Diagnostics of high-pressure plasmas generated in helium and molecular impurities

In experimental plasma studies, it is impossible to generate plasma in perfectly pure gas atmosphere. Molecular impurities such as water vapor and/or ambient air (N₂/O₂) exist in the discharge space. The influence of molecular impurities is critical, especially in high-pressure plasmas, owing to high-frequency collisions between excited species and background gas species. For instance, in atmospheric-pressure plasmas in He gas, nonelastic collisions of excited He atoms in metastable states are key to controlling the reaction systems, and the presence and fraction of molecular impurities significantly affect the generation of ions and excited species in and around the discharge space. The influence of molecular impurities has been extensively studied through numerical studies; however, experimental studies are still limited owing to difficulties in controlling small-fraction molecular impurities.

The author has attempted to understand the influence of molecular impurities through experimental approaches. One is to control the gas temperature reducing it below the boiling point of the molecular impurities. Significant transitions in the discharge behavior were observed, particularly at the boiling points of H₂O, N₂, and O₂. It was also possible to observe a nearly pure He discharge at cryogenic temperatures. Another approach is to control molecular impurities in a glass cell filled with He gas at room temperature. Based on our understanding of impurity behaviors in the He-gas discharge setup, we fabricated a closed gas cell with molecular impurities at levels below several tens of ppm. The results of the experimental studies are expected to examine reaction models proposed by numerical studies and gain understandings of discharge dynamics for further development of high-pressure plasma technologies.