Rebuilding Optical Emission Spectroscopy Measurements in a Low Density Plasma Facility using a Collisional-Radiative Model

In recent years, Air Breathing Electric Propulsion (ABEP) has emerged as a promising technology for the efficient exploitation of Very Low Earth Orbits (VLEO) for Earth observation satellites. In the context of the development and performance assessment of this technology, the DRAG-ON facility (Dual-chamber for RArefied Gases and ON-ground testing) was constructed at the von Karman Institute for Fluid Dynamics. It has the purpose to replicate on-ground the rarefied flow conditions encountered by a satellite flying at VLEO, in order to test the efficiency of intakes specifically designed for ABEP. This flow is generated by a Particle Flow Generator (PFG), which generates a partially ionized plasma plume with ions reaching orbital speeds.

The present work implements a first iteration of a non-invasive diagnostics method for the characterization of the rarefied plasma encountered in DRAG-ON, specifically for the PFG running on Argon gas. The employed method is Optical Emission Spectroscopy (OES), complemented with a Collisional-Radiative (CR) model for the prediction of emission line intensities in chemical non-equilibrium conditions, based on a set of plasma parameters. The main objectives are to assess the degree of non-equilibrium of the plasma in DRAG-ON, and to provide insight on how to improve the fidelity of the CR modelling.

The work is divided into three main parts. The first consists of the construction of the CR model and the identification of the key plasma parameters influencing the relative populations of the energy levels of Argon. The electron temperature and density stand out as the parameters of interest. The second part corresponds to the experimental measurement of the radiative signature of the plasma in the DRAG-ON facility, and the extraction of emission line intensities related to Argon atomic transitions. The last step in the developed methodology consists of comparing the experimental and predicted line intensities. Through this comparison, the electron temperature and density leading to the minimum discrepancy between the experimental and predicted results can be found.

The developed method is applied to early experimental results. This first experiment allowed to identify the sensibilities of the experimental intensities to the setup, showing that the robustness of the setup can be improved with automatization. The results obtained for the best fitting parameters between the CR model predictions and experimental intensities show the highly out-of-equilibrium plasma conditions encountered in the facility. The apparent overestimation of the plasma parameters also suggests that the usual assumption of a Maxwellian electron energy distribution function might not be applicable to the studied rarefied plasma. The results of this work lead to the construction of a roadmap to refine the method for higher fidelity results and more robustness in the experimental setup.