Excitation and Detection of Acoustic Waves in Plasmas.

摘要

This report contains initial results of an experimental study of acoustic disturbances in weakly ionized gaseous plasmas which exist in the positive column of dc discharges. One phase of the research concerns itself with the effects of externally perturbing such a medium with acoustic waves emanating from transducers that operate in the audio and ultrasonic frequency ranges. The balance of the effort is devoted to the observation of sound waves generated by natural and uncontrolled low-frequency oscillations which commonly occur in laboratory discharges. Compact transducers incorporated into cylindrical discharge tubes provide the source of acoustic signals and serve as microphones in detecting sound waves. Modulation of electromagnetic waves by the intentionally disturbed plasma was measured in a novel rectangular cavity whose output was detected and fed to a wave analyzer. Resultant wave-analyzer responses indicate that the electron collision frequency was modulated in addition to the electron plasma frequency, and that the variations in both were proportional to the magnitude of loudspeakerdiaphragm deflection, as predicted by simple acoustic theory. (Author)