How air impurities affect plasma in helium microdischarges at atmospheric pressure

The influence of trace air admixtures on the kinetics and macro-response of atmospheric-pressure helium microdischarges with a short 0.2 mm gap has been analyzed using a one-dimensional drift-diffusion fluid model that includes electron energy balance and incorporates air impurity traces within a helium plasma chemistry set. The baseline helium scheme resolves atomic states up to n = 4 levels and includes elastic scattering, excitation/de-excitation, direct and stepwise ionization, associative and Penning ionization, excimer formation/quenching, ion conversion, three-body and dissociative recombination, and radiation. Simulations show that ppm-level air can leave the current–voltage characteristic nearly unchanged while qualitatively rerouting the underlying kinetics: rapid charge transfer and Penning channels shift the cation composition from helium molecular and atomic ions to impurity ions, quench helium metastables/excimers, and modestly cool both electrons and gas via inelastic molecular pathways. These results explain why ‘pure-He’ models sometimes match bulk electrical curves but not species-level physics. They also show impurity regimes where precise He–air chemistry is necessary for understanding diagnostics and for controlling and designing microplasma devices.