Microplasmas as vacuum ultraviolet source for Cl-atom density measurements by resonance absorption spectroscopy

International audienceA micro-hollow cathode discharge was used to generate radiation on the chlorine atom resonance lines. Such radiation could be used to measure, by resonance absorption spectroscopy, the density of chlorine atoms in either ground state (3p5 2P3/2) or in the fine structure metastable state (3p5 2P1/2), which is located at 882.35 cm-1. Among the nine analysed lines in the 132-142 nm spectral region, only those at 137.953 and 139.653 nm, which are strong enough and are not affected by the self-absorption, can be used for the resonance absorption diagnostic of the ground state and the metastable state, respectively. The best operating conditions of the lamp source are 0.5% of Cl2 in argon at 150 mbar and 4 mA discharge current. The measured 800 ± 30 K gas temperature of the microplasma, indicates that under these specific conditions, these two lines are dominantly Doppler broadened. So their profile is Gaussian shaped with full widths at half maximum of (4.7 ± 0.1) × 10-4 nm

Periodic forced flow in a nanosecond pulsed cold atmospheric pressure argon plasma jet

International audienceThis paper is devoted to the study of the argon flow modification in a cold atmospheric pressure plasma jet driven by nanosecond high voltage (HV) pulses, from single to multiple HV shots applications. A schlieren optical bench has been designed in order to visualize the argon flow downstream expansion in quiescent air, for moderate flow rates below 1 standard liter per minute. A coupled approach is used between charge coupled device (CCD) schlieren imaging and intensified CCD (ICCD) plasma plume imaging, both time-resolved. It is shown that the application of only one HV pulse (i.e. single HV shot) is enough to disturb the flow. The disturbed flow exhibits ripple propagation, on a timescale similar to the flow velocity. When operating in double HV shots, the second ionization wave can be used as a probe, to instantly visualize the flow structure any time after the first HV pulse application. For some flow rates, the ripple can increase in amplitude up to the point when it strongly deforms, or even stops, the plasma plume expansion, after which it is entrained by the flow and the plasma plume retrieves its full usual expansion. When a series of HV pulses are applied, the maximal disturbance of the flow is achieved for a certain pulse repetition frequency (PRF), specific of each flow rate. It is associated with ripples alternation in the plasma plume, in a 3D helicallike arrangement. For greater PRF, the ripples progressively vanish, and the flow is clearly less disturbed. Once the ripples have vanished, increasing further the PRF does not change the plasma plume and flow structures. We suggest that the repetitive plasma ignition mechanically forces the flow inside the capillary with consequences on the global flow structure, similarly to a forced backward-facing step flow with actuator

Dynamics of colliding microplasma jets

International audienceBecause of their capabilities to generate plasmas that are not confined between electrodes, low-temperature plasma jets offer unique opportunities for applications such as material processing and biomedicine. The need to generate multiple jets in order to cover larger treatment areas has recently become desirable. However, the interaction between neighbouringjets is unavoidable. It is therefore crucial to elucidate the physical processes that occur between jets. In this paper we present the case of two counter-propagating jets generated bytwo DBD-based devices. We show that the plasma bullets emitted by the two jets interact with each other as soon as they leave their respective DBD device, resulting in a decrease in their velocities. The bullets do not actually meet but rather approach each other at a minimum approach distance. The location of the region of minimum approach is not midway between the nozzles of the jet devices but rather depends on the operating conditions. In addition, we discovered the emergence of a ‘secondary’ discharge exactly in the region of minimum approach. This discharge exhibits a pink glow, reminiscent of the pink afterglow occurring in some nitrogen discharges. Time-resolved spectroscopic measurements and current measurement analysis showed that the pink glow is a transient negative glow discharge thatcannot be attributed to kinetic processes associated with re-excitation of nitrogen molecules. It is rather ignited by electrons accelerated from both jets towards the region of minimum approach. This process is found to be exactly timed with the measured current reversal

Space-time resolved density of helium metastable atoms in a nanosecond pulsed plasma jet: influence of high voltage and pulse frequency

International audienceUsing tunable diode laser absorption spectroscopy, the spatio-temporal distributions of the helium He(23 S1) metastable atoms’ density were measured in a plasma jet propagating in ambient air. The plasma jet was produced by applying short duration high voltage pulses on the electrodes of a DBD-like structure, at a repetition rate in the range 1–30 kHz. In addition to the metastable density, the spatial distribution of helium 587 nm emission intensity was also investigated to give insight into the excitation mechanisms of the He(33 D) excited state inside the dielectric tube, in which no laser measurement can be performed. It is demonstrated that the shape of the radial distribution of helium He(23 S1) metastable atoms strongly depends on the polarity of the applied voltage and on the repetition frequency. For positive applied voltages, a dramatic constriction of the excited species production is observed whenever the pulse repetition frequency is higher than 6 kHz, and the voltage higher than 5 kV. This shrinking of the jet structure induces an increase by one order of magnitude of the metastable atoms’ density in the jet centre which reaches values as high as 1014 cm−3. Beyond a critical distance, associated to a transition between a positive streamer and a negative one, the distribution of the excited atoms gets back to an annular structure. For the negative polarity, no shrinking effect correlated to the pulse repetition frequency was observed. The on-axis constriction of the excited species for the high repetition rate and positive polarity is attributed to a memory effect induced by the negative ions, having a lifetime of hundreds of microseconds, left between successive pulses at the periphery of the helium gas flow

Experimental investigation of a ns-pulsed Ar plasma jet for the fast desorption of weakly volatile organic compounds deposited on glass substrates at variable electric potential

International audienceA ns-pulsed argon atmospheric pressure plasma jet (APPJ) was studied for the fast desorption of weakly volatile organic molecules (bibenzyl) deposited on glass substrates at variable electric potential (floating-potential or grounded) [1]. The experiments focused particularly on thin resistant bibenzyl films (a large thick bibenzyl deposit was also studied), which are more difficult to be desorbed when using a substrate at a floating-potential. The APPJ was probed by means of high-resolution laser absorption spectroscopy to map the Ar(1s5) metastable absolute density (spatially and temporally resolved) at the close vicinity of the glass plate where bibenzyl was deposited. Furthermore, the electrical, optical and thermal features of the APPJ were investigated systematically. In this way, the plasma desorption efficacy on thin resistant bibenzyl deposits was evaluated for the envisaged application, by varying the exposure time of the molecules to the APPJ (texp, from 10 s up to 180 s). The obtained results confirm the relatively low desorption efficacy in the case of a floating-potential substrate, which improves to some extent with increasing texp. However, when the substrate is grounded, the effect of the plasma becomes much more significant (i.e. much higher desorption efficacy). Besides, contrary to the case of a floating-potential substrate, an almost complete desorption of bibenzyl is achieved for texp = 180 s. Similar effects of the APPJ were recorded on a thick bibenzyl deposit, validating the previous results. For both operating conditions (floating-potential and grounded substrate), the plasma action should be due to the production close to the glass surface of relatively high densities of Ar(1s5) (up to 2×1013 cm−3) and of oxidative species, such as atomic oxygen, hydroxyl radical and ozone. Thermal effects might play a synergistic role only when the substrate is grounded, since relatively high gas and glass-surface temperatures (>60 °C) are reached only in this case. The present results are of interest for public-security applications related to the fast detection of resistant prohibited substances, such as narcotics and explosives.[1] K. Gazeli et al., J. Phys. D: Appl. Phys. 53, 475202 (2020)

Investigation of gas flow pattern in a Micro-Hollow Cathode Discharge-based deposition reactor using planar Laser Induced Fluorescence

International audienceA micro-hollow cathode discharge (MHCD), ignited in an Ar/N2 gas mixture, is used to produce atomic nitrogen in a Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor for hexagonal boron nitride synthesis. To expand the plasma volume of the MHCD and transport the species onto the substrate, a pressure differential is introduced between the two sides of the MHCD, creating a plasma jet. Two dimensional spatial mappings of the atomic nitrogen density in the deposition chamber showed an unexpected density profile, presumably due to the gas flow pattern. To better understand the gas flow effect, Planar Laser Induced Fluorescence is used on acetone to visualize the gas flow structure in the deposition chamber

Investigation of gas flow pattern in a Micro-Hollow Cathode Discharge-based deposition reactor using planar Laser Induced Fluorescence

International audienceA micro-hollow cathode discharge (MHCD), ignited in an Ar/N2 gas mixture, is used to produce atomic nitrogen in a Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor for hexagonal boron nitride synthesis. To expand the plasma volume of the MHCD and transport the species onto the substrate, a pressure differential is introduced between the two sides of the MHCD, creating a plasma jet. Two dimensional spatial mappings of the atomic nitrogen density in the deposition chamber showed an unexpected density profile, presumably due to the gas flow pattern. To better understand the gas flow effect, Planar Laser Induced Fluorescence is used on acetone to visualize the gas flow structure in the deposition chamber

Cross-comparison of diagnostic and 0D modeling of a micro-hollow cathode discharge in the stationary regime in an Ar/N 2 gas mixture

International audienceAbstract A micro-hollow cathode discharge (MHCD) operated in Ar/N 2 gas mixture, working in the normal regime, was studied both experimentally and with a 0D (volume-averaged) model in this work. This source provides high electron densities (up to 10 15 cm −3 ) at low injected power (1 W). To understand the mechanisms leading to the production of N atoms, the densities of electrons, N atoms and argon metastable atoms (Ar*) were monitored over a wide range of experimental conditions. Electrons, N atoms and Ar* densities were probed by means of optical emission spectroscopy, vacuum ultra violet Fourier transform spectroscopy and tunable diode laser absorption spectroscopy, respectively. Measurements showed that using a smaller hole diameter enables to work with less injected power, while increasing the power density inside the hole and, subsequently, increasing the densities of excited species. Varying the percentage of N 2 in the gas mixture highlighted that, up to 80%, the density of N atoms increases although the dissociation rate drops. Looking at the processes involved in the production of N atoms with the help of the 0D model, we found that at very low N 2 fraction, N atoms are mostly produced through dissociative electron-ion recombination. However, adding more N 2 decreases drastically the electron density. The density of N atoms does not drop thanks to the contribution of Ar* atoms, which are the main species dissociating N 2 between 5% and 55% of N 2 in the gas mixture. A reasonable agreement is found between the experiments and the model results. This study shows that, with this MHCD, it is possible to significantly modify the production of N atoms when modifying the physical parameters, making it particularly relevant for applications requiring a N atoms source, such as nitride deposition

Development of a drift tube mass spectrometer associated with plasma microjets

International audienceThe objective of our instrument is the detection of Volatile Organic Compounds (VOCs) at trace level either in air or deposited on surfaces. The mass analyzer is a drift tube associated with a linear quadrupole and we are designing a linear ion trap for a more compact version. We will use Plasmas microjets in order to desorb molecules of low volatility deposited on surfaces.In the first version of our instrument precursor ions are formed in a glow discharge, then react with the air at a pressure of about 1 mbar in the drift tube and are then mass analyzed with a quadrupole mass filter.Compounds of low volatility are deposited on surfaces. In order to desorb them we will use plasmas microjets operated with Ar