Initial Radical Oxygen Concentration Measurements Using Catalytic Probes in an RF Plasma

Electric propulsion (EP) systems are vital for space exploration and satellite station keeping. Traditionally, xenon has been the preferred propellant for these systems due to its high mass efficiency and performance. However, its growing demand and limited supply have driven the need to explore alternative propellants. These alternative propellants pose challenges, particularly the generation of reactive species, such as radical oxygen neutrals, which can significantly degrade spacecraft components and affect the operational lifetime of EP systems. This study uses a radio-frequency (RF) oxygen plasma to generate radical oxygen neutrals for catalytic probe development for the validation of catalytic probe use in determining atomic oxygen density measurements in future EP systems. This was done by employing nickel catalytic probes to characterize the atomic neutral density over a power range of 10 to 600 W within an RF plasma. The average atomic neutral density measured in the RF plasma source ranged from 6.1 × 10¹⁸ m^-3 to 3.8 × 10²⁰ m^-3 over the RF power range. Furthermore, this investigation presents a comparative analysis of 99.9% pure nickel and 98.9% shim nickel as catalytic surfaces. The results reveal an average of 18% difference between the atomic oxygen neutral density measured by the pure and shim nickel. These results suggest that catalytic recombination is sensitive to material properties indicating that minor variations in purity can influence recombination rate and probe temperature. This study demonstrates that catalytic probes are a promising approach for measuring atomic oxygen neutral densities in EP systems.