Double plasma and grid excitation of sinusoidal ion acoustic waves

Sinusoidal ion acoustic waves are excited in a double plasma device by applying a signal either between the driver and the target plasma (DP mode) or directly to the grid that separates the two plasmas (grid mode). The amplitude of the waves close to the grid is not a smooth function of frequency but it has distinct extrema. The experimental results are in qualitative agreement with a simple model, which assumes that the waves are excited by modulated ion beams that pass symmetrically in both directions through the grid.