LIF Diagnostics of Molecular Propellants: A Study of N 2 + in a Hidden Anode Plasma Source

Molecular propellants are increasingly relevant for future electric propulsion systems, yet quantitative plasma diagnostics for these species remain underdeveloped. This contribution presents further advances in laser–induced fluorescence (LIF) on N2+ in a plasma–thruster–relevant device, using a Hidden Anode Plasma Contactor as a testbed. An improved excitation scheme based on the ��2Σ+�� → ��2Σ+�� (0, 0) ��(9) transition is implemented to reduce intrinsicspin–rotation splitting and minimize the need for spectral deconvolution. Spatially resolved Ion Velocity Distribution Functions (IVDFs) measurements up to 40 mm downstream reveal that significant ion acceleration begins only beyond 10–15 mm. Strong, low velocity tails indicate delayed ionization in the expanding molecular plume. The inferred potential drops are compared with emissive probe measurements of the plasma potential and with far plume ion energy distributions from a Retarding Potential Analyzer (RPA) probe, showing close agreement. These results validate the diagnostic in a molecular plasma and establish N2+ LIF as a robust tool for velocity–resolved characterization of nitrogen–based thruster concepts.