Fs-TALIF for Low Pressure Interfacial Plasmas

Plasma-surface interaction has been an important area of research for many aerospace applications. Therefore, there is a need to advance its fundamental knowledge using advanced laser diagnostics by measuring plasma-generated species with high spatial and temporal resolutions close to a solid surface. This work presents a study on low-pressure (≤ 1 torr) plasmas using femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) of nitrogen around a alumina ceramic substrate. The experimental setup consists of an air low-pressure plasma produced by a radio-frequency power source. The TALIF signal is found to follow a square dependence on pump laser energy up to ∼ 20 µJ2 before reaching saturation. One-dimensional spatial distribution results for atomic nitrogen concentration were found to be in the range of 1012 − 1014 cm-3 for different plasma source powers. Concentration values are compared to literature data, showing good agreement. Additionally, the dissociation degree is found to be increasing with decreasing pressure which compares well with literature. Ultimately, the spatial distribution profiles indicated an increase in nitrogen concentration with power and towards the alumina ceramic substrate. However, a threshold in concentrations was observed at lower pressures where further increases in power did not proportionally increase atomic nitrogen, possibly due to low molecular nitrogen concentration density and wall quenching effects due to higher diffusivity.