Time-averaged electron number density measurement of a one atmosphere uniform glow discharge plasma (OAUDGP) by absorption of microwave radiation

Abstract

Summary form only given. A simple and accurate technique for measuring the time-averaged electron number density of a plasma at atmospheric pressure is described. It is based on the angle-of-incidence dependent response of the plasma absorption to the incident microwave signal. The method is applied to determine the time-averaged electron number density of a One Atmosphere Uniform Glow Discharge Plasma (OAUDGP) generated in air on a dielectric-coated reflective substrate on which a series of parallel, plasma-generating electrodes are located. In the case of microwave propagation through this plasma, the refractive index of the propagation medium is complex, and the penetrating wave drops off exponentially from the boundary of incidence. The flux density drops by a factor of e/sup -1/ after the wave has propagated a distance /spl delta/, known as the skin depth. The skin depth is inversely proportional to the square root of the electron number density. The author of reference has developed a steady-state, uniform glow discharge plasma at one atmosphere pressure that is generated in a thin surface layer in air and other gases. This OAUDGP layer is generated by applying an RF signal of 1-20 KHz on a dielectric flat panel covered with parallel strip electrodes. The electron number density of the plasma layer is periodic with the applied RF signal. If microwave radiation propagates through the plasma layer, the attenuation is proportional to the round-trip path length of the reflected wave. A 20 dB horn antenna, operating at 12 to 18 GHz, was used both at the transmitter and the receiver end. We compared the change in the intensity of the reflected signal with the plasma layer on and off. At normal incidence, our experimental results should agree with a relatively straightforward numerical calculation based on Appleton's equation. Finally, we hope to extend the experiment to stroboscopic electron number density measurement of the electron number density as a function of time.