Measurements are reported on low-frequency self-excited oscillations in an inhomogeneous afterglow plasma, in which there is a radial density gradient perpendicular to an axial homogeneous magnetic field. The plasma was produced either by using a short pulse of electrons emitted from a hot cathode to ionize the background gas, or by pulsing off an rf discharge. The plasma so formed had an initial density of ∼ ${10}^{10}$ ion ${\mathrm{cm}}^{-3\mathrm{}}$ and a temperature $T_e\sim 1000$ °K. The experiment was carried out using tubes of 2.5 and 5.0 cm diam, giving inverse density scale lengths for the radial density gradient of ∼ 2.3 and ∼ 1.3 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. The oscillations occurred mainly as $m=+1\mathrm{}$ azimuthal modes although some results for $m=+2\mathrm{}$ were obtained. The oscillations were shown to be standing waves in the axial direction $z$ with a wavelength ${\lambda }_z$ approximately equal to the column length. It was found that the frequency of oscillation $\omega$ was dependent on the column length and by varying this, a dispersion relationship $\omega$ vs $k_z=\left(\frac{2\pi }{{\lambda }_z}\right)$ could be plotted under various conditions. It was found that the frequency was independent of time (or density) in the afterglow and that the frequency of oscillation tended to zero as $k_z\to 0$ and to ${\omega }^{*}$ as $k_z$ assumed larger values. The simple theory of drift dissipative instabilities has been extended to include effects due to electron-neutral and ion-neutral collisions and the $\mathrm{Re}\left(\omega \right)$, and growth rates $\mathrm{Im}\left(\omega \right)$, vs $k_z$ have been calculated for the relevant experimental cases. Comparison of theory and experiment shows remarkably good agreement considering the possible errors in some of the experimental quantities and the assumptions present in the theory.

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