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.
Beam Velocity and Density Influences on Ion-Beam Pulses Moving in Magnetized Plasmas