Nom de l'entreprise / du laboratoire: LAPLACE, Université de Toulouse

In many plasma-based applications, it is essential to be able to fully characterize the composition of the plasma, particularly the energy distributions of its constituent species. Intrusive diagnostics such as Langmuir probes or laser spectroscopy techniques are often required, but these are complex to implement in many configurations. An alternative approach consists in using collisional-radiative (CR) models coupled with non-intrusive measurements of emission spectra of the plasma (1,2). This approach is appealing because it makes it easier to obtain a detailed description of the plasma. In practice, emission spectra are acquired in order to infer the number density of certain excited states. We then run a CR model that solves the plasma kinetics as a function of a reduced number of free parameters (e.g., the electron temperature), to obtain the best fit with the experimental data. Good agreement provides confidence in the model and enables us all to use all of its other results to characterize the plasma. Therefore, this approach enables the indirect measurement of key quantities that would otherwise be very challenging to measure, such as the number density and temperature of electrons, as well as the translational temperature, and the rotational, vibrational, and electronic excitation temperatures.

CR models coupled with emission spectra are intended to improve our understanding of plasma physics, for example in atmospheric re-entry plasmas or electric propulsion for space applications. They can also be used to monitor and optimize industrial plasmas such as fusion plasmas, thin film deposition, CH4 plasmalysis, CO2 and NH3 reforming.
In order to exploit the full capabilities of CR models, it is deemed necessary to ensure of their validity through rigorous experimental validation. The goal of this research project is therefore to develop and validate CR models for argon and argon-molecular gas mixtures (N2, H2, CO2) with the help of advanced optical diagnostics.
Previous work within the group has already developed a CR model for argon, which will form the basis of the current project (3). The experimental work will focus on characterizing the plasma jet of a surfaguide-type microwave torch delivering power from 0.5 to 5 kW. In particular, optical emission spectroscopy will be applied to measure temperatures and the number density of excited states. Electron temperature and number density will be measured using laser Thomson scattering (4). In mixtures containing molecular gases, rotational Raman scattering will be employed to measure the number density of the ground state and the rotational temperature of molecular species. These experimental results will be used to improve and validate existing CR models, as well as to develop new models for molecular mixtures. The modeling aspect of this work can be adapted according to the Ph.D. student’s preferences.

References
1. Durocher-Jean A, Desjardins E, Stafford L. Characterization of a microwave argon plasma column at atmospheric pressure by optical emission and absorption spectroscopy coupled with collisional-radiative modelling. Physics of Plasmas. 2019 Jun 1;26(6):063516. doi:10.1063/1.5089767
2. Khazem F, Durocher-Jean A, Hamdan A, Stafford L. 2D spatial mapping of the electron temperature, electron number density, and argon molecular ion number density in a microwave argon plasma jet. Plasma Sources Sci Technol. 2026 Feb 10. doi:10.1088/1361-6595/ae4451
3. Annaloro J, Teulet P, Bultel A, Cressault Y, Gleizes A. Non-uniqueness of the multi-temperature law of mass action. Application to 2T plasma composition calculation by means of a collisional-radiative model. Eur Phys J D. 2017 Dec;71(12):342. doi:10.1140/epjd/e2017-80284-5
4. Vincent B, Tsikata S, Mazouffre S, Minea T, Fils J. A compact new incoherent Thomson scattering diagnostic for low-temperature plasma studies. Plasma Sources Sci Technol. 2018 May 8;27(5):055002. doi:10.1088/1361-6595/aabd13