Low frequency electrostatic waves in a magnetized plasma with heavy negative ions

We have observed large amplitude, low frequency (well below any cyclotron or plasma frequencies) electrostatic waves in a magnetized Q-machine plasma containing positive potassium ions (39 amu), electrons, and heavy negative ions (350 amu). The negative ions were produced by leaking $C_7 F_{14} $ (perfluoromethylcyclohexane) vapor into the Q-machine. $C_7 F_{14} $ has a large attachment rate for low energy electrons (in the Q-machine, $T_e \approx 0.2\,\,\mbox{eV})$, so that a relatively large fraction (${n_- } \mathord{\left/ {\vphantom {{n_- } {n_e }}} \right. \kern-\nulldelimiterspace} {n_e }>10^3)$ of magnetized $C_7 F_{14}^- $ negative ions are formed at neutral pressures $\approx 10^{-5}$Torr. The waves propagate in the azimuthal direction of the cylindrical plasma column. The frequency spectrum of the waves contains narrow features at the fundamental (m=1) and several harmonics. Possible excitation mechanisms being considered are the negative ion-modified drift instability driven by the radial density gradient, and radial shear in the azimuthal ($\vec {E}\times \vec {B})$ drift velocity.