Observation of modulational instability in a multi-component plasma with negative ions

The non linear evolution of ion-acoustic waves in a multi-component plasma with negative ions has been studied experimentally using a double-plasma device. At a critical concentration of negative ions the phase velocity of the ion-acoustic waves increases with the wave amplitude, and the wave has negative dispersion, which show that the wave satisfies Lighthill's condition. The initial small modulation in amplitude is found to grow spatially. The observed phenomena are compared with the relevant theory using the nonlinear Schroedinger equation. The fluid equations are integrated to confirm the validity of the nonlinear Schroedinger equation. (author)