Quantification of molecular impurity ratio in high-pressure helium dielectric barrier discharge by laser absorption spectroscopy

In high-pressure low-temperature plasmas in rare gas flow, presence of impurities has critical influence on the discharge characteristics. Simulation studies have indicated that ppm-level variation of molecular impurities affects major positive-ion species and reactive neutral species. Therefore, in experimental studies on the high-pressure plasmas, it is indispensable to monitor the ratio of molecular impurities with a method not disturbing the discharge. In this study, we investigated molecular impurities in a helium (He) gas flow utilized in a dielectric barrier discharge (DBD). The DBD configuration is a tubular type that widely used in plasma jet studies. We estimated species of the molecular impurity from optical emission spectroscopy (OES) diagnostics. OH, H, and O emission suggested that a major molecular impurity in our setup was H₂O vapor. A laser absorption spectroscopy (LAS) was performed to measure lifetimes of excited He atoms in a metastable state (He^m). In order to consider the influence of H₂O dissociation by the DBD, we measured dependence of the He_m lifetime on the applied voltage frequency by the LAS method. From the frequency dependence, we calculated the H₂O ratio in the He gas flow using a low-frequency-limit He^m lifetime.