Non-uniform (dust) Plasma Ion Acoustic Wave Excitation And Propagation

The thesis focused on the excitation and propagation of the ion-acoustic waves in inhomogeneous (dusty) plasmas, including theories and experiments.By assymmetric discharges in a double (dusty) plasma device, an inhomogeneous plasma, in which the axial density was inhomogeneous while the radial density almost remained uniform, was produced at the right side (experimental region). The scale length as well as the distribution character of the plasma density can be controlled by adjusting the gas pressure or introducing an separating grid between the source and the experimental regions.The ion-acoustic waves were excited by using the grid excitation method. Several kinds of voltage signals, including the single period or double periods sine waves, step or ramp signals, low-frequency signals with irregular shapes and other arbitary signals, were applied externally to the excitation grid. The evolution of the wave form when propagating along axial direction and its relation with the externally applied signals were checked. The experimental results showed that some other wave modes, such as the slow ion-acoustic wave and the ion beam mode, were excited spontaneously with the ion-acoustic wave. Generally, the slow ion-acoustic wave as well as the ion beam mode was damped much faster than the ion-acoustic wave, and a relatively pure ion-acoustic mode can be excited by selecting the proper external parameters. The experiments also showed that, for an applied signal with much lower frequency than the ion plasma oscillation rate, the shape of the received waves closely followed that of the time derivative of the applied signals, which was interpreted theoretically by using a quasi-steady sheath model.