Large-Amplitude Ion Acoustic Waves in a Plasma

Experiments are performed on collisionless damping of large-amplitude ion acoustic waves excited externally in a thermally ionized cesium plasma. The wave damping of small-amplitude ion acoustic waves is independent of the wave amplitude, and the waves are damped by Landau damping. For large-amplitude ion acoustic waves, the wave damping is not exponential along the plasma column. Over a few wavelengths from the wave exciter, the waves are damped almost exponentially, though the damping rate increases with the increase of the exciting voltage. After this strong damping, the wave damping begins to saturate and the damping rate becomes smaller than that of the small-amplitude waves. The results are in reasonable agreement with those of the numerical computation carried out by Armstrong on large-amplitude electron plasma waves. Amplitude oscillations are observed for $\frac{e\varphi }{K{T}_i}\gtrsim 0.4$, where $\frac{e\varphi }{K{T}_i}$ denotes fluctuating electric energy normalized with thermal energy of ions. The distance between the amplitude maxima becomes shorter by increasing the exciting voltage or frequency, and is several times longer than the wavelength. The phenomena are well interpreted by the same mechanism as predicted by the nonlinear theories of Al'tshul' and Karpman, and O'Neil on electron plasma waves.