Effects of a slow ion beam on ion-acoustic waves

The influence of a slow ion beam on an ion-acoustic wave is studied theoretically and experimentally. A kinetic dispersion relation of an ion-beam-plasma system predicts a critical value of the ion-beam density above which the ion-acoustic wave is transformed into the fast mode of the ion beam. Below the critical value, the ion-acoustic wave propagates without transforming into the fast mode of space charge waves for any beam velocity. A dispersion relation based on a fluid model does not provide such a critical value for the ion-beam density. Experimentally, the ion-beam density is controlled carefully to test the presence of the critical density at which the change in topology of dispersion roots appears