Abstract-Experimental results are presented for studies of low (2-20 kHz) and intermediate-frequency (20-100 kHz) oscillations in crossed-field closed electron-drift Hall discharges. Conditional sampling using two electrostatic probes is used to identify and extract properties of coherent structures associated with the propagation of azimuthal and longitudinal instabilities within the discharge channel. The azimuthal component phase velocities are determined for a wide range of wave frequencies and over characteristic regimes of operation of these devices. A variety of propagation modes are observed and analyzed, including the appearance of an induced mode due to the presence of the probes themselves. This later result is believed to be the first direct evidence of how fluctuations can be influenced in these Hall discharges using relatively simple actuation methods.

Using recent experimental data on the time-averaged, spatially varying plasma properties within a Hall discharge plasma, we present in this article, a theoretical study of the response of this plasma to small (linear) perturbations in its properties. As a starting point for this analysis, we assume a two-dimensional fluid description that includes a simplified equation for the electron energy, and constrain the azimuthal wave vector such that we excite only the dominant (m=1) azimuthal modes. The growth rate and frequencies of predominantly axial and azimuthally propagating plasma disturbances are obtained by numerical solution of the resulting eigenvalue problem under a quasiuniform plasma condition, along the entire discharge channel. The results identify the persistence of a low frequency instability that is associated with the ionization process, concentrated largely in the vicinity of the exit plane, where the magnetic field is at its maximum value, consistent with experimental observations for the relatively low operating voltages (∼100 V) considered in this study.

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