Nonlinear Development of Electromagnetic Instabilities in Anisotropic Plasmas

TLDR

The study presents theory and simulations of transverse electromagnetic instabilities in anisotropic plasmas with various sources. Experiments confirm quasilinear predictions, demonstrate conservation of nonlinear energy constants, and show that magnetic energy saturates when the bounce frequency equals the linear growth rate, implying magnetic trapping limits amplitude while leaving residual anisotropy.

Abstract

Theory and simulation experiment are presented for a wide variety of transverse electromagnetic instabilities in plasmas with different sources and degrees of anisotropy. In each of the electron bi-Maxwellian, electron-pinch, and ion-pinch experiments, the bulk response of the system during the initial stages of instability is in good agreement with the predictions of quasilinear theory. Furthermore, the two independent energy constants which derive from the fully nonlinear Vlasov-Maxwell equations are found to remain constant to very good accuracy, even when the magnetic field energy reaches a substantial fraction of the total system energy. In each simulation experiment it is found that the magnetic energy saturates once the magnetic bounce frequency has increased to a value comparable to the linear growth rate prior to saturation, i.e., when ω¯B∼γ¯k. It is concluded that amplitude limitation for Weibel instabilities is a result of magnetic trapping for a broad range of system parameters. In many experiments a large remnant anisotropy in kinetic energy persists long after saturation.