Influence of small-fraction N₂ impurity on the lifetime of metastable He atoms experimentally investigated with a 10-ppm resolution

Excited helium atoms in metastable states (He^m) play a crucial role in high-pressure He plasmas owing to their long lifetimes and high excitation energies. In the afterglow, energy transfer from He^m to other gas species is key to the whole reaction systems. The influence of small-fraction impurities on high-pressure plasmas has been mainly studied by numerical simulation, which can tune the impurity fraction artificially. However, comparisons between numerical and experimental results have been insufficient due to the difficulties in precisely controlling impurities in experiments. In this study, we developed an experimental setup for dielectric barrier discharge (DBD) that can control N₂ density in He gas while maintaining other impurity levels below several tens of ppm. The discharge experiments were conducted over an N₂ concentration range from near zero to 3000 ppm with a 10-ppm resolution, while He pressures were set to 10, 50, and 100 kPa. We measured the decay rate of He^m density (He^m decay) in the afterglow of the He-DBD using time-resolved laser absorption spectroscopy. At 10 kPa, the He^m decay increased linearly with respect to N₂ density, and its slope showed good agreement with the reported rate coefficients for He^m + N₂ → He + N₂⁺ + e⁻_. At pressures higher than 50 kPa, the dependence of He^m decay on N₂ density suggests that the generation of He dimer (He₂) affects He^m decay under nearly pure He conditions.