Olivier Eichwald

Experimental analysis and modelling of positive streamer in air: towards an estimation of O and N radical production

This paper is mainly devoted to the comparison between the calculation and experimental results of primary and secondary streamer development in a point-to-plane positive corona discharge in dry air at atmospheric pressure. The qualitative agreement between experimental and calculation results based on the hydrodynamics approximation shows that the O radical is mainly produced in the secondary streamer which is in good agreement with the recent literature measurements using TALIF diagnostics. However, the O radical production yield (in terms of radicals produced per energy injected) is more efficient in the primary streamer than in the secondary one. The main positive corona discharge characteristics are revisited using fast electrical and optical ICCD and streak camera measurements. The calculation shows two streamer radii of, respectively, 10 µm (associated with the radial extension of a high electron density region) and 200 µm (corresponding to the extension of the radial space charge electric field).

Low-temperature plasmas at atmospheric pressure: toward new pharmaceutical treatments in medicine.

This article concerns a new field covered by low‐temperature plasmas at atmospheric pressure for medical treatments. This is based on the very attractive possibility to tune and design plasmas as possible pharmaceutical products using selectively some active species (charged particles, radicals, atomic and molecular agents, UV radiations) and even electric fields self‐generated by the plasma. The delivery of active species occurs at the gaseous level. This means that there is no need for a carrier medium, and the treatment of living tissue or surface is optimal because plasmas can penetrate small pores, spread over rough surfaces, and reach both prokaryotic and eukaryotic cells. The present article gives first a review on the main low‐temperature plasma setups potentially usable for medical treatments with an emphasis on the setups as, for instance, plasma jets developed in our laboratory. Then, the present article gives a review of the current state of the art of such plasmas as pharmaceutical products or therapeutic tools in medicine with a light on a selection of forefront researches particularly in the field of chronic wounds, blood coagulation, and cancer treatment.

Critical Analysis on Two-Dimensional Point-to-Plane Streamer Simulations Using the Finite Element and Finite Volume Methods

Two different categories of high-order numerical methods are adopted for the electro-hydrodynamic modeling of the streamer: the finite-element method (FEM) with flux-corrected-transport (FCT) technique and the finite-volume method (FVM) with monotonic upwind-centered scheme for conservation-law (MUSCL) algorithm. Specific numerical tests on the transport equation are used to investigate the efficiency of the two methods to propagate sharp density gradients in stationary uniform and nonuniform velocity fields. The streamer simulations are performed in a positive point-to-plane electrodes filled with air at room temperature and atmospheric pressure. The influence of the numerical schemes on the 2-D streamer modeling is analyzed. Both FVM-MUSCL and FEM-FCT accurately describe the streamer propagation and the morphology of the discharge channel, thereby giving similar axial and radial density profiles. Furthermore, the computational cost is higher for the FEM-FCT solver as compared to the FVM-MUSCL one, which is at least twice as fast. However, the unstructured-grid approach adopted by FEM-FCT proves to be very efficient in describing nonuniform geometries.

Low-temperature plasma-induced antiproliferative effects on multi-cellular tumor spheroids

Biomedical applications of low-temperature plasmas are of growing interest, especially in the field of plasma-induced anti-tumor effects. The present work is aimed at investigating the regionalized antiproliferative effects of low-temperature plasmas on a multicellular tumor spheroid (MCTS), a model that mimics the 3D organization and regionalization of a microtumor region. We report that a low-temperature plasma jet, using helium flow in open air, inhibits HCT116 colon carcinoma MCTS growth in a dose-dependent manner. This growth inhibition is associated with the loss of Ki67, and the regionalized accumulation of DNA damage detected by histone H2AX phosphorylation. This regionalized genotoxic effect leads to massive cell death and loss of the MCTS proliferative region. The use of reactive oxygen species (ROS), scavenger Nacetyl cysteine (NAC) and plasma-conditioned media demonstrate that the ROS generated in the media after exposure to low-temperature plasma play a major role in these observed effects. These findings strengthen the interest in the use of MCTS for the evaluation of antiproliferative strategies, and open new perspectives for studies dedicated to demonstrate the potential of low-temperature plasma in cancer therapy.

Effect of order fluid models on flue gas streamer dynamics

The present paper shows that in the case of a micro-discharge modelling using the hydrodynamics assumption, the second order fluid model involving the complete electron momentum conservation equation must be used in order to better quantify the radical formation in a micro-discharge applied to air pollution control. The present results show large differences in the micro-discharge parameters (such as velocity and electron density) between the three tested hydrodynamics models: the classical first order model using the local electric field approximation and two second order models using the local energy approximation with or without the drift–diffusion approximation. The tests have been carried out in the case of a wire-to-plane corona reactor filled with a typical flue gas (76% N2, 12% CO2, 6% O2, 6% H2O) at atmospheric pressure and ambient temperature. The simulation of the micro-discharge dynamics is performed using a 1.5D numerical streamer model coupled with a simple chemical kinetics model involving 31 species (charged and neutral particles in their fundamental or metastable state) reacting following 29 selected chemical reactions.

Electrical analysis of positive corona discharge in air and N2, O2, and CO2 mixtures

This paper presents an experimental analysis of the electrical behavior of positive point-plane corona discharges. The corona current, streamer velocity, mean discharge frequency, and current-voltage characteristic are studied, firstly in synthetic air as a function of experimental parameters such as gap distance and tip radius. Different electrical diagnostics are used in order to better understand the streamer development as well as the dependence of its characteristics on the previous listed parameters. Then the influence of gas mixture (several proportions of N2 and O2 with or without CO2) is analyzed. When the gas concentration is varied the shape and amplitude of the corona current are significantly affected due to the variation of the gas electronegativity following its composition and concentration. The ionization and attachment coefficients are calculated from the electron energy distribution function in the case of these different gas mixtures in order to quantify the critical electric field val...

Atmospheric Pressure Dielectric Barrier Discharges Under Unipolar and Bipolar HV Excitation in View of Chemical Reactivity in Afterglow Conditions

This paper deals with atmospheric pressure DBDs driven under unipolar and bipolar pulsed excitation, in a context of chemical reactivity in flowing afterglow conditions. The resulting discharges in air and nitrogen are examined via electrical and optical diagnostics. Then, their yield in the production of active species (ozone for the air afterglow-atomic Nitrogen and N2(A) metastables for the N2 afterglow) is compared. An effort is made to illustrate the key discharge parameters that are likely to influence the concentrations of neutral active species in afterglow conditions.

Electro-hydrodynamics and kinetic modelling of polluted air flow activated by multi-tip-to-plane corona discharge

The present paper is devoted to the 2D simulation of an Atmospheric Corona Discharge Reactor (ACDR) involving 10 pins powered by a DC high voltage and positioned 7 mm above a grounded metallic plane. The corona reactor is periodically crossed by thin mono filamentary streamers with a natural repetition frequency of some tens of kHz. The simulation involves the electro-dynamic, chemical kinetic, and neutral gas hydrodynamic phenomena that influence the kinetics of the chemical species transformation. Each discharge stage (including the primary and the secondary streamers development and the resulting thermal shock) lasts about one hundred nanoseconds while the post-discharge stages occurring between two successive discharge phases last one hundred microseconds. The ACDR is crossed by a lateral air flow including 400 ppm of NO. During the considered time scale of 10 ms, one hundred discharge/post-discharge cycles are simulated. The simulation involves the radical formation and thermal exchange between the discharges and the background gas. The results show how the successive discharges activate the flow gas and how the induced turbulence phenomena affect the redistribution of the thermal energy and the chemical kinetics inside the ACDR.

Simulation of Expansion of Thermal Shock and Pressure Waves Inducaed by a Streamer Dynamics in Positive DC Corona Discharges

This paper is devoted to the simulation of the thermal shock and the induced pressure-waves expansion, generated by a dc pin-to-plan corona discharge in the air at ambient temperatures and under atmospheric pressure. The positive dc voltage applied to the tip generates a monofilamentary streamer that crosses the gap from the tip toward the plan. The simulation models are based on the coupling of a 2-D dynamics streamer model with the hydrodynamics conservation equations of a compressible gas. The source term for the gas dynamics equations takes into account the fast-energy relaxation from excited molecules to the random thermal energy. The simulation shows that the streamers generate a thermal shock near the anodic tip, which induces high pressure gradients and finally the gas expansion. The thermal shock is located just in front of the anodic tip, where the injected energy density is the highest. After 0.3 μs, the mean gas temperature increases up to around 800 K in a small volume just in front of the anodic tip while the maximum temperature reaches 1200 K. In addition, two pressure waves, a spherical and a cylindrical one, are induced with a propagation velocity of 370 m s-1 i.e., close to the speed of sound in air.

Primary and Secondary Streamer Dynamics in Pulsed Positive Corona Discharges

Preliminary comparisons between numerical simulation and experimental results are obtained under pulsed applied voltages in the case of positive point to plane corona discharges in dry air at atmospheric pressure. The experimental conditions correspond to 7 mm interelectrode distances with a tip radius of 20