Propagation of Ion Free Streaming Waves

Propagation of two ion waves, a fast ion wave and a slow one, was observed in an ion drifting plasma. When the exciting frequency was above the ion plasma frequency, the slow wave disappeared but the fast one did not. The two waves were identified as the ion acoustic waves riding on the ion drifting plasma in the frequency range much below the ion plasma frequency. The fast wave, at frequencies above the ion plasma frequency, was essentially the ion free streaming wave. Dispersion relation of ion free streaming wave was derived on the assumption that the drifted Maxwellian distribution as the initial distributed. The computed phase velocity and damping rate were in agreement with the experimental results.