Effect of ion acoustic instabilities on the energy-angle distributions of the ions impacting on the wall of a finite-size plasma

In this work, we have numerically investigated the effect of ion acoustic instabilities on the energy-angle distributions of the ions impacting on the wall of a finite-size plasma. Ion acoustic instabilities develop in non-equilibrium plasmas having electron temperature much larger than the ion temperature. As revealed by a dispersion analysis, this instability is also affected by the drift motion of the ions, normally encountered in the sheath and presheath of a finite-size plasma interfaced with a material wall. The analysis was performed using the hPIC Particle-in-Cell code. Simulations show that at large temperature ratios the ion distribution develops a high-energy tail triggered by finite-amplitude acoustic waves. The instability has the net effect of increasing the ion impact energy on the walls above the expected thermal ranges, and of reducing the ion pitch angle at the time of impact. A systematic parametric analysis as a function of the temperature ratio is reported, showing the gradual onset of the instability and its effect on the energetic tails of the distributions.