time off (i.e., time without discharge) between each pulse, a progressive increase in n(Arm) from 1011 cm−3 up to 1012–1013 cm−3 was observed. When the time off was reduced, this increase was measured in sync with the current peak. The chemical composition of the coatings was not significantly affected by using a pulse signal, whereas the topography was strongly influenced and led to powder formations when reducing the time off.By comparing time-resolved optical emission spectroscopy measurements and the predictions of a collisional-radiative model, the evolutions of electron temperature (Te) and number density of argon metastable atoms [n(Arm)] were determined in argon-ethyl lactate dielectric barrier discharges. The influence of a square pulse power supply on Te, n(Arm), and discharge current is evaluated and correlated with the chemistry and the topography of plasma-deposited coatings. Pulsed discharges were found to have shorter (100 ns) but stronger (1 A) current peaks and higher electron temperatures (0.7 eV) than when using a 35 kHz sinusoidal power supply (2 μs, 30 mA, 0.3 eV). The n(Arm) values seemed to be rather stable around 1011 cm−3 with a sinus power supply. In contrast, with a pulse power supply with long time off (i.e., time without discharge) between each pulse, a progressive increase in n(Arm) from 1011 cm−3 up to 1012–1013 cm−3 was observed. When the time off was reduced, this increase was measured in sync wi...

Deposition of TiO 2 -SiO 2 nanocomposite coatings using atmospheric-pressure plasmas

The synthesis of nanocomposite thin films (NCTFs) by atomization of a colloidal solution in atmospheric-pressure discharges (APDs) has revealed a great potential. In this work, Scanning Electron Microscopy (SEM) is used to analyze the relationship between the voltage excitation waveform in a plane-to-plane Townsend dielectric barrier discharge and the transport dynamics of TiO 2 nanoparticles (NPs) as a function of position along the gas flow lines.