Electron density and collision frequency measurements in a pulsed, high power microwave discharge in low pressure air

The authors have measured electron densities and collision frequencies in low pressure microwave discharges using microwave interferometric techniques. Microwave discharges were formed at the constructive interference regions of an S-band microwave beam which was incident on an inclined metal plate. The discharge normally consisted of ionization layers parallel to the plate. With suitable choices of pressure, microwave power and pulse duration, it was possible to form single layer breakdowns. The range of pressures investigated was from 0.01 Torr to 3.0 Torr. They employed an X-band (9.60 GHz) Mach-Zehnder interferometer which probed normal to the plane of the discharge. Analysis of the data indicates that under single plasma layer formation conditions, the electron densities frequently reached overdense levels (for the S-band frequency), and that the collision frequencies (primarily dependent on interferometer scene power attenuation) were observed to be up to an order of magnitude higher than expected based on known momentum transfer cross section data. This presentation describes the interferometer technique. The results of the microwave and optical measurements are discussed