Experiemental study of interaction of microwaves with a nonmagnetized pulsed-plasma column

Abstract

The time evolution of backscattered and sidescattered microwaves from a plane conductor covered by a pulsed-plasma column has been studied; transmitted signals along the axial and transverse directions of the column have also been studied. The incident microwaves are at a cw power level varying from a few milliwatts to a few watts. The cw nature of the signal makes it possible to observe the fine structure in the time evolution of the interactions. The plasma is generated by an array of plasma guns embedded in the conducting plate. The plasma density has a gradient in the axial direction and contains regions of subcritical and supercritical plasma density. Reductions in the backscattered radiation down to the lowest measurable levels with available equipment are observed for durations that are an order of magnitude longer than the duration of the current pulse that generated the plasma. Variations caused by changes in the charging potential on the plasma gun capacitor bank, the pressure of the background gas, and the frequency and power level of the incident microwaves are reported. The attenuation in the backscattered radiation decreases with increasing pressure, especially at lower charging potentials. There is a close correlation betwe~n the timing and location of sidescattered signals and the transition of the critieal density layer across the axial position of the detectors for the scattered signal. The amplitude of the scattered signal was also measured. Estimates have been made for the percentage of incident power accounted for by scattering. Models are discussed for absorption of microwaves in the plasma.