Laser induced fluorescence study of plasma-insulator wall interaction involving secondary electron emission

In Hall-effect ion thrusters, the interaction between the plasma and the ceramic walls has an impact on the devices' lifetime and performances. This is partially due to the secondary electron emission (SEE), a phenomenon that may cool down the plasma, resulting in a lower ionization rate, and may trigger turbulence and instabilities. For these reasons, it is necessary to understand the plasma-ceramic wall interaction and evaluate the impact of the SEE. This study mainly focuses on plasma sheaths - the fundamental mechanism involved in plasma-wall interaction - standing in front of Hall thrusters' ceramics. Those sheaths are compared to other materials' ones used in plasma devices, and the influence of the SEE on them is studied. The experimental part of the study mainly relies on the laser induced fluorescence diagnostic that allows to probe plasma sheaths in a nonintrusive way. The sheaths' structure, the ion density variations and the ion distribution functions' shape are presented and discussed. It is shown that the sheath is material dependent and that the thrusters' ceramics' SEE is lower than for the other studied ceramics, which is coherent with previous measurements. Also, a peak in the ion density is observed near the sheath entrance, a result not captured by the classical sheath theory. These experimental results are compared with a 1D kinetic sheath model and kinetic simulations that use the SEE yields found in the literature. Additionally, a discussion about emissive probes is given, which deals with the uncertainty determination of the plasma potential measurement using the inflection point technique. Finally, the first step of highly emissive plasma sheath measured with laser induced fluorescence is presented. (author)