Nonlinear Phenomena Associated with Large Amplitude Whistler Pulses

Abstract
In a magnetized laboratory plasma (n≈10¹¹cm^-3, ,kTe≥1eV, Bo≥10G,1 m diam x 2.5 m) large amplitude current pulses (150 A, 0.2 µs) are excited in the parameter regime described by Electron MHD (EMHD ;ω_ci≤ω≤ω_ce.). The currents are transported by low-frequency whistlers forming wave packets with topologies resembling 3D spherical vortices. The generalized vorticity, [MATH], is shown to be frozen into the electron fluid, [MATH]. The nonlinearity in is negligible since v and Ω(r,t) are found to be nearly parallel. Thus, large amplitude pulses [[MATH]] show the same behavior as small amplitude pulses. However, the associated large currents with [MATH] lead to strong electron heating which can modify the damping of whistlers in collisional plasmas (ω≈V_eiα T^-3/2_e). Observations show that a heated flux tube provides a filament of high Spitzer conductivity which permits a nearly collisionless propagation of whistler pulses. This filamentation effect is not associated with density modifications as in modulational instabilities but arises from conductivity modifications. The heated flux tube also generates a quasi-dc magnetic field driven by thermoelectric currents. These assume self-consistently a Taylor state with ∇xB≈kB.