NONLINEAR EVOLUTION OF A STRONGLY SHEARED CROSS-FIELD PLASMA-FLOW

A study is presented of the nonlinear evolution of a magnetized plasma in which a localized electron cross-field flow is present. The peak v elocity of the flow is denoted by V0; L(E) represents the flow's shear scale length; and the regime rho(e) < L(E) < rho(i) is considered, wh ere rho(i) and rho(e) denote the ion and electron Larmor radii, respec tively. It is shown that if the shear frequency omega(s) = V0/L(E) is larger than the lower-hybrid frequency, omega(LH), then the system dyn amics is dominated by the onset of the electron-ion-hybrid (EIH) mode which leads to the formation of coherent (vortexlike) structures in th e electrostatic potential of the ensuing lower-hybrid waves. The wavel ength of these structures is on the order of L(E), and correlates well with that predicted by the linear theory of the EIH mode. Since the c haracteristic wavelength is longer than rho(e), the corresponding phas e velocity is low enough that there results significant direct resonan t ion acceleration perpendicular to the confining magnetic field. When omega(s) > 3omega(LH), the system exhibits significant anomalous visc osity (typically an order of magnitude larger than that due to Coulomb collisions), which increases as the shear frequency is increased. As omega(s) is reduced below omega(LH), shear effects are no longer domin ant and a smooth transition takes place in which the system dynamics i s governed by the short wavelength (on the order of rho(e)) lower-hybr id drift instability.