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Julien COSIMI’s Thesis Defense

par Laurence Laffont - publié le

Julien COSIMI conducted his thesis on the "Characterizations of a cold helium plasma jet at atmospheric pressure" on Monday, October 19th 2020 at the amphitheater 2 of the Maison de la Recherche et de la Valorisation (MRV) on the campus of Université Paul SABATIER.

His thesis has been supervized by Mr Frédéric MARCHAL and Mr Nofel MERBAHI.

Jury :
- G2Elab - GreEn-ER - Rapporteur
Mr Jérôme PULPYTEL - LISE - CNRS - Rapporteur
Mr Nofel MERBAHI - Université Toulouse III - Paul Sabatier - LAPLACE - Thesis Supervisor
Mr Olivier EICHWALD - Université Toulouse III - Paul Sabatier - LAPLACE - Reviewer
Mr Ahmed KHACEF - GREMI, Groupe de Recherches sur l’Énergétique des Milieux Ionisés (Research Group on the Energetics of Ionized Environments) - Guest
Mr Frédéric MARCHAL - Université Toulouse III - Paul Sabatier - Guest


Cold atmospheric pressure plasma jets are a subject of great interest in many biomedical fields for the past decade. In the various applications of these jets, the plasma generated can interact with many types of surfaces. Plasma jets influence the treated surfaces, but it is now well known that the treated surface also influences the plasma according to their characteristics. The work carried out in this thesis therefore aims to characterize a cold helium atmospheric pressure plasma jet in contact with three surfaces (dielectric, metallic and ultrapure water) by means of different electrical and optical diagnostics in order to understand the influence of the nature of the surfaces on the physical properties of the plasma and the chemical species generated.
The first part of this thesis is focused on the study of the influence of surfaces on the plasma jet. Different parameters are studied, such as the nature of treated surfaces, the gas flow, the distance between the outlet of the device and the surface or the composition of the injected gas. For this purpose, helium flow at the outlet of the device is followed by Schlieren imagery with and without the discharge. Emission spectroscopy is used to determine the emissive species generated by the plasma. ICCD imagery is employed to follow the generation and the propagation of the discharge and the distribution of several excited species in the jet by using band-pass interference filters. A dielectric target causes the ionization wave to spread over its surface and a conductive target leads to the formation of a conduction channel. The evolution of excited species densities (OH*, N2*, He* and O*) increases with the relative permittivity of the treated surface.
As well known, active species generated by plasma jets play a fundamental role in the kinetics and the chemistry of the mechanisms linked to plasma processes. The second part of the present work therefore relates to the spatial and temporal evaluation of the densities of the hydroxyl radical OH which plays a major role in many cellular mechanisms. The spatial mapping and the temporal evolution of the absolute and relative densities of OH are obtained by LIF and PLIF laser diagnostics. The density of OH generated increases with the electrical conductivity of the treated surface. It can be noted that the OH molecules remain present in the helium channel between two consecutive discharges (several tens of microseconds).
Finally, we focus on the production of chemical species in ultrapure water treated with plasma. The influence of different parameters on the concentration of species in the treated water has been studied to optimize the production of chemical species. In experimental conditions, grounding the ultrapure water during treatment increases the concentration of H2O2. Furthermore, the grounding induces a decrease in the NO2- concentration.

Keywords :
Laser-Induced Fluorescence, Radical Species, Plasma Jet, ICCD Images, Schlieren Imaging, Plasma/Surface Interaction