Measurement of electron densities in weakly ionized atmospheric pressure air

Summary form only given, as follows. Research on weakly ionized atmospheric air is motivated by applications such as reflectors and absorbers of electromagnetic radiation (plasma ramparts), large volume material processing, gaseous pollution remediation, and field enhanced combustion. The electron densities for these applications range between 10/sup 11/ and 10/sup 15/ cm/sup -3/. Particularly, applications as plasma ramparts require electron densities on the order of 10/sup 13/ cm/sup -3/. Measurements of electron densities with sufficient spatial and temporal resolution to explore the development of instabilities, such as glow-to-arc transitions, are essential for the characterization of these air plasmas. Two diagnostic techniques have been identified as best suited for this application: interferometric techniques, and techniques based on emission spectroscopy. Microwave interferometry is well suited for plasmas where the electron density integrated over the path length is less than 10/sup 15/ cm/sup -2/. For higher values of the density times path length integral far infrared lasers are required. Emission spectroscopy measurements of the Stark-broadened Balmer /spl beta/ line of hydrogen represent a very effective technique for the determination of electron densities in excess of 5/spl times/10/sup 13/ cm/sup -3/ with high spatial resolution. This technique is however more difficult to apply at electron number densities below 5/spl times/10/sup 13/ cm/sup -3/ as instrumental and Doppler broadenings become comparable to Stark broadening. Other diagnostic techniques such as Thomson scattering and electric probe measurements are well established for low pressure plasmas. Their potential for the measurement of electron densities in weakly ionized atmospheric pressure air has yet to be explored. We will, in this paper, discuss the potential and the limitations of the various electron density diagnostic techniques for the characterization of weakly ionized atmospheri- air.