Groupe de Recherches sur l'Energétique des Milieux Ionisés

Abstract Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO 2 conversion into value-added chemicals and fuels, CH 4 activation into hydrogen, higher hydrocarbons or oxygenates, and NH 3 synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NO x removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over ‘conventional’ catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.

Non-thermal plasma (NTP) is generated by ionizing neutral gas molecules/atoms leading to a highly reactive gas at ambient temperature containing excited molecules, reactive species and generating transient electric fields. Given its potential to interact with tissue or cells without a significant temperature increase, NTP appears as a promising approach for the treatment of various diseases including cancer. The aim of our study was to evaluate the interest of NTP both in vitro and in vivo. To this end, we evaluated the antitumor activity of NTP in vitro on two human cancer cell lines (glioblastoma U87MG and colorectal carcinoma HCT-116). Our data showed that NTP generated a large amount of reactive oxygen species (ROS), leading to the formation of DNA damages. This resulted in a multiphase cell cycle arrest and a subsequent apoptosis induction. In addition, in vivo experiments on U87MG bearing mice showed that NTP induced a reduction of bioluminescence and tumor volume as compared to nontreated mice. An induction of apoptosis was also observed together with an accumulation of cells in S phase of the cell cycle suggesting an arrest of tumor proliferation. In conclusion, we demonstrated here that the potential of NTP to generate ROS renders this strategy particularly promising in the context of tumor treatment.

Seed germination and plants growth are significantly improvement by combining plasma activated water and plasma treated seeds.

The generation and behavior of particles in a low-pressure silane-argon discharge have been analyzed under continuous and pulsed radio-frequency (rf) excitation conditions. In the continuous rf excitation regime, the influence of parameters such as gas temperature and silane partial pressure are determined. By using rf pulsed excitation, it is shown that gas-flow effects play a predominant role for particle dynamics when the excitation is stopped. Radio-frequency regimes with short and adjustable rf off sequences are used to study both the inhibition of particle formation and the elimination of particles from the dusty plasmas. The electrical properties of the discharge are shown to be sensitive to the presence of the particles. Simple models for particle trapping in the plasma edge and for particle dynamics when the discharge is turned off are presented.

The growth of particle size has been measured in a low-pressure argon-silane plasma using high-resolution transmission electronic microscopy. The results show that formation and growth of dust particles is an homogeneous process; the first generation size distribution is monodispersed; and the growth kinetics reveals a three-step process from molecular ions to large particles. Together with measurements of particle concentration obtained by laser light scattering, these measurements give a clear indication that the growth proceeds through three successive steps: (i) 'rapid' formation of crystalline clusters (as shown by dark-field high-resolution transmission electron microscopy) with concentrations of up to 1010 cm-3; (ii) formation of aggregates, of diameters up to 50 nm, by coagulation (during coagulation the particle concentration decreases dramatically); and (iii) growth of the particles with a constant concentration by surface deposition of SiHx radicals, whilst the numerical density remains constant. Laser-induced particle explosive evaporation has been performed using a XeCl (308 nm) laser. This experiment allowed detection of nanocrystallites and also the beginning of their coagulation and gave clear evidence of the temperature effect on particle formation.

Abstract This work was conducted to evaluate the potential antitumor effect of an in vivo plasma treatment on a U87‐luc glioma tumor. A pulsed DBD with µs pulses at moderate power was used for treatment. Electrical analyses and spectroscopic measurements were realized to characterize plasma properties. We showed that the plasma treatment was safe for mice. However, the reiterated long time plasma treatment (20 min, 3 consecutive days) produces a superficial burn. During plasma treatment, we observed an increase of subcutaneous temperature and a cutaneous skin pH reduction. After 5 d of plasma treatment, we observed a dramatical U87 bioluminescence decrease associated with a reduction of tumor volume in U87 glioma‐bearing mice. magnified image

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In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The validity and application ranges of the different models that can be used to calculate these parameters are critically discussed.

Pancreatic tumors are the gastrointestinal cancer with the worst prognosis in humans and with a survival rate of 5% at 5 years. Nowadays, no chemotherapy has demonstrated efficacy in terms of survival for this cancer. Previous study focused on the development of a new therapy by non thermal plasma showed significant effects on tumor growth for colorectal carcinoma and glioblastoma. To allow targeted treatment, a fibered plasma (Plasma Gun) was developed and its evaluation was performed on an orthotopic mouse model of human pancreatic carcinoma using a MIA PaCa2-luc bioluminescent cell line. The aim of this study was to characterize this pancreatic carcinoma model and to determine the effects of Plasma Gun alone or in combination with gemcitabine. During a 36 days period, quantitative BLI could be used to follow the tumor progression and we demonstrated that plasma gun induced an inhibition of MIA PaCa2-luc cells proliferation in vitro and in vivo and that this effect could be improved by association with gemcitabine possibly thanks to its radiosensitizing properties.

Measurements are reported for the radiation pressure and gas drag forces acting on a single melamine-formaldehyde microsphere. The radiation pressure force coefficient q, which would be unity if all incident photons were absorbed, has the value q=0.94±0.11. For argon, the Epstein gas drag force coefficient δ, which would be unity if impinging molecules underwent specular reflection, has the value δ=1.26±0.13 as measured with our single-particle laser acceleration method, or δ=1.44±0.19 as measured using the vertical resonance method.

Résumé Les travaux à partir des notions « d’instrument d’action publique » ou de « technologies de gouvernement » ont connu ces dernières années une forte recrudescence dans l’espace francophone. Ils s’appuient sur une tradition anglo-saxonne de plus de quarante années et aujourd’hui très diversifiée, mais ils l’ont enrichi de différents apports allant des théories du pouvoir à la sociologie des sciences. L’article présente ainsi un panorama de la littérature en mettent en valeur la diversité des perspectives qui coexistent et leur évolutions. L’approche par les instruments est un bon traceur de changement de l’action publique, mais aussi des régimes et des styles politiques. Elle incite aussi à saisir l’action publique dans sa matérialité. Comprendre l’instrumentation est une façon de saisir les transformations de l’État en envisageant ses pratiques, et les recompositions qu’elles connaissent, en particulier dans la tension permanente entre contrainte et incitation.

Atmospheric pressure plasma propagation inside long dielectric tubes is analyzed for the first time through nonintrusive and nonperturbative time resolved bi-directional electric field (EF) measurements. This study unveils that plasma propagation occurs in a region where longitudinal EF exists ahead the ionization front position usually revealed from plasma emission with ICCD measurement. The ionization front propagation induces the sudden rise of a radial EF component. Both of these EF components have an amplitude of several kV/cm for helium or neon plasmas and are preserved almost constant along a few tens of cm inside a capillary. All these experimental measurements are in excellent agreement with previous model calculations. The key roles of the voltage pulse polarity and of the target nature on the helium flow patterns when plasma jet is emerging in ambient air are documented from Schlieren visualization. The second part of this work is then dedicated to the development of multi jet systems, using two different setups, based on a single plasma source. Plasma splitting in dielectric tubes drilled with sub millimetric orifices, but also plasma transfer across metallic tubes equipped with such orifices are reported and analyzed from ICCD imaging and time resolved EF measurements. This allows for the design and the feasibility validation of plasma jet arrays but also emphasizes the necessity to account for voltage pulse polarity, target potential status, consecutive helium flow modulation, and electrostatic influence between the produced secondary jets.

Kinetics of the many-stage process of particle nucleation and growth in low-pressure rf discharge in silane SiH4–Ar is considered. The particle growth is considered in an analytical model as a chain of negative-ion molecular reactions, stimulated by vibrational excitation. In the framework of this model, a limitation of first generation particle size is explained as well as the strong temperature effect on cluster growth. A theory of critical phenomena of cluster trapping in discharge area has been elaborated to describe the neutral particle selection by size, and the particle concentration increases during a period exceeding the residence time in plasma. Finally, an analytical model of critical phenomena of particle coagulation and its influence on plasma parameters is developed to explain the latest experimental results on supersmall 2–10 nm cluster kinetics. All theoretical results are presented in comparison with corresponding new experimental data and with results of an especially made computer simulation.

Presents an overview of research on a dusty plasma as obtained in a low-pressure discharge. Some results have been already published in parts and this review includes recent insights obtained in the properties of dusty plasmas, as given in an invited talk at XIth ESCAMPIG Conference in St Petersburg. Different aspects connected to the formation of particulates in an argon-silane RF reactor have been studied. The first one concerns the formation of the particulates: their size kinetics and structural characteristics give evidence of a multistep growth process. The second concerns the influence of the presence of particles on plasma parameters and discharge properties: the experimental data are in good agreement with predictions of theoretical models and both reveal the strong influence of the dust on plasma physics and chemistry. The last aspect concerns the specific interest of dusty plasmas as a source of new basic problems in plasma physics.

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The time evolution of the size of particles produced in a silane-argon low-pressure radio-frequency discharge has been determined in the nanometer to the micrometer range using transmission electron microscopy. Highly ‘‘monodisperse’’ distributions are observed and their temporal evolution is followed. These studies are combined with laser light scattering measurements to obtain particle densities, which are of the order of 107 cm−3. For short plasma durations (Δt<5 s), a fast (10 nm/s) linear increase in diameter is measured.

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The maintenance of skin integrity is crucial to ensure the physiological barrier against exogenous compounds, microorganisms and dehydration but also to fulfill social and aesthetic purposes. Besides the development of new actives intended to enter a formulation, innovative technologies based on physical principles have been proposed in the last years. Among them, Cold Atmospheric Plasma (CAP) technology, which already showed interesting results in dermatology, is currently being studied for its potential in skin treatments and cares. CAP bio-medical studies gather several different expertise ranging from physics to biology through chemistry and biochemistry, making this topic hard to pin. In this review we provide a broad survey of the interactions between CAP and skin. In the first section, we tried to give some fundamentals on skin structure and physiology, related to its essential functions, together with the main bases on cold plasma and its physicochemical properties. In the following parts we dissected and analyzed each CAP parameter to highlight the already known and the possible effects they can play on skin. This overview aims to get an idea of the potential of cold atmospheric plasma technology in skin biology for the future developments of dermo-cosmetic treatments, for example in aging prevention.

The energy loss in a plasma target was measured for different heavy-ion species, ranging from $^{40}\mathrm{Ca}$ to $^{238}\mathrm{U}$ at an energy of 1.4 MeV/u. A discharge tube was used to generate a hydrogen plasma with a high degree of ionization and temperatures between 1 and 2 eV. An on-line diagnostic of the plasma was performed to measure the free-electron density and the electron temperature. Compared to neutral hydrogen of the same particle density, the plasma target shows an enhanced stopping power due to the increased energy transfer to the free plasma electrons, and a higher effective charge of the projectiles inside the plasma target. Theoretical predictions based on the Bethe-Bohr-Bloch stopping theory are in good agreement with the experimental results.