Instability in a Weakly Ionized Afterglow Plasma with a Magnetic Field in a Short Tube

Abstract

In a very short discharge tube, the oscillation with a frequency of a few kHz appears in the afterglow with a relatively weak magnetic field. The frequency of the observed oscillation is much lower than the ion cyclotron frequency. The critical magnetic field B c , when the instability sets up, is proportional to the pressure P and B c / P becomes smaller in the shorter tube. The diffusion deviates from the classical value, when the magnetic field exceeds B c . The oscillation is theoretically analysed to be due to an instability enhanced by the large axial density gradient. The theory is based on the two fluid model which takes the axial density gradient into account, neglecting the ion and electron inertia terms. The observed frequency, the relation between B c and P or the tube length, and the relation between P and the oscillation amplitude can be explained by the present theory.