Observations of Ionic Sound Waves in Plasmas: Their Properties and Applications

This paper discusses extensive experimental studies of the ionic sound waves predicted by Tonks and Langmuir. Self-excited oscillations have been observed both in magnetically supported plasma columns and in spherical discharge tubes. The fundamental frequency and the overtones of each system agree with those predicted by the ionic-sound-wave formula in terms of the electron temperature, the ion mass, and the dimensions of the system. A simple equation is used to predict when damping by the neutral gas prevents a given mode of plasma oscillation from being observed. The thermal electron pressure, required for the existence of an ionic sound wave, has been measured mechanically. Resonance effects have been observed between the ionic-sound and the ion cyclotron frequency in the magnetically supported plasma column. Practical applications of ionic sound waves are as follows: The electron temperature of a plasma was found by measuring the frequency of a standing ion-wave mode. The neutral gas density was estimated by observing the gas damping of various standing ion-wave modes.