The influence of the Ar/O2 ratio on the electron density and electron temperature in microwave discharges

The aim of this work is to study the properties of Ar-O-2 microwave driven surfatron plasmas as a function of the Ar/O-2 ratio in the gas mixture. The key parameters are the plasma electron density and electron temperature, which are estimated with Thomson scattering (TS) for O-2 contents up to 50% of the total gas flow. A sharp drop in the electron density from 10(20) m(-3) to approximately 10(18) m(-3) is estimated as the O-2 content in the gas mixture is increased up to 15%. For percentages of O-2 lower than 10%, the electron temperature is estimated to be about 2-3 times higher than in the case of a pure argon discharge in the same conditions (T-e approximate to 1 eV) and gradually decreases as the O-2 percentage is raised to 50%. However, for O-2 percentages above 30%, the scattering spectra become Raman dominated, resulting in large uncertainties in the estimated electron densities and temperatures. The influence of photo-detached electrons from negative ions caused by the typical TS laser fluences is also likely to contribute to the uncertainty in the measured electron densities for high O-2 percentages. Moreover, the detection limit of the system is reached for percentages of O-2 higher than 25%. Additionally, both the electron density and temperature of microwave discharges with large Ar/O-2 ratios are more sensitive to gas pressure variations.