Microinstability of a Mirror-Confined Plasma

Abstract

Infinite uniform magnetized plasmas with non‐Maxwellian particle distributions are subject to electrostatic longitudinal instabilities of the type discussed by Harris. Mirror confinement leads to depletion of low energy ion populations and thus to the Rosenbluth‐Post ``maser'' instability at frequencies ω » ω_ci, the ion gyrofrequency, and to the flute‐like instability of Dory, Guest, and Harris with ω ≃ ω_ci and wave vector normal to the external magnetic field. This study is concerned with unstable waves which propagate obliquely with ω ≃ ω_ci and wavelength ≲ ρ_g, ion gyroradius. For their appearance it is necessary that ω_pe/ω_ci exceed a threshold value of order n(1 + 8a²v²_∥e / v²_⊥i)^½, where n is the harmonic number of the unstable wave, and v_∥e and v_⊥i are the components of electron and ion thermal velocity, parallel and perpendicular to the external magnetic field. Over a wide range of interesting conditions, the parameter a lies between 2 and 3. Furthermore, the plasma is stable if nω_ci ≲ ak_∥v_∥e or if ω_pe ≲ ak_∥v_∥e, where k_∥ is the parallel component of the wave propagation vector and ω_pe is the electron plasma frequency. These waves are not fully stabilized by increasing ion temperature parallel to the field as are the anisotropy modes in a two‐temperature Maxwellian plasma.