Understanding the Impact of Buffer Gases on the Radar REMPI Diagnostic
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2015, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. This paper has been motivated by a need to gain insight into the effect of buffer gases on the radar REMPI signal. It was observed in a previous study that our peak signal appeared to be inexplicably dependent on the choice of buffer gas despite the target species concentration remaining unchanged. While this unwanted dependence can be avoided by maintaining the same gas mixture and density, such a constraint is especially undesirable for applications like combustion, where the gas composition is sometimes unknown and challenging to replicate. We examine three possible reasons for this phenomenon, namely the effect of (i) a limited system response time, (ii) quenching collisions and (iii) the electron-neutral collisional frequency. Our results show that all three explanations are valid to varying degrees. While the first two reasons are readily understood and can be potentially avoided, the effect of the electron collisional frequency is more far-reaching. If its impact is real, it represents an additional parameter that needs to be considered when analyzing signals obtained from different gas mixtures.
