Thèse Diagnostics Laser Ultrarapides d'Écoulements Réactifs et Hors Équilibre à l'Interface Gaz-Liquide H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Sciences de l'ingénierie et des systèmes École doctorale : Sciences Mécaniques et Energétiques, Matériaux et Géosciences Laboratoire de recherche : EM2C - Energétique Moléculaire et Macroscopique, Combustion Direction de la thèse : Gabi-Daniel STANCU ORCID 000000031743916X Début de la thèse : 2026-10-01 Date limite de candidature : 2026-09-30T23:59:59 In this PhD thesis, we propose to apply unique non-linear ultrafast laser techniques (fs-TALIF, fs/ps-CARS) to investigate the same plasma device, namely a microwave plasma jet, which can be driven in continuous or pulsed mode at atmospheric or intermediate pressures. The plasma non-equilibrium degree (vibration, rotation, translation), its reactivity (radical fluxes) and its interaction with a water liquid surface will be controlled spatially or temporarily. Mapping of the vibrational non-equilibrium distribution (non-Boltzmann) in electronic ground states of N, O2 and H molecules is essential for example for understanding vibration-dependent kinetics in applications such as plasma-assisted nitrogen fixation, where vibrational ladder is a key dissociation mechanism. Radical density (e.g. O, N, H) and temperature mappings at the plasma-liquid interface provide access to gradients and thus to species and heat transfer fluxes. This knowledge is important for understanding fundamental processes in non-equilibrium reactive flows, such as plasma kinetics under various degrees of humidity, evaporation, or radical diffusion in liquid phase, for applications such as plasma water depollution (e.g. PFAS abatement) or plasma-assisted two-phase combustion. To foster innovation in aerospace propulsion, energy and environment plasma applications, accurate measurements of the reactive flow field are essential. Experimental diagnostics are necessary to understand complex media such as non-equilibrium reactive flows at gas-liquid interfaces in plasmas and combustion. Mapping flow field parameters is required for validating numerical models. Among the available techniques, laser spectroscopic diagnostics offer reliable, in situ and non-intrusive, quantitative measurements with high selectivity and high spatial and temporal resolution. In particular, Two-Photon Absorption Laser Induced Fluorescence (TALIF) and Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy stand out as the gold standard for the detection of key atomic radical such as O, N and H, and for thermometry and sensing of vibrational distributions in the electronic ground states of homo-nuclear molecules such as N2, O2, H2. Although, the nanosecond laser based TALIF was employed for characterization of plasma and combustion for about four decades, its implementation remains complex and is currently subject to large uncertainties even at intermediate pressure. At present, the EM2C group at CentraleSupélec is developing a TALIF technique based on femtosecond laser technology. One of the novelties, as demonstrated theoretically in our article [1], lies in the possibility of developing sensitive (due to favourable photon statistics), photolytic-free [2] and quench-free TALIF technique, thanks to fs-lasers. This allows for a considerable improvement in the accuracy of the atomic radical detection in plasmas and combustion environments, as shown recently in [3]. Ultrashort fs lasers which exhibit intensities up to 1 PW cm-2, require density matrix description that accounts for coherent excitation and Stark detuning and new calibration methods, as recognised in [1,4]. This approach differs radically from the classical rate equation description used until now by the plasma and combustion communities.Complementarily, ONERA, a pioneer of CARS, is developing high-speed instruments based on femtosecond ultrafast lasers. Recent progress in hybrid fs/ps-CARS have pushed the limits of the technique up to 5 kHz thanks to an innovative Yb:YAG laser architecture [5,6], and even at higher rates (100 kHz-1 MHz) [7] in intermittent burst mode. A continuous-duty instrument functioning at 50 kHz is currently under development within ONERA's, DPHY lab. This will enable high-fidelity diagnostics, the acquisition of extensive datasets over short periods and the capture of extended temporal dynamics, relevant to non-stationary reactive flows.In this PhD thesis, we propose to apply these unique non-linear ultrafast techniques to investigate the same plasma device, namely a microwave plasma jet, which can be driven in continuous or pulsed mode at atmospheric or intermediate pressures. Here, the plasma non-equilibrium degree (vibration, rotation, translation), its reactivity (radical fluxes) and its interaction with a water liquid surface will be controlled spatially or temporarily. Mapping of the vibrational non-equilibrium distribution (non-Boltzmann) in electronic ground states of N, O2 and H molecules is essential for example for understanding vibration-dependent kinetics in applications such as plasma-assisted nitrogen fixation, where vibrational ladder is a key dissociation mechanism. Radical density and temperature mappings at the plasma-liquid interface provide access to gradients and thus to species and heat transfer fluxes. This knowledge is important for understanding fundamental processes in non-equilibrium reactive flows, such as plasma kinetics under various degrees of humidity, evaporation, or radical diffusion in liquid phase, for applications such as plasma water depollution (e.g. PFAS abatement) or plasma-assisted two-phase combustion. An originality of this project lies in its ability to investigate a controlled complex reactive non-equilibrium flow in boundary layers at the gas-liquid interface using two complementary, cutting-edge spectroscopic techniques. This particularly demanding environment will challenge our techniques in terms of sensitivity, selectivity, spatial and temporal resolution, and accuracy (e.g. temperature evaluation from a non-Boltzmann distribution, photolytic enhancements due to water vapours, quenching processes with characteristic time less than 1 ns, spatial resolution below 100 micrometers). This project will also provide a benchmark for validating plasma models, revealing microscopic properties inaccessible to conventional diagnostic methods. This PhD project focuses on the implementation of fs-TALIF and hybrid fs/ps-CARS and the characterization of the non-equilibrium microwave plasma jet. The three-year research is structured around the following objectives: (i) Development of a transportable microwave plasma jet device, with controlled non-equilibrium and reactivity, with and without interaction with a water liquid surface. Divers gas mixtures (air, nitrogen, argon), flows, powers, and gap distances will be tested (1st year, S1)(ii) Implementation of fs-TALIF & mapping atomic radicals (e.g. O, N) and implementation of fs/ps-CARS & mapping temperatures (Tvib, Trot/Tgas) and vibrational distribution (e.g. N2(X,v)) in the microwave free jet (1st year S2, 2nd year, S1)(iii) Mapping of atomic radicals by fs-TALIF at mapping of temperatures and vibrational distribution by fs/ps-CARS at the plasma-liquid interface (2nd year, S2, 3nd year, S1)(iv) Thesis and articles writing (3nd year, S2) Two-Photon Absorption Laser Induced Fluorescence (TALIF) and Coherent Anti-Stokes Raman Scattering (CARS)

This thesis project is intended for engineer/M2 graduates in fields of engineering sciences or physics. Knowledge of lasers, spectroscopy, plasmas, or transport phenomena is appreciated. If you are highly motivated by challenging experiments, passionate about research in applied physics and engineering then you're the one we're looking for!

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