Heating of Counterstreaming Ion Beams in an External Magnetic Field

TLDR

Ion heating by a strong ion‑ion two‑stream instability perpendicular to a magnetic field occurs in a cold electron background, with straight ion trajectories and electrons bound to field lines. The study examines the potential role of this instability in high Mach number shocks. The authors develop quasilinear and nonlinear electrostatic theories for the instability and compare them with computer simulations. Quasilinear theory accurately describes the system until ion trapping becomes significant, after which nonlinear stabilization occurs when the trapping period matches the inverse growth rate, and the directed beam energy is largely converted into random ion energy.

Abstract

Ion heating by a strong ion-ion two-stream instability perpendicular to a magnetic field in the presence of a relatively cold electron background (Te≪miVd2) is considered. The magnetic field strength is such that the ion trajectories are straight, whereas the electrons are bound to the field lines (krLe≪1≪krLi). Theory is presented for both quasilinear and nonlinear stages of the evolution of the system for the case that the instability is electrostatic [(B2/8π) (1+β) >nmiVd2/8] and is compared with a series of computer simulation experiments. It is found that the quasilinear theory gives a fairly accurate description of spatially averaged plasma properties until the ion beams have been sufficiently modulated for ions to be trapped by the waves. In the subsequent nonlinear stage, stabilization occurs when the ion trapping period is equal to the reciprocal growth rate associated with the instability. The directed ion beam energy is mainly converted into random ion energy. The possible role of this instability in high Mach number shocks is discussed.