Search

International audience; The breakdown voltage (Vb) of gases can be reasonably estimated using Paschen's law, which is based on assumptions such as a perfectly uniform electric field, and infinite electrode size. This analytical approach has demonstrated remarkable capability in redicting Vb in numerous cases, by scaling it by the product of pressure and nter-electrode distance (p·d). This description is quite efficient and convenient over wide ranges of pressure and gap distance. However, the robustness of Paschen's law in predicting consistent values of Vb is severely compromised at gap distances shorter than typically 1 micrometer at atmospheric pressure. Several studies have provided experimental, theoretical, and simulation evidences highlighting these limitations. This raised significant concerns, both theoretical and practical, about the insulation properties of gases at short distances. This study focuses on experimentally investigating Vb of air in short electrode gaps, ranging from 100 nm to 6 μm. The electrode system consists of a silicon wafer coated with gold, connected to ground. The anode connected to high voltage is a sharp CuBe needle coated with gold, with 20 μm tip radius of curvature. The gap distance is controlled by a piezoelectric actuator. Ambient air is used at atmospheric pressure, allowing for comparisons with data from existing literature. Complementary experiments are also performed in a closed chamber allowing to vary the pressure from 10-4 mbar to 3.0 bar, and control the gas purity and water content.

The simulation of corona discharges usually requires an excessive amount of time. The objective of this work is therefore to reduce the CPU time of corona discharges. The profound disparity between physical time scales in a gas discharge renders explicit time integration impractical in large time scale applications, for example in flow controls. We propose an implicit time strategy in the scope of the Local Field Approximation plasma model. We investigate some algorithms to solve the discretized plasma model, which are adapted to take into consideration a constraint on the electron density. The constraint is introduced to compensate for unaccounted sources of electrons; it is expressed by a minimum/floor density imposed on the electrons. Therefore, simplified plasma models can be employed for discharge simulations, for instance a four-specie, four-reaction model is used in this article. The investigation is done on a one-dimensional wire-to-wire corona discharge. After having identified the most suitable algorithm, we carry on to two-dimensional wire-to-wire discharges and compare the numerical results with experiment data for different floor densities.

International audience; This paper is a broad overview and a summary of the recent progress achieved in Ion Cyclotron Range of Frequency (ICRF) Systems, Experiments and Modeling during the last twenty years in magnetic confinement fusion experiments. It has been written mainly for students or newcomers in this exciting field and introduces the different applications and lists the main detrimental effects associated with the use of ICRF in magnetic fusion experiments. Modelling techniques and state-of-the-art ICRF systems are described. Recent results obtained with ICRF in fusion plasma experiments are summarised. Planned ICRF projects such as ITER or future fusion reactors and the remaining issues that remain to be solved are detailed.

International audience; The frequency-resolved terahertz (THz) beam profile characteristics of a two-color air-plasma THz source were investigated in the broadband frequency range (1-15 THz). The frequency resolution is achieved by combining THz waveform measurements and the knife-edge technique. Our results show that the THz focal spot size is strongly frequency dependent. This has important implications on nonlinear THz spectroscopy applications where accurate knowledge of the applied THz electrical field strength onto the sample is important. In addition, the transition between the solid and hollow beam profile of the air-plasma THz beam was carefully identified. Far from the focus, the features across the 1-15 THz range have also been carefully examined, revealing the characteristic conical emission patterns at all frequencies.

International audience; Knowledge of thermodynamic properties as well as parameters such as energy density and power flow isimportant for modeling thermal plasmas of fluoroalkylamine-air mixtures. In this paper, these thermodynamic properties of fluoroalkylamine-air mixture plasmas are calculated in a temperature range of 500 K to 20,000 K at atmospheric pressure and local thermodynamic equilibrium (LTE). The Gibbs free energy minimization method is used to determine the chemical equilibrium compositions of the plasmas that are needed to calculate the thermodynamic properties. These thermodynamic properties are then used to calculate the energy density and power flow of these plasmas. The variation of the energy density is related to the variations of the density and mass enthalpy. We notice that, this energy density increases with the percentage of air in the mixture for temperatures higher than 7000 K. The power flow, which depends also on density, enthalpy mass and sound speed, increases with the percentage of air in the same temperature range. Energy density and power flow results show that increasing air percentage in the mixture can be more interesting for damaging gaseous chemical species such as CF2, CO, HCN, and HF appearing at low temperatures with high concentrations.

International audience; Methods to follow in real time complex processes occurring along living cell membranes such as cell permeabilization are rare. Here, the terahertz spectroscopy reveals early events in plasma membrane alteration generated during photodynamic therapy (PDT) protocol, events which are not observable in any other conventional biological techniques performed in parallel as comparison. Photodynamic process is examined in Madin-Darby canine kidney cells using Pheophorbide (Pheo) photosensitizer alone or alternatively encapsulated in poly(ethylene oxide)-block-poly(ε-caprolactone) micelles for drug delivery purpose. Terahertz spectroscopy (THz) reveals that plasma membrane permeabilization starts simultaneously with illumination and is stronger when photosensitizer is encapsulated. In parallel, the exchange of biological species is assessed. Over several hours, this conventional approach demonstrates significant differences between free and encapsulated Pheo, the latter leading to high penetration of propidium iodide, Na+ and Ca2+ ions, and a high level of leakage of K+, ATP, and lactate dehydrogenase. THz spectroscopy provides, in a single measurement, the relative number of defects per membrane surface created after PDT, which is not achieved by any other method, providing early, sensitive real-time information. THz spectroscopy is therefore a promising technique and can be applied to any biological topic requiring the examination of short-term plasma membrane permeabilization.

International audience; 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, Te, is observed during each gas injection with the sudden pressure rise. The opposite trend, with unrealistic Te 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.

International audience; In this chapter, we focus on the application of plasma structures to realize reconfigurable antennas. Several approaches are presented to dynamically control the beamwidth and radiation gain of circularly polarized helical antennas based on plasma reflectors. Ideas and design principles were discussed and confirmed by full-wave simulations and measurements of realized prototypes. It is shown that plasma reflectors can be effectively used to design reconfigurable helicone antennas with controllable gain and beamwidth. The chapter also presents a reconfigurable slotted antenna using a plasma tube inside the metallic waveguide. It is shown that the radiation pattern of the antenna can be readily reconfigured by changing the state of the plasma column. In short, it is shown that in contrast to conventional methods based on electronic or mechanical devices, reconfigurable antennas based on plasma media benefit from simple and relatively low-cost structures as well as high performance.

International audience

International audience

International audience

International audience

International audience

International audience; The high-dimensional and multiscale nature of fusion plasma flows require the development of reduced models to be implemented in numerical codes capable of capturing the main features of turbulent transport in a sufficiently short time to be useful during tokamak operation. This paper goes further in the analysis of the dynamics of the κ − ε model based on the turbulent kinetic energy κ and its dissipation rate ε [Baschetti et al., Nuc. Fus 61, 106020 (2021)] to improve the predictability of the transverse turbulent transport in simulation codes. Present 1D results show further capabilities with respect to current models (based on constant effective perpendicular diffusion) and on the standard quasi-linear approach. The nonlinear dependence of D in κ and ε estimated from two additional transport equations allow to introduce some non-locality in the transport model. This is illustrated by the existence of parameter ranges with turbulence spreading. The paper also addresses another issue related to the uncertainties on the inherent free parameters of such reduced model. The study proposes a new approach in the fusion community based on a variational data assimilation involving the minimisation of a cost function defined as the distance between the reference data and the calculated values. The results are good, and show the ability of the data assimilation to reduce uncertainties on the free parameters, which remains a critical point to ensure the total reliability of such an approach.

International audience; Plasma micro reactors are in their whole elaborated in clean room by MEMS fabrication techniques. They are usually made on silicon platform and constituted by SiO2 dielectric grown by thermal oxidation and nickel coated by PVD process. Electrical and optical characterizations are the main diagnostics used to determine the properties of the discharge and the robustness of the device. SEM images are also made before and after plasma operation. These DC microdischarges can operate continuously during more than 24 hours in stable regime from 100 Torr up to the atmospheric pressure. In these conditions, a high density of reactive species is produced and can be used for gas, liquid or surface treatment.

This paper investigated thin films deposition processes of silica-like based on the injection of liquid droplets in Atmospheric Pressure Plasma Jet–APPJ operated in open air. An aerosol of hexamethyldisilane is produced by a syringe-pump and injected in a nitrogen post-discharge for different liquid precursor and carrier gas flow rates. For high carrier gas flow, this process enables to form silica-like without addition of oxygen in the plasma phase. Furthermore, this process offers a thin film dynamic deposition rate from 500 to 1400 nm.m.min−1 depending on the carrier gas flow and the film structure departs from silica-like to organosilicon layers for the lowest flow rates. These evolutions are attributed to plasma–droplets interactions related to the transport of droplets, the evaporation of liquid and plasma polymerization.

International audience

National audience; Climate change is predicted to cause severe loss in agricultural production by increasing disease epidemics and intensifying abiotic stresses. Therefore, there is a need for sustainable methods to alleviate plant stress, such as non-thermal plasma. Here we review the role of non-thermal plasma for plant treatment, with focus on the control of viruses, bacteria, fungi and other diseases. We present factors influencing the microbicidal activity of non-thermal plasma. Application of non-thermal plasma for combating abiotic stresses such as drought, metal toxicity, nanoparticles and salinity are discussed. Plasma-generated reactive species trigger the activity of stress-responsive genes in plants. The hypothetical mechanisms involved in triggering the activity of different stress-responsive genes controlling diseases as well as abiotic stresses, are also presented and discussed. The mechanism of plant-plasma interaction is similar to priming, hormesis or adaptive response, and resembles vaccination in animals and humans.

International audience; A new low-Mach algorithm for the thermal lattice Boltzmann method (LBM) is proposed aiming at reducing the computational cost of thermal flow simulations in the low Mach number limit. The well-known low Mach number approximation is adopted to accelerate the simulations by enlarging the time step through re-scaling the psuedoacoustic speed to the same order of the fluid motion velocity. This specific process is inspired by the similarity between the artificial compressibility method and the isothermal LBM and is further extended to its thermal counterpart. It must be emphasized that such low-Mach acceleration strategy is in a general form, thus can be easily applied to other compressible LB methods. The present method overcomes the drawback of the classical pressure gradient scaling method due to the pressure gradient changing. The new algorithm is validated by various well-documented academic test cases in laminar [one dimensional gravity column, 2D (two dimensional) rising thermal bubble, and 2D differentially heated square cavity] and turbulent [3D (three dimensional) Taylor–Green vortex and 3D heated cylinder] regimes. All the results show excellent agreement with the reference data and high computational efficiency.

7èmes Journées Scientifiques 2022 GT Matériaux Diélectriques, GdR SEEDS, Grenoble"Décharges électriques et matériaux isolants : Phénomènes et Applications"; National audience; La rupture dielectrique d’un gaz est un aspect critique pour les applications reposant sur l’isolation électrique telles qu’utilisée dans les dispositifs de coupures électriques ou autres systèmes de contact. La tension de claquage peut elle, être estimée en bonne approximation grâce à la loi de Paschen établie il y a plus d’un siècle. Cette dernière est basée sur des fondements théoriques simples moyennant un certains nombres d’hypothèses (champ électrique rigoureusement uniforme, dimensions infinies des électroqdes, rugosité de surface nulle…). Cette approche analytique a montré sa capacité remarquable de prédiction de la tension de claquage pour un nombre incalculable de systèmes non-idéaux. S’artilant autour du produit pression·distance (p·d), la loi de Paschen s’exploite sur une large échelle de pression couplée à des distances relativement adaptées à leurs utilisations; des distances macroscopiques. Indépendament des incertitudes autours des constantes présentes dans formalisme mathématique, la fiabilité que l’on connait de cette loi faillie dès lors que les distances inter-électrodes se rapprochent du micromètre et en dessous. Plusieurs études ont téhoriques et expérimentales ont déjà mis en évidence cette limitation qui remet en question le caratère isolant d’un gaz même à des préssions de lors de l’atmosphère ou au-dessus.Cette contribution rapporte les premiers résultats du d’étude expérimentale visant à mieux comprendre les mécanismes de claquages lors de très faibles distances inter-électrodes. Les distances typiques sont de l’ordre de la centaine de nanomètres entre une pointe de rayon de courbure de quelques micromètres et un plan de faible rugosité relativement au gap. Des conditions expérimentales spécifiques permettront de mettre en évidence la possibilité d’initier un claquage à des tensions de 30V ce qui ouvre de nouvelles perspectives notamment dans le domaine de microsystèmes de type MEMS.

International audience; In this study, the SPS of 0.1 µm zirconia has been performed and shows high densification rate behavior in the final stage. This work consists of modeling the densification behavior of this powder especially at the intermediate/final stage transition. To model this, a Master Sintering Curve (MSC) is combined with an analytic model to identify the activation energy and the sintering moduli. The modeling of the behavior of the powder during the sintering tests is done via the Skorohod-Olevsky model. It appears from this study that the modeling of final stage sintering requires a regime transition to model the exceptionally high densification rate of the powder. The final stage grain growth seems not to decrease the final stage sintering kinetics.

International audience; This work examines the combination of pulsed direct-liquid injections with dielectric barrier discharge at atmospheric pressure for the deposition of organosilicon coatings using hexamethyldisiloxane (HMDSO) as the precursor and nitrogen as the carrier gas. In such conditions, deposition relies on the charging of micrometer droplets and their transport toward the substrate by the Coulomb force. The thin-film morphology and extent of precursor fragmentation are strongly linked to the amount of energy provided by the filamentary discharge to HMDSO droplets. While cross-linked and smooth coatings were achieved at low energies as in standard gas phase plasma polymers, viscous and droplet-like structured thin films were deposited at higher energies. The latter material is attributed to the soft polymerization of HMDSO droplets related to plasma–droplet interactions.

International audience

International audience; A generalized Scharfetter-Gummel method is proposed to construct the numerical flux for one-dimensional drift-diffusion equations. Instead of taking a constant approximation of the flux as Scharfetter and Gummel did in [1], we consider a p-degree polynomial with p ≥ 1. The high order moments of the approximating flux function serve as intermediaries to bring numerical correction to the Scharfetter-Gummel flux, that the other end turns out to be the solution derivatives. Therefore, local solution reconstructions are required. The resulting schemes are high order and discretize at the same time the convective and diffusive fluxes without having to employ separately different methods to do so. The new schemes with p = 1 and p = 2 are employed to simulate atmospheric pressure discharge where they are applied to the continuity equations for electrons and ions, and solved simultaneously with Poisson's equation. Numerical results indicate that our method are robust and highly accurate.

International audience; A new OpenFOAM solver is presented, for the simulation of plasma cutting torches. The mathematical model that is introduced is based on the compressible Navier-Stokes equations coupled via source terms to the electric current conservation equation. Due to the conservative and hyperbolic nature of the model, a Godunov-type scheme is used for the first time in the context of plasma cutting simulation. The numerical method consists of a second-order Total Variation Diminishing (TVD) integration with flux Harten-Lax-van Leer-Contact (HLLC) Riemann solver for the flow conservation equations, coupled with a Laplace solver for the current conservation equation. An efficient formulation for the equation of state, accurately taking into account the plasma properties, is also presented. The solver is validated through a set of canonical test cases (shock tubes and 2D Riemann problems) and it is used to simulate a three-dimensional plasma cutting torch. Good agreement is found with the literature, with an improvement in the ability to deal with the shocks occurring during plasma cutting.

Mass leakage at boundaries can be a critical issue for the reliability of the lattice Boltzmann (LB) method based on Cartesian grids. Despite numerous works based on the LB method, the intrinsic macroscopic mechanisms causing mass leakage are still not fully characterized but are essential to improve the mass conservation of LB simulations. In this paper, an original theoretical investigation of mass leakage at boundaries is proposed within the general LB framework. It is demonstrated that the mass leakage originates from the intrinsic deficiency of the wall-cut LB links at boundary nodes in recovering macroscopic momenta. From a mesoscopic-level definition, i.e., the net loss of distribution functions during the streaming process, the local mass leakage at individual boundary nodes, and its averaged value along smooth boundaries are mathematically expressed using macroscopic variables. The local mass leakage is shown to be dominated by terms proportional to the tangential momentum component. In contrast, the averaged mass leakage is shown to be contributed by various terms, including the boundary curvature, the tangential momentum, and the gradients of density, momentum, and momentum flux. Meanwhile, the amplitude of the averaged mass leakage is theoretically estimated to be proportional to the local grid spacing based on which a first-order accurate correction scheme is proposed. In addition, both the local and averaged mass leakage are demonstrated to be significantly dependent on boundary orientation with respect to the grid. The proposed theoretical analysis is assessed by performing numerical experiments. Two-dimensional weakly compressible flows through straight and curved moving channels are considered to estimate each term appearing in the theoretical analysis. The numerical results are in very good agreement with the proposed analysis, and the proposed mass correction scheme based on the averaged mass leakage effectively cures the mass leakage problems in the considered test cases.