Nonlinear modulation of ion plasma waves via parametric processes

Nonlinear modulation of the open-quotes slowclose quotes ion-beam modes in a beam-plasma system is experimentally shown to occur above a threshold of wave amplitude and to progress during propagation. The threshold is found to change depending on the wave frequency at a fixed plasma density and on the plasma density at a fixed frequency. The wave modulation is also studied by means of analysis of frequency spectra. The result reveals that the frequencies of the lower or upper sidebands, generated as a result of the wave endash wave interaction, vary roughly in proportion to the ion plasma frequency fpi, when the plasma density is changed. Finally, on the basis of experimental data obtained here, the nonlinear modulation of the wave is tentatively interpreted to result from the parametric process, based on the three wave interaction satisfying relations ω0=±ω1+ω2 and k0=±k1+k2. copyright 1997 American Institute of Physics