Influence of water vapour on the propagation speed and mean energy of an atmospheric non-equilibrium diffuse discharge in air

We report results on the influence of humidity on the propagation and the energy of a pin-to-plane nanosecond pulse discharge at atmospheric pressure. Water vapour only impacts discharges in saturated gas mixtures, for which propagation is first slowed down, but accelerates faster than usual close to the plane. Energy is unchange

Picosecond two-photon absorption laser induced fluorescence (ps-TALIF) in krypton: the role of photoionization on the density depletion of the fluorescing state Kr 5p´[3/2]2

International audienceThe present study focuses on the application of a picosecond (ps) TALIF technique in krypton (Kr) at variable pressure (0.1-10 mbar). The laser intensity (I, units W.cm-2) is tuned between 1 and 480 MW.cm-2 , and the depletion of the density of the Kr 5p´[3/2]2 fluorescing state through photoionization (PIN) and amplified stimulated emission (ASE) is investigated. This is done by combining TALIF experiments with a simple 0D numerical model. We demonstrate that for a gas pressure of 3 mbar and 15 < ≤ 480 MW.cm-2 , a saturated fluorescence signal is obtained, which is largely attributed to PIN, ASE being negligible. Also, a broadening of the two-photon absorption line (i.e. 4p 6 1 S0→→5p´[3/2]2) is recorded due to the production of charged species through PIN, inducing a Stark effect. For I≤15 MW.cm-2 , though, PIN is significantly limited, the absorption line is noticeably narrowed, and the quadratic dependence of the TALIF signal intensity versus the laser energy is obtained. Thus, in this case, the investigated Kr TALIF scheme, using the 5p´[3/2]2→5s[3/2]1 fluorescence channel, can be used for calibration purposes in ps-TALIF experiments. These results are of interest for fundamental research since most ps-TALIF studies performed in Kr do not investigate in detail the role of PIN and ASE on the depletion of the Kr 5p´[3/2]2 state density. Moreover, this work contributes to the development of ps-TALIF for determining absolute densities and quenching coefficients of H and N atoms in plasmas. This is useful in numerous plasma-based applications (e.g. thin film synthesis, biomedical treatments, plasma-assisted combustion, …), for which the knowledge of the density/kinetics of reactive atoms is essential

Investigation on streamers propagating into a helium jet in air at atmospheric pressure: Electrical and optical emission analysis

International audienceThe plasma produced due to streamers guided by a dielectric tube and a helium jet in atmospheric air is herein studied electrically and optically. Helium streamers are produced inside the dielectric tube of a coaxial dielectric-barrier discharge and, upon exiting the tube, they propagate into the helium jet in air. The axisymmetric velocity field of the neutral helium gas while it penetrates the air is approximated with the PISO algorithm. At the present working conditions, turbulence helium flow is avoided. The system is driven by sinusoidal high voltage of variable amplitude (0-11 kV peak-to-peak) and frequency (5-20 kHz). It is clearly shown that a prerequisite for streamer development is a continuous flow of helium, independently of the sustainment or not of the dielectric-barrier discharge. A parametric study is carried out by scanning the range of the operating parameters of the system and the optimal operational window for the longest propagation path of the streamers in air is determined. For this optimum, the streamer current impulses and the spatiotemporal progress of the streamer UV-visible emission are recorded. The streamer mean propagation velocity is as well measured. The formation of copious reactive emissive species is then considered (in terms of intensity and rotational temperatures), and their evolution along the streamer propagation path is mapped. The main claims of the present work contribute to the better understanding of the physicochemical features of similar systems that are currently applied to various interdisciplinary engineering fields, including biomedicine and material processing. © 2013 AIP Publishing LLC

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

International audienceNanosecond-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 demonstrate the validity of the method in our conditions and applied it for the straight forward monitoring of N-atom densities during the transients of a pulsed ECR plasma. We especially 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 attributing this enhancement 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

Interpretation of the gas flow field modification induced by guided streamer ('plasma bullet') propagation

International audienceAtmospheric-pressure non-equilibrium plasmas of noble gases in the form of 'bullets' have attracted considerable attention, against cold low-pressure or thermal atmospheric-pressure plasmas, for multidisciplinary scientific fields such as material science and biomedicine, due to their unique compatible features. A key factor for the efficiency of most of these systems is the interaction between the noble-gas channel, where the 'bullets' (streamers) propagate, and the plasma itself. It is the object of this paper to demonstrate this interaction and to provide the explanation on the gas flow field modification induced by the plasma ignition. A three-dimensional numerical model incorporating most of the governing equations, schlieren imaging and UV-visible high-resolution optical emission spectroscopy are applied. In accordance with the present results, the mechanism leading to the flow field alteration is clearly related to the electrohydrodynamic force, while it is demonstrated that the gas temperature plays a minor role. © 2014 IOP Publishing Ltd

Absolute N-atom density measurement in an Ar/N₂ micro-hollow cathode discharge jet by means of ns-two-photon absorption laser-induced fluorescence

International audienceIn this work, nanosecond Two-photon Absorption Laser Induced Fluorescence (TALIF) is used to probe the absolute density of nitrogen atoms in a plasma generated using a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime and the plasma is ignited in an Ar/N 2 gas mixture. First, we study a MHCD configuration having the same pressure (50 mbar) on both sides of the electrodes. A good agreement is found between the density of N atoms measured using TALIF in this work, and previous measurements using vacuum ultraviolet Fourier transform absorption spectroscopy. Then, we introduce a pressure differential between the two electrodes of the MHCD, creating a plasma jet. The influence of the discharge current, the percentage of N 2 in the gas mixture and pressures on both sides of the MHCD is studied. The current has a small impact on the N-atoms density. Furthermore, an optimal N-atom density is found at around 95% of N 2 in the discharge. Finally, we demonstrate that the pressure has a different impact depending on the side of the MHCD: the density of N-atoms is much more sensitive to the change of the pressure in the low pressure side when compared to the pressure change in the high pressure side. This could be due to several competing phenomena: gas residence time in the cathodic region, recirculation or recombination of the N-atoms at the wall. This study contributes to the optimization of MHCD as an efficient N-atom source for material deposition applications

Atmospheric-pressure guided streamers for liposomal membrane disruption

International audienceThe potential to use liposomes (LIPs) as a cellular model in order to study interactions of cold atmospheric-pressure plasma with cells is herein investigated. Cold atmospheric-pressure plasma is formed by a dielectric-barrier discharge reactor. Large multilamellar vesicle liposomes, consisted of phosphatidylcholine and cholesterol, are prepared by the thin film hydration technique, to encapsulate a small hydrophilic dye, i.e., calcein. The plasma-induced release of calcein from liposomes is then used as a measure of liposome membrane integrity and, consequently, interaction between the cold atmospheric plasma and lipid bilayers. Physical mechanisms leading to membrane disruption are suggested, based on the plasma characterization including gas temperature calculation

Effect of the gas flow rate on the spatiotemporal distribution of Ar(1s(5)) absolute densities in a ns pulsed plasma jet impinging on a glass surface

This work presents spatial (axial-z and transversal-y) and temporal distributions of Ar(1s(5)) metastable absolute densities in an atmospheric pressure argon micro-plasma jet impinging on an ungrounded glass surface. Guided streamers are generated with a DBD device driven by pulsed positive high voltages of 6 kV in amplitude, 224 +/- 3 ns in FWHM and 20 kHz in frequency. The argon flow rate is varied between 200 and 600 sccm. The glass plate is placed at 5 mm away from the reactor's nozzle and perpendicular to the streamers propagation. At these conditions, a diffuse stable discharge is established after the passage of the streamers allowing the quantification of the Ar(1s(5)) absolute density by means of a conventional TDLAS technique coupled with emission spectroscopy and ICCD imaging. The good reproducibility of the absorption signals is demonstrated. The experiments show the strong dependence of the maximum density (0.5-4 x 10(13) cm(-3)) on the gas flow rate and the axial and transversal position. At 200 sccm, high maximum densities (>2.4 x 10(13) cm(-3)) are obtained in a small area close to the plasma source, while with increasing flow rate this area expands towards the glass plate. In the transversal direction, density maxima are obtained in a small zone around the propagation axis of the streamers. Finally, a noticeable increase is measured on the Ar(1s(5)) effective lifetime close to the glass surface by varying the flow rate from 200 to 600 sccm. In overall, the effective lifetime varies between similar to 25 and similar to 550 ns, depending on the gas flow rate and the values of z and y coordinates. The results obtained suggest that the present system can be implemented in various applications and particularly in what concerns the detection of weakly volatile organic compounds present in trace amounts on different surfaces

Characterization of a helium micro-plasma jet by means of ps-TALIF and a streak camera

Atmospheric pressure plasma jets (APPJ) present rich gas-phase chemistry,transient electric fields and electron densities, low gas temperatures, etc., whichmake them very promising in various novel applications such as ambient ionizationmass spectrometry [1]. Due to their operation at atmospheric pressure, collisionalquenching of generated reactive species becomes significant, and, in some cases(e.g., excited atomic hydrogen), the corresponding effective lifetimes may fall to subnstimescales [2]. In most published studies, classic optical diagnostics (such asnanosecond –ns– TALIF and ICCD cameras) are employed to understand thekinetics of reactive atoms in APPJ. However, precise measurements of theirdensities using ns-TALIF may become challenging at atmospheric and higherpressures [2]. In this case, the use of picosecond (ps) or femtosecond (fs) TALIF aswell as ultrafast detectors for atomic density and lifetime determination is a bettersolution [2,3].This work focuses on the investigation of a μs-pulsed microtube helium APPJ bymeans of ps-TALIF (laser: Ekspla®; pulse width: ~10 ps) and a streak camera(C1091005, Hamamatsu®; few ps time resolution). The achieved spatial resolutioncan be as low as 400 μm, while we were able to measure laser-excited H-atomlifetimes down to a few hundred ps (Figure 1). The obtained H-atom density andlifetime values depend on the distance from the tube exit and the APPJ parameters(voltage, helium flow rate, …). These results are supported by electrical and OESdiagnostics. This study provides useful information for APPJ kinetic models while ithelps in the optimisation of APPJ for novel applications [1].[1] S. Brandt et al., Anal. Chim. Acta 951, 16–31, 2017[2] K. Gazeli et al., Plasma 4, 145–71, 2021[3] S-J. Klose et al., Plasma Sources Sci. Technol. 29, 125018, 202