Excitation and Measurement of Acoustic Ion Waves

Abstract

Measurements of the phase velocity and the attenuation constant of acoustic ion waves in weakly ionized gases in the frequency range from 0.1 to 6 MHz are described. The experiments were performed in continuous rf discharges having ion plasma frequencies of the order of 1 MHz. A beam of plane ion wares was excited and received by grids with diameters much greater than the wavelengths. At frequencies well below the ion plasma frequency, the phase velocity is frequency-independent and approximately given by the Tonks-Langmuir speed; the predicted dependence on ion mass is verified. The attenuation in this frequency range is primarily due to ion-neutral collisions. At higher frequencies, the phase velocity increases slightly, whereas the attenuation per wavelength falls somewhat off. These results can be explained by assuming non-Maxwellian velocity distribution of the ion gas. The measured phase velocity and attenuation were found to be independent of the geometrical dimensions of the grids, independent of the signal voltage applied to the excitation grid, and not influenced by drifts of the charged particles.