Electromagnetic ion beam instabilities

TLDR

Linear theory predicts electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma. The authors solve the full dispersion relation numerically and analyze oblique propagation properties. They identify four parallel‑propagation instabilities: two right‑hand modes (resonant and nonresonant) for energetic beams, a left‑hand ion‑cyclotron anisotropy mode at low beam speeds with high \(T_{\perp}/T_{\parallel}\), and a left‑hand resonant mode for hot beams, and show that growth‑rate maxima occur at cyclotron harmonics as drift increases.

Abstract

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right-hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T⊥/T∥ gives rise to the left-hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left-hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both right and left elliptically polarized modes.