Pulsed currents carried by whistlers. VI. Nonlinear effects

Abstract

In a large magnetized laboratory plasma (n≂1011 cm−3, kTe≥1 eV, B0≥10 G, 1 m × 2.5 m), current pulses in excess of the Langmuir limit (150 A, 0.2 μs) are drawn to electrodes in a parameter regime characterized by electron magnetohydrodynamics (ωci≪ω≪ωce). The transient plasma current is transported by low-frequency whistlers forming wave packets with topologies of three-dimensional vortices. The generalized vorticity, Ω, is shown to be frozen into the electron fluid drifting with velocity v, satisfying ∂Ω/∂t≂∇×(v×Ω). The nonlinearity in v×Ω is negligible since v and Ω(r,t) are found to be nearly parallel. However, large currents associated with v≥(2kTe/me)1/2 lead to strong electron heating which modifies the damping of whistlers in collisional plasmas. Heating in a 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 companion paper [Stenzel and Urrutia, Phys. Plasmas 3, 2599 (1996)] shows that, after the decay of the transient wave magnetic field, magnetic helicity remains in the plasma due to temperature-gradient driven currents.