Low-Frequency Instabilities of a Highly Ionized Magneto-Plasma

Abstract

Measurements are made of the low-frequency oscillations occurring in a low-pressure helium arc plasma immersed in a magnetic field. A spectrum analyzer is employed in the frequency range below 200 kHz to study frequency spectra of signals from magnetic probes, a Rogowskii coil for the discharge current, and a monochromator selecting spectral lines of helium ions in the spatially definite regions of the plasma column. A drift mode instability arises from density inhomogeneity of the plasma in the boundary region and an ion acoustic mode instability occurs in the central region. The two modes are coupled, presenting the spectra similar to that of frequency modulation. The relevant collision effects are negligible and the pertinent instabilities develop due to an axial electron stream.