Effective ionization coefficient in mixtures of Ar and O2 determined using the Townsend discharge

Precise knowledge of the fundamental ionization properties of gases, such as the effective ionization coefficient, is crucial for discharges in mixtures of Ar:O2, which are significant for a wide range of plasma applications. This study determined the effective ionization coefficient in electronegative gas mixtures of Ar:O2 in the pressure range of 10–800 Torr and reduced electric field strength E/N range of 40–1200 Td utilizing a steady-state non-self-sustaining Townsend discharge. The reduced effective ionization coefficient αe/N increased with E/N and decreased with increasing O2 content in the gas mixture. The experimental results were compared with a model which was based on calculating the ionization and attachment coefficients with BOLSIG+. The ion conversion of O− to O2−, detachment from O2−, and formation of O3 were accounted for similarly as has been done with N2:O2 mixtures. Reasonably good agreement between the measurements and the model calculations was achieved for Ar:O2 mixtures with the O2 content between 20% and 70%. A discrepancy of more than 20% between measurement and calculations was observed at low E/N values when the O2 content was below 20% and at high E/N values when the O2 content was above 70%. Several possible explanations were proposed for the observed discrepancy; however, more elaborate models are required. The reduced critical electric field E/Ncrit, where the apparent effective ionization coefficient is zero, was determined as a function of the O2 content in the Ar:O2 mixtures. E/Ncrit increased with increasing O2 content in the mixture.