This study investigates electron temperature (T_e) and density (n_e) in atmospheric-pressure plasma jets by considering quenching between metastable helium and nitrogen/oxygen molecules in the air.
Optical emission spectroscopy was performed in the plasma plume region. Based on the framework by Lin et al. (2023), the collisional-radiative model was extended to include Penning ionization. Parameters (Te, ne, metastable densities, and He/air ratio) were optimized via grid search by minimizing the discrepancy between measured and calculated population densities of helium excited states.
The He+Air model yielded Te = 1.21 eV and ne = 3.9 × 10^{11} cm-3. In contrast, the Pure He model gave an unrealistic 0.23 eV. Metastable densities dropped by ~6 orders of magnitude in the He+Air model due to quenching. Although the He+Air model had a higher fitting error, its results are physically consistent with active streamer discharges. The He+Air model provides the physically correct solution.