Antoine Durocher-Jean

We investigate a low-power (<100 W), low-flow (0.3 slm) argon planar gliding discharge (GD) open to ambient air using spatiotemporally resolved optical emission spectroscopy coupled with a collisional-radiative model to characterize electron and neutral gas temperatures under two discharge current conditions. At low current (≈0.05 A), the electron temperature reaches 1.7 ± 0.2 eV, while the neutral gas temperature remains at 810 ± 20 K, indicating a strongly non-equilibrium state. At higher current (≈0.5 A), the neutral gas temperature increases to 1280 ± 10 K, whereas the electron temperature remains nearly unchanged, revealing distinct energy coupling pathways for electrons and heavy species. Temporal scans of the electron temperature at fixed spatial positions along the discharge axis show little variation, while measurements near the cathode exhibit substantial variations (1.5–4 eV), indicating varying conditions within the near-cathode region. These results demonstrate that the present GD operates in a purely non-equilibrium regime and stress the importance of diagnostic platforms enabling localized, spatiotemporal characterizations.

A detailed characterization of electron parameters, particularly electron temperature and electron number density, is essential to understand the physics driving non-equilibrium plasma jets sustained by microwave electromagnetic fields. These parameters are often extracted from optical emission spectroscopy, a widely used and non-intrusive diagnostic method. Single point measurements along a line-of-sight are the most applied techniques, while performing 2D spatially resolved measurements continues to be a challenge. Although filter-based imaging techniques offer spatial resolution, spectral information is limited to only a few emission lines. Conversely, acquiring full spectra across a 2D region requires sequential point-by-point scanning, making real-time 2D mapping complex. In this study, we address these limitations by coupling hyperspectral imaging with collisional-radiative modelling to simultaneously extract and map, with an exceptionally high spatial resolution, the electron temperature, electron number density, and argon molecular ion number density. The results are obtained in both contracted (100% Ar) and expanded (Ar + 0.35% Xe) microwave plasma jets expanding into ambient air. The electron temperature, electron density, and molecular ion density exhibit distinct and structured spatial distributions, with electron temperature peaking and molecular ions accumulating at the edges.

Optical emission spectroscopy (OES) is widely used for non-invasive plasma characterisation. OES data is often coupled with a collisional-radiative model (CRM) to get insight on the fundamental plasma properties. A CRM comes with many adjustable parameters, whose value must provide the best agreement between the measured and simulated spectra. In this work, a CRM is used to analyse time-resolved OES measurements of low-pressure ‘misty’ plasmas, in this case Ar plasmas in which liquid droplets are injected as aerosols in a pulsed fashion. It is shown that series of liquid pulses can lead to significant quenching of Ar(1 s ), even though a single injection does not. The search for the best-matching spectrum is framed in terms of probability distributions, taking full advantage of the principles of Bayesian inference. Some of the tools provided by Bayesian analysis are illustrated in the specific case of OES-CRM analysis of Ar 2 p -1 s transitions. Both experimental and modelling uncertainties are considered in a rigorous mathematical framework, and results are expressed as probability distributions instead of single values. The question of model selection is addressed through the estimation of the Bayes factor, applied as a quantitative filter for superfluous adjustable parameters.

Misty plasmas have recently emerged as a promising tool for nanocomposite thin films deposition. However, aerosol-plasma interactions remain poorly documented, especially at low working pressure. In this work, optical emission spectroscopy is used to probe the temporal evolution of three fundamental plasma parameters during pulsed liquid injection in an inductively coupled argon plasma at low-pressure. Time-resolved values of metastable argon density, electron temperature, and electron density are determined from radiation trapping analysis and particle balance equations of selected argon 1s and 2p levels. Pulsed liquid injection is found to induce a sudden drop in metastable density and electron temperature, and an increase in electron density. These results are attributed to the lower ionization thresholds of the injected molecular species compared to the one of argon. In addition, upstream liquid temperature is found to affect the transitory kinetics for non-volatile solvents more than volatile ones, in accordance with a previously reported flash boiling atomization mechanism.

Non-equilibrium plasmas at atmospheric pressure are often characterized by optical emission spectroscopy. Despite the simplicity of recording optical emission spectra in plasmas, the determination of spatially resolved plasma properties (e.g., electron temperature) in an efficient way is very challenging. In this study, spatially resolved optical images of a microwave argon plasma jet expanding into the ambient air are recorded over a wide range of wavelengths using a hyperspectral imaging system based on a tunable Bragg-grating imager coupled to a scientific complementary metal–oxide–semiconductor camera. The system’s working principle is detailed, along with the necessary post-processing steps. Further analysis of the spatial–spectral data, including the Abel transform used to determine 2D radially resolved spatial mappings, is also presented. Overall, the proposed approach provides unprecedented cartographies of key plasma parameters, such as argon and oxygen line emission intensities, Ar metastable number densities, and argon excitation temperatures. Considering that all these plasma parameters are obtained from measurements performed in a reasonable time, Bragg-grating-based hyperspectral imaging constitutes an advantageous plasma diagnostic technique for detailed analysis of microwave plasma jets used in several applications.

Pulsed gas injection in a plasma can affect many fundamentals, including electron heating and losses. The case of an asymmetric RF magnetron plasma with a pulsed argon injection is analyzed by optical emission spectroscopy of argon 2p-to-1s transitions coupled with collisional-radiative modeling. For a fully detailed population model of argon 2p levels accounting for direct and stepwise electron-impact excitation in optically thick conditions, a rapid decrease in the electron temperature, T_e, is observed during each gas injection with the sudden pressure rise. The opposite trend, with unrealistic T_e values before and after each pulse, is observed for analysis based on simple corona models, thus emphasizing the importance of stepwise excitation processes and radiation trapping. Time-resolved electron temperature variations are directly linked to the operating parameters of the pulsed gas injection, in particular the injection frequency. Based on the complete set of data, it is shown that the instantaneous electron temperature monotonously decreases with increasing pressure, with values consistent with those expected for plasmas in which charged species are produced by electron-impact ionization of ground state argon atoms and lost by diffusion and recombination on plasma reactor walls.

This work examines the rotational–translational equilibrium in non-thermal, argon-based plasmas at atmospheric pressure. In particular, rotational temperatures ( T _rot ) and neutral gas temperatures ( T _g ) are compared along the axis of plasma columns sustained by either radiofrequency (RF) or microwave (MW) electromagnetic fields. Water vapours or N ₂ admixtures are added to the high-purity argon plasmas to record the rotational temperatures from the emission spectra of either the OH(A ² Σ ⁺ − X ² Π _i ) or the N ₂ ⁺ (B ² Σ _u ⁺ − X ² Σ _g ⁺ ) rovibrational systems. T _g values are also deduced from the line broadening of selected Ar emission lines using an hyperfine spectrometer. In the MW Ar/H ₂ O plasma, T _g decreases from ∼2100 K close to the wave launcher to ∼1600 K near the end of the plasma column, while T _rot is mostly constant in the 1500 K range. In presence of N ₂ admixtures instead of water vapours, T _g is higher by about 300 K (from ∼2400 K to ∼1900 K), while T _rot decreases from ∼3200 K to ∼2750 K along the plasma column. A discrepancy between T _g and T _rot is also observed in the much colder RF plasmas with T _g ∼ 400 K and T _rot ∼ 515 K. Such departure from the rotational–translational equilibrium in both plasmas is ascribed to the influence of electrons competing with neutrals to impose their own temperature on the distribution of rotational levels of both ground and excited states.

Optical absorption spectroscopy of non-equilibrium plasmas using a supercontinuum laser combined with a laser line tunable filter and a HyperFine spectrometer is examined. Owing to the 2 and Ar 1s ₄ (Paschen notation) number densities. At a pressure of 1 Torr, a discharge current of 20 mA, and an absorption length of 18 cm, the analysis of the Ar 2p ₃ -1s ₂ and Ar 2p ₈ -1s ₄ absorption lines at respectively 840.8 nm and 842.5 nm resulted in number densities of n 1 s 2 = 5.5 × 10 ¹⁵ m ⁻³ and n 1 s 4 = 1.1 × 10 ¹⁶ m ⁻³ and a neutral gas temperature of T _g = 340 K. These values are typical of DC discharges operated under similar experimental conditions. While the fractional absorption of both lines decreased with decreasing optical absorption path, the number densities remained the same, as expected. Finally, the number densities increased with increasing discharge current, a result also coherent with the literature.