Nonlinear evolution of the ion-ion streaming instability in single- and multi-ion species plasmas*

When two streams of ions flow through one another, the relative flows may drive the growth of large-amplitude ion acoustic waves via the ion-ion streaming instability (IISI). We study the linear and nonlinear evolution of the IISI using a 2D+2V high-order continuum method novel to this problem. The electrostatic energy generated by the IISI is converted into ring-like velocity distributions of ions that are both heated and slowed. Due to variation in the ion trapping conditions for species of differing charge-to-mass ratio, we find that the plasma streaming velocity may be altered radically by the IISI. Here, this process causes the collisionless stopping of a lighter ion species by a heavier ion species. When the two streams each contain a mixture of species, the differing ion trapping conditions cause a velocity separation of the ion species. We observe that the heavier ion species emerges from the interaction significantly hotter than the lighter ion species, and can even be heated to a temperature significantly above that of the electrons.