Non-equilibrium steady-state kinetics of He-air atmospheric pressure plasmas

A non-equilibrium, steady-state collisional-radiative kinetics model is developed to study atmospheric pressure discharges produced in He mixed with dry air (79% N-2 and 21% O-2). The model is based on a self-consistent solution of the Boltzmann equation for the electron energy distribution function coupled to a system of non-linear equations for species that govern plasma chemistry (electrons, ions, radicals, atoms, and molecules in ground and excited states). The main plasma parameters, including the maintaining electric field and species densities, are obtained as a function of He-to-air ratio. The results indicate that the concentration of air strongly influences the plasma. Notably, the He metastables and ion densities collapse at air concentrations above 0.1%, while the power required to maintain the plasma sharply increases as the concentration of air exceeds 1%. The model is applied to study the plasma characteristics along the length of an atmospheric pressure plasma jet using He as a carrier gas.