Shear Alfven waves in a magnetic beach and the roles of electron and ion damping
Experiments are performed in the Large Plasma Device (LaPD) [Gekelman , Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles to study the propagation of the shear Alfven 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, {delta}=c/{omega}{sub pe}. 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, {omega} equals one-half the local ion-cyclotron frequency, {omega}{sub ci} and the local Alfven speed, v{sub A}, is approximately equal to the electron thermal speed, {bar v}{sub e}. The wave propagates along a slowly decreasing background field to where {omega}={omega}{sub ci} and v{sub A}{approx}{bar v}{sub e}/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.
- Sponsoring Organization:
- (US)
- DOE Contract Number:
- FG03-00ER54598
- OSTI ID:
- 40277861
- Journal Information:
- Physics of Plasmas, Vol. 8, Issue 9; Other Information: DOI: 10.1063/1.1389092; Othernumber: PHPAEN000008000009003884000001; 030109PHP; PBD: Sep 2001; ISSN 1070-664X
- Publisher:
- The American Physical Society
- Country of Publication:
- United States
- Language:
- English
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