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Simulation and numerical experiments
Theory, modeling, and simulations complement experimental studies by providing access to quantities that are difficult or impossible to determine experimentally, and allowing for a better understanding of the underlying physics. Validation of the developed codes is achieved both through quantitative comparisons with experiments and through modeling reference cases between several codes. The sources present a wide variety of conditions and are highly non-equilibrium, making the kinetic effects on transport, chemistry, and interaction with surfaces fundamental. Therefore, modeling work relies on a combination of ’fluid’ models (based on moments of the Boltzmann equation) and fully kinetic models such as Monte Carlo methods and Particle-In-Cell/Monte Carlo Collisions (PIC/MCC) simulations.
A significant part of LPP’s modeling activities focuses on verifying and validating numerical approaches against experiments, which allows for revisiting the fundamental physics of discharges. Notably, LPP conducted an international benchmark where seven independently developed PIC codes for a Hall effect thruster were compared. This comparison analyzed the convergence of PIC models and ensured the reproducibility of identified instabilities. A major and original advancement of these codes is the ability to perform ’digital diagnostics,’ simulating both the plasma and the response that an experimental diagnostic might have to this plasma. This technique has, for example, been used to study a virtual collective Thomson scattering diagnostic on PIC data to investigate these small scales related to anomalous transport and to bridge simulation and experiments.
On the other hand, the physics of atmospheric pressure plasma jets and their interactions with surfaces have been studied both numerically and experimentally. A thorough comparison was presented, including the electric field in the plasma as well as in dielectric surfaces exposed to the plasma, the mean electron energy, the electron density, and the charge of the targets, allowing for the assessment of model fidelity and providing perspectives for future improvements for both models and experiments. Another example, at low pressure this time, is a theoretical model compared to measurements of striations in radiofrequency plasmas, providing a completely new interpretation of these instabilities.
The team’s activities also include theoretical work addressing fundamental problems such as the development of a macroscopic model capable of capturing kinetic effects in low-temperature discharges. A high-order fluid model was derived from kinetic theory, allowing closure terms with few collisions and efficient modeling of non-local transport effects. Similarly, efforts to derive new numerical schemes based on cutting-edge numerical analysis have been pursued, enabling more efficient numerical simulations
