Ion phase space vortices in ion beam-plasma systems and their relation with the ion acoustic instability: numerical and experimental results

Numerical simulations and experiments were performed in order to clarify the relation between ion phase space vortices, the ion acoustic instability and shock-like perturbations in ion beam-plasma systems. The simulations show that the formation of vortices in the ion phase space occurs only when the ion beam-plasma system is linearly unstable. The size and characteristic formation time of these vortices are in good agreement with predictions based on the wavelengths and growth rates of the linear one-dimensional ion acoustic instability; this confirms that vortices result from the saturation of the instability. Experiments on step-like perturbations in the ion beam-plasma system were performed in a DP device. They show the formation of extended 'hump and dip' regions behind the beam-induced shock front. Inside these regions localized occurrences of the ion acoustic instability have been seen. In contrast with the ion vortices, these structures do not result from the instability. These findings are in qualitative agreement with the results of numerical simulations designed after the experiment.