A rarefied atmosphere-particle collisional-radiative model: Investigating active oxygen and multicomponent ion densities using optical emission spectroscopy in an atmosphere-breathing electric thruster system

Satellites in Very-Low Earth Orbit (VLEO) have prominent advantages, including low launch cost, a secure operating environment and high observational resolution. However, atmospheric drag in this orbit is a critical factor that restricts the on-orbit lifetime of satellites. The atmosphere-breathing electric thruster (ABET) system, which captures rarefied atmospheric particles as propellant, acts as a pivotal device to extend satellite service life. Unlike the conventional electric propulsion systems for high-orbit satellites that operate under rated conditions with noble propellants, the ABET systems are required to achieve a high degree of ionization of atmospheric particles and continuous performance adjustment. These requirements drive the development of the monitoring method for determining the key active oxygen density and multicomponent ion densities in the ABET system. In this work, an atmosphere-particle collisional-radiative (AP-CR) model is built, which includes the molecular ions, atomic ions, as well as the excited states of ions and atoms. Based on the model, a self-actinometry method for determining the number densities of active oxygen and multicomponent ions is presented, supported by the diffusion mechanisms of ions and neutral particles. In addition, a line-ratio method for determining the electron temperature and electron density is also presented. Furthermore, an ion beam current model is developed with the above-obtained plasma parameters as input variables to evaluate the ion beam current of the thruster. The proposed methods are validated using an electron cyclotron resonance (ECR) ion thruster with nitrogen and oxygen propellants, and the ion beam current predicted by the model is in good agreement with the measured values. In the future, our methods could be incorporated into the density monitoring module of ABET systems. This would enable ground-based assessment of the degree of ionization and assist in optimizing thruster performance, ultimately supporting the operational stability and continuous adjustability of ABET systems in the challenging VLEO environment.