Excitation and reflection of ion-acoustic waves by a gridded plate and a metal disk

Reflection coefficients and excitation efficiency of a gridded plate (a bipolar potential structure where the grid is separate but parallel to the juxtaposed plate) and a metallic disk for ion-acoustic waves have been measured experimentally. When the grid is biased negatively with respect to the plasma potential, reflection in phase with the incident wave and excitation of an outgoing wave are observed only when the bias of the plate is positive. When the bias of the gridded plate is positive, reflection occurs when the grid bias is either negative or positive. For the metal disk, reflection and excitation are observed when the bias is positive. The small reflection detected when the bias is negative is found to be due to scattering of the incident wave at the edge of the disk. (Author)