Ion-acoustic instability in the positive column of a helium discharge

Abstract

The ion-acoustic instability of the helium positive column in the pressure regime 0.05 to 0.25 Torr is studied. Instability of the column is predicted by a kinetic theory which includes the effects of collisions with neutrals, but not by a hydrodynamic theory. In the regime of the experiment the ion-acoustic instability arises from the resonant interaction of those electrons having velocities close to the phase velocities of the ion-acoustic waves (inverse Landau damping) even though the electron-neutral collision frequency exceeds the ion-acoustic frequency. The helium positive column is well suited to the study of this instability mechanism because the important velocity hierarchy ion thermal velocity ≪ ion-acoustic phase velocity ≪ electron drift velocity ≪ electron thermal velocity is naturally well established. The detection of the spontaneously excited waves is found to be crucially dependent on the bias of the detecting probe. The onset characteristics, spectral properties, spatial distribution, and saturation intensity are measured and compared with theoretical predictions.