Ion-cyclotron instabilities resulting from a thermally anisotropic plasma

Abstract

Electromagnetic instabilities occurring in a hot-ion, cold- electron plasma are investigated. The instabilities occur in the neighborhood of the ion-cyclotron frequency and result from a left-hand circularly polarized wave propagating parallel to the magnetic field. This mode interacts with the thermally anisotropic ions to produce the instability. Two ion velocity distributions are discussed. The first assumes a monoenergetic distribution transverse to the magnetic field with a Lorentzian velocity shape parallel to it. The second is an anisotropic Maxwellian velocity distribution. Dispersion relations for the two cases are derived and the presence of instabilities is illustrated by several examples. It is found that the instability can be suppressed by lowering the plasma density. Thermal ion velocity spread parallel to the magnetic field is also shown to suppress the instability and specific criteria are presented. For unstable cases the resulting growth rates are compared with those for the electrostatic modes of Harris. Examples applicable to neutral injection machines are discussed and it is shown that the electrostatic modes are dominant.