Experimental study on self-modulation of ion plasma oscillations

Experimental study on the self-modulation of externally amplitude-modulated ion plasma oscillations is described. Oscillations were excited by applying an rf voltage with a frequency near the ion plasma frequency to a grid being a wall of the plasma container. The envelopes of such oscillations were observed to break up into several spikes if their amplitudes were above a value. Individual spikes thus formed were found to have similar properties to those of envelope solitons, although the spikes were observed not to move in the laboratory frame. The number of spikes per group was observed to depend strongly on various parameters. Frequency spectra of excited oscillations were studied over various amplitudes of the applied rf voltage. Comparison of these experimental results with theoretical ones indicates that the self-modulation of oscillations observed here occurs due to the modulational instability. (auth.)