Experimental observation of parametric decay instability for ion acoustic waves

We observed, by microwave scattering method, that two ion acoustic waves with frequencies f 0 /3 and 2 f 0 /3, and wave numbers k 0 /3 and 2 k 0 /3, were simultaneously excited in an argon positive column, when the amplitude of the pump ion acoustic wave with f 0 and k 0 exceeded a certain value. The pump wave was excited externally by means of a grid. The value of f 0 was well below the ion plasma frequency f pi. The threshold value agreed roughly with a theoretical one calculated from the coupled equations for the subharmonic waves.