Nonlinear evolution of a strongly sheared cross-field plasma flow

Abstract

A study is presented of the nonlinear evolution of a magnetized plasma in which a localized electron cross-field flow is present. The peak velocity of the flow is denoted by V0; LE represents the flow’s shear scale length; and the regime ρe<LE<ρi is considered, where ρi and ρe denote the ion and electron Larmor radii, respectively. It is shown that if the shear frequency ωs=V0/LE is larger than the lower-hybrid frequency, ωLH, then the system dynamics is dominated by the onset of the electron–ion-hybrid (EIH) mode which leads to the formation of coherent (vortexlike) structures in the electrostatic potential of the ensuing lower-hybrid waves. The wavelength of these structures is on the order of LE, and correlates well with that predicted by the linear theory of the EIH mode. Since the characteristic wavelength is longer than ρe, the corresponding phase velocity is low enough that there results significant direct resonant ion acceleration perpendicular to the confining magnetic field. When ωs≳3ωLH, the system exhibits significant anomalous viscosity (typically an order of magnitude larger than that due to Coulomb collisions), which increases as the shear frequency is increased. As ωs is reduced below ωLH, shear effects are no longer dominant and a smooth transition takes place in which the system dynamics is governed by the short wavelength (on the order of ρe) lower-hybrid drift instability.