Shear Alfvén waves in a magnetic beach and the roles of electron and ion damping

Abstract

Experiments are performed in the Large Plasma Device (LaPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles to study the propagation of the shear Alfvén wave in a parallel gradient of the background magnetic field. The waves are excited by modulating a field-aligned electron current drawn to a disk antenna with a radius on the order of the electron skin-depth, δ=c/ωpe. The resulting shear waves have a nonzero parallel electric field and propagate both parallel and perpendicular to the background magnetic field. In this experiment, the wave is launched in a region where its frequency, ω equals one-half the local ion-cyclotron frequency, ωci and the local Alfvén speed, vA, is approximately equal to the electron thermal speed, v̄e. The wave propagates along a slowly decreasing background field to where ω=ωci and vA≈v̄e/2. The wave thus propagates from a region where Landau damping is significant to where ion-cyclotron damping dominates. Detailed two dimensional measurements of the wave magnetic field morphology are presented. The measured wavelength decreases in accord with WKB solutions of a modified wave equation. Wave damping is also observed and dissipation by both ions and electrons is required in the WKB model to fit the data. Suppression of the damping via electrons in the model results in a predicted wave magnetic field amplitude twenty times larger at the ion-cyclotron resonance point than observed.

The hot hELicon eXperiment (HELIX) and the large experiment on instabilities and anisotropy (LEIA)

“…The second theoretical expression (the blue curve) includes additional kinetic effects and off-diagonal dielectric tensor elements as described in Ref. (Vincena et al 2001). The excellent agreement between the measurements and the more complete theoretical prediction provide strong evidence of the successful excitation of Aflvén waves in HELIX plasmas (Houshmandyar et al 2011).…”

Currents and shear Alfvén wave radiation generated by an exploding laser-produced plasma: Perpendicular incidence

“…35,36 The kinetic shear Alfvén wave occurs when the electron thermal speed is greater than the Alfvén speed, namely v te /v Alfvén Ͼ1. When v te /v Alfvén Ͻ1, the wave is termed an inertial shear Alfvén wave.…”

Shear Alfvén Wave Perpendicular Propagation from the Kinetic to the Inertial Regime

“…This approximation is valid in the LaPD for frequencies as high as ! 0:95 [12]; however, it cannot necessarily be made for plasmas such as the Earth's equatorial magnetosphere or those within the interiors of tokamaks. Also, the plasma is assumed to be collisionless, so that the only damping of wave energy will be through electron Landau damping.…”

“…This change in sign has been invoked by Streltsov et al [10,11] to explain trapping of dispersive Alfvén waves on auroral magnetic field lines between the ionospheres. Subtle evidence for electron acceleration [12] and wave refraction [13] have been observed in laboratory parallel e gradients. Experiments on Alfvén waves in the intermediate regime have been minimal and mainly limited to measurements of the phase velocity [14,15].…”

“…The highly reproducible discharge is repeated once per second which allows for the collection of ensemble data sets. The wave launching antenna is the same design used in other experiments [9,12,17], and the radiation has been modeled theoretically in both the inertial [18] and kinetic [19] regimes. The antenna is simply a 1 cm diameter, circular copper mesh of 50% optical transparency.…”