Nonlinear saturation of the drift cyclotron loss-cone instability

Abstract

The nonlinear behavior of the drift cyclotron loss-cone instability close to a stability boundary is considered. The time-asymptotic small-amplitude saturation of a single mode is determined by a variation of the Bogoliubov method. A collisionless plasma model is used with no mirror losses or particle sources. Flute-like local perturbations are considered. The stability boundary in density, density-gradient space is determined for several different ion loss-cone distributions. The modified equilibrium ion distribution function, the electric field, and the amplitude of the fluctuating electrostatic potential are then obtained as a function of the fractional increase in density gradient above the critical value. The nonlinear frequency shift remains indeterminate. In all cases, stable saturation occurs and is due to a slight filling-in of the loss cone. Comparison with numerical simulation results show good agreement when the high thermal level of neighboring modes is artificially suppressed.