Electromagnetic ion cyclotron instability driven by ion energy anisotropy in high-beta plasmas

Low frequency (ω≈ωci=eiB0/mic) transverse electromagnetic perturbations propagating parallel to a confining magnetic field B0êz are shown to exhibit instability in the presence of ion energy anisotropy with Ti⊥≳Ti∥. The characteristic maximum growth rate for Ti⊥≫Ti∥ is γM≈(βi⊥/2)1/2ωci, where βi⊥=8πniTi⊥/B20, and the wavelengths corresponding to instability are of order c/ωpi, where ωpi is the ion plasma frequency. Within the context of a quasi-linear model, it is shown that the characteristic time scale for energy isotropization through nonlinear response of the ions to the instability is several γ−1M. Since γ−1M≪τii (the ion-ion binary collision time) in typical high-density pinch experiments, this instablity appears to provide a viable collective mechanism for ion energy isotropization during the implosion or post-implosion phases of these experiments. It is also shown that the instability persists in the limit of weakly magnetized ions (‖ω+iγ‖≫‖ωci‖, k2zr2Li≫1) and strongly magnetized electrons (‖ω+iγ‖≪‖ωce‖, k2zr2Le≪1) provided βi⊥≫1. The instability also has applications to astrophysical plasmas with Ti⊥≳Ti∥ as well as laboratory plasmas heated by neutral-particle injection perpendicular to B0.