Thèse Étude des Plasmas Micro-Ondes pour la Fixation de l'Azote par des Diagnostics Laser Avancés 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 : 2026-04-21T23:59:59 For continuous plasma generation, a very promising approach recently developed at the EM2C laboratory consists of generating a microwave (MW) discharge in jets or capillaries (see Figures a and b) in attachment [1]. Even at very low power levels (e.g., a few watts), microwave plasmas can be generated over lengths ranging from a few millimeters to a few centimeters. The main advantage of capillary or jet microwave discharges over standard pulsed electrode discharges lies in their continuous nature (continuous production of reactive species), non-invasive character (absence of electrodes), extended transport of radicals and efficient energy transfer.
The MW plasma generated in capillaries is a high-potential candidate for improving the efficiency of the Birkeland-Eyde (BE) process. Controlling species density, temperature, and their gradients should be advantageous in capillary plasmas, where fine tuning of plasma parameters and significant production of nitric oxide (NO) radicals through heat transfer have recently been demonstrated [2]. The efficiency of the discharge in producing NO has been significantly improved (i.e., by an order of magnitude), offering an original means of reducing the high cost of the BE process.
To achieve the necessary technological innovations, there are a wide variety of complex phenomena that need be studied in these MW plasmas. To date, the physical properties, the reactivity and transport, the energy coupling and power budget are poorly known. The understanding of these plasmas is limited by the lack of fine discharge diagnostics.
The objective of this thesis is to characterize microwave plasmas using high-end spectroscopic laser diagnostics in order to understand the key mechanisms of plasma nitrogen fixation and to optimize the efficiency of these plasmas.
Plasma nitrogen fixation is currently considered a potentially greener alternative to the Haber-Bosch (HB) process, which sustains 40% of the world's population but has a global environmental impact, consuming 2% of the world's energy and emitting hundreds of millions of tons of carbon dioxide each year [3]. Efficient production of nitric oxides for fertilizers using plasma could reduce plant size by employing local renewable energy and air, resulting in a carbon-free technology with a planetary impact.
Various plasma sources, reactor configurations, and catalytic materials have been studied in order to improve the Birkeland-Eyde process responsible for nitric oxide (NO) production [3,4]. Plasma nitrogen fixation is now considered to be almost as competitive as the commercial HB process, with a cost approximately four times higher.
The first objective of this thesis is to adapt existing MW reactors to a broader functional domain (including parameters such as launcher type, power, flow regime, gas mixtures, capillary nature and size, pressure, heat transfer, electromagnetic excitation frequency) and to high-end spectroscopic laser diagnostics.
The characterization of key radical productions, including atomic species such as N and O and molecular species such as NO and NO2 produced by plasmas, will be the main objective of the thesis. It should be emphasized that mapping radical densities and gradients in microwave plasmas is experimentally challenging and is essential for understanding species kinetics in plasma, such as enhanced vibrational dissociation of N (the main limiting process), transport mechanisms (essential for preventing reverse reactions), and energy branching (key for understanding BE efficiency).
Atomic radical species will be measured by Two-Photon Absorption Laser Induced Fluorescence (TALIF) [5] using femtosecond (fs) laser systems [6] (see figure (c) for example of TALIF processes in attachment). The molecular radicals can be monitored by Cavity Ring-Down Spectroscopy (CRDS) technique [7] or Mid-IR Quantum Cascade Laser Absorption Spectroscopy (QCLAS) [8].
Note that these advanced laser-based methods have exceptional features and are state of the art diagnostic techniques for fundamental plasma understanding. For instance, the remarkable temporal resolution (e.g. 100 fs) of fs-TALIF allows to investigate ultrashort transient phenomena, which were inaccessible till recently, these techniques having the potential to revolutionize the diagnostics of reactive flows.
The final objective is to optimize MW discharges for maximum nitrogen fixation efficiency. Here the study is expected to provide new solutions for increasing the efficiency of the BE process and give ideas for the design of new reactors.
Microwave discharges generated in jets or capillaries
Advanced laser spectroscopic diagnostics including:
Femtosecond Two-Photon Absorption Laser Induced Fluorescence (TALIF)
Mid-IR Quantum Cascade Laser Absorption Spectroscopy (QCLAS)
Cavity Ring-Down Spectroscopy (CRDS)

This thesis project is intended for engineer/M2 graduates in fields of engineering sciences or physics. Knowledge of plasmas, lasers, spectroscopy or transport phenomena is appreciated.

If you are highly motivated by challenging experiments, passionate about research in applied physics and engineering, and interested in a greener planet, then you're the one we're looking for!