Ion-Wave Instabilities in Mercury-Vapor Plasma

The Landau dispersion equation $2k^2={k_{\mathrm{De}}}^2{Z}^{\prime }\left({\zeta }_e\right)+{k_{\mathrm{Di}}}^2{Z}^{\prime }\left({\zeta }_i\right)$ is studied experimentally for $\omega ={\omega }_r+i{\omega }_i$, where ${\omega }_i=0\mathrm{}$ determines the stable-unstable boundary of ion waves, $k_{\mathrm{De}}$ and $k_{\mathrm{Di}}$ are the electron and ion Debye wave numbers, and $Z\left(\zeta \right)$ is the plasma dispersion function. The decrease of the phase velocity with the frequency predicted by dispersion relation is observed for the spontaneously excited ion waves in the mercury-vapor discharge. The cutoff frequency beyond which no ion waves appear is also observed. The cutoff frequency increases with the electron drift velocity in the plasma. The dependence of the cutoff frequency on the electron drift is explained by the ion-wave instability in a two-Maxwellian-component plasma. One-way propagation of externally excited ion waves is also shown.