Low-Frequency Oscillations in a Thermal Cesium Plasma

Abstract

The oscillation modes present in the ``TOPSY'' thermally generated cesium plasma device have been investigated experimentally; oscillation frequency spectra and three-axis (r,θ,z) maps of relative phase have been studied as functions of axial magnetic field strength, electron density, axial drift current, plasma column length, and ionizer sheath polarity. Both conducting and nonconducting cold end plates were used. Two distinct modes were observed: a low-frequency oscillation (∼1 kHz) which was identified as an ion acoustic wave excited by axial circulating currents in the periphery of the plasma column, and a higher-frequency oscillation (7 to 50 kHz) whose frequency behavior showed close agreement with the theory of the ``universal'' resistive drift-wave instability, but which failed to show the azimuthal propagation predicted by this theory. The lack of observed azimuthal phase shift could not be explained in terms of the transit-time-controlled potential-relaxation oscillations predicted for thermally generated plasmas, by sheath oscillations, or by the reflection of the drift waves at the plasma/end plate sheaths. Both modes were spontaneously present in the ``quiescent'' plasma (i.e., with no net drift current) and were neither further excited nor significantly altered in the presence of strong drift currents.