Current driven electromagnetic ion cyclotron instability

Abstract

When an electron distribution drifts relative to the ions along a d.c. magnetic field, it is known that, above some critical drift velocity, a nearly field-aligned electromagnetic ion cyclotron instability may be excited. We extend the study of this instability over wide variations in plasma parameters, ion β in particular, and beyond marginal stability.Above threshold the most unstable waves propagate very obliquely to the ambient d.c. magnetic field at wavenumbers of the order of an inverse ion Larmor radius. At low ion β the critical electron drift normalized to the ion thermal velocity scales inversely as β _i- ½ while, for β _i>10-₂, the critical drift scales as the ion thermal velocity. For the T_e≈T_ielectromagnetic ion cyclotron instabifity begins to have a lower threshold than the corresponding electrostatic instability at β _i≈m_e/M_i. In a moderately high β _i, homogeneous, collisionless plasma the electromagnetic ion cyclotron instability appears to have the lowest threshold of any current driven instability.