Effects of background gas composition on stability and properties of free space microwave-driven plasmas

The Air Force Research Laboratory is investigating the characteristics of a free space plasma sustained by a focused high power microwave beam. A multi-kW, 4.7 GHz microwave system is used to generate quasi-stable and stable plasmas with background gas of various compositions of argon, oxygen, and nitrogen at pressures ranging from 100 to 200 mTorr. The composition of the background gas has been previously shown to have a major impact on the stability of the plasma, however there is limited data with regards to how the gas composition affects plasma parameters of the various discharge modes. Using various plasma diagnostic techniques, the background gas’ effect on the plasma density and temperature are quantified and their relation to the stability properties of the plasma are presented.