Ion-beam–plasma electromagnetic instabilities

Abstract

It is well known that ion-beam–plasma interactions can destabilize right- and left-hand polarized electromagnetic waves. Owing to the fact that these instabilities have mostly been studied numerically by solving the hot-plasma dispersion relation, their fluid nature has often gone unnoticed. Choosing the ion background to be the rest frame, it is shown that the right-hand polarized instabilities are the result of a merging of the magnetosonic/electron-cyclotron branch of the dispersion relation with the ion beam. For any given ion-beam density and sufficiently large beam velocity, there are always two right- and two left-hand polarized instabilities leading to forward-propagating electromagnetic waves. It is also shown that all right-hand polarized instabilities are resonant instabilities, satisfying ω−kU+Ωp ≈ 0 around their maximum growth rate (ω and k are the frequency and the wavenumber respectively, U is the beam velocity, and Ωp is the proton gyrofrequency). Likewise, when the two left-hand instabilities are simultaneously present, they are also resonant instabilities satisfying ω ≈ Ωp. The high-frequency right-hand resonant instability (ω [Gt ] Ωp) has a maximum growth rate that depends only on the ratio between the beam density and the total density. The range of the unstable spectrum decreases with increasing beam velocity, leading to highly monochromatic radiation.