Investigation of He spectra using a collisional-radiative model for plasma diagnostics of dielectric barrier discharges

We have performed optical emission spectroscopy to measure He spectra from pure He, or N₂ and/or H₂ mixed He dielectric barrier discharge plasmas for near atmospheric pressures (APs) up to 200 Torr. Two reactor configurations, coaxial cylinder and parallel plates, were examined for an applied AC voltage of 1-5 kV at 20 kHz. The obtained He line intensity ratios were investigated using a He collisional-radiative (CR) model to determine the plasma parameters T_e and n_e, which are critical parameters to understand plasma-assisted catalysis for ammonia synthesis. The He CR model, originally developed by Goto [1], was integrated with AP reactions such as heavy-particle collisions between He atoms and molecules [2]. Our model analysis shows that He excited-state populations for AP plasma are strongly affected by the population density of metastables He(2¹S) and He(2³S), which were treated as free parameters determined together with T_e and n_e. We utilized a zero-dimensional kinetic model developed for N₂-H₂-He AP plasmas to estimate these metastable populations. [1] M. Goto, J. Quant. Spectrosc. Radiat. Transfer 76, 331 (2003). [2] W. Lee et al., Phys. Plasmas 27, 073502 (2020).