Experimental Studies of Explosive Instability in an Ion Beam-Plasma System

When the ion beam-plasma system is stimulated by a pump field with frequency f o near the ion Langmuir frequency f pi , the ion sound wave ( f s ) is enhanced and, at the same time, a new wave ( f b ) corresponding to the slow wave of the beam appears, and among their frequencies, the resonant condition, f b = f pi + f s , is satisfied. The resonant condition for the wave numbers, k b = k pi + k s , is ascertained by a test wave method. The test wave grows temporally at the frequencies coinciding with the frequencies of f b and f pi and, at the same time, the wave with the frequency of f b - f pi = f s grows also temporally. Their growing is affected by an ion beam energy, an initial intensity of the pump field and its frequency. These phenomena will be discussed with a nonlinear theory of the explosive instability for the coherent three-wave interaction.