Theoretical and experimental laser-induced fluorescence studies on plasma thrusters

This dissertation explores the potential of laser-induced fluorescence (LIF) spectroscopy as a minimally intrusive diagnostic tool for plasma thrusters. The work aims to develop robust methodologies for determining time-averaged and time-resolved ion velocity distribution functions and recovering in-situ magnetic field vectors. Both Xenon and Krypton were used as propellants across the multiple experimental campaigns. In the first part, a generalized theoretical model for LIF spectra is developed, incorporating Doppler, lifetime, isotope shifts, hyperfine interaction, saturation effects, and, notably, the anomalous Zeeman effect. This framework avoids assumptions about magnetic field geometry or laser polarization, allowing accurate evaluation of the Zeeman effect across diverse laser–plasma interaction configurations. Validation was carried out using a Rubidium vapour cell and hollow cathode lamps containing Xenon and Krypton. Minor discrepancies were observed in the Rubidium media, hence validating the theoretical model. On the contrary, substantial broadening mechanisms were observed in the hollow cathode lamps, later attributed to pressure-broadening mechanisms. Magnetic field analyses and neutral flow analysis were pursued in two experimental campaigns. In the first, a Highly Efficient Multistage Plasma Thruster (HEMPT) was investigated, where the presence of strong magnetic fields enabled the observation of significant Zeeman splitting. This allowed for the precise determination of the Landé g-factor and refinement of the resonance wavelength of the 4𝑑4 𝐷7/2 → 5𝑝4𝑃◦5/2 Kr II transition. A second campaign focused on in-situ magnetic field vector measurements and neutral dynamics in a laboratory-model Hall thruster operating with Krypton. By probing three Kr I transitions (826 nm, 828 nm, and 830 nm), a comparative analysis was conducted using a double-polarization strategy. Bayesian inference enabled accurate flow parameter extraction and magnetic field measurements with precisions within a few Gauss above 100 G. An apparent axial acceleration of the neutral population was also identified. The phase-resolved LIF (PR-LIF) technique was developed and validated through two complementary campaigns. In the first, a laboratory Hall thruster operating with Krypton was used to demonstrate a real-time implementation of PR-LIF, based on Hilbert transform phase tracking and FFT-based spectral demodulation. In the second, conducted within the CHEOPS project on the PPS®X00 Hall thruster developed by Safran, the PR-LIF technique was extended through advanced post-processing and applied across three magnetic field configurations. The measurements revealed a progressive contraction of the ionization and acceleration regions with increasing field intensity, and confirmed the ability of the technique to resolve time-dependent ion dynamics with high temporal and spatial resolution.