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

Citation
S. Vincena et al., Shear Alfven waves in a magnetic beach and the roles of electron and ion damping, PHYS PLASMA, 8(9), 2001, pp. 3884-3896
Citations number
36
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physics
Journal title
PHYSICS OF PLASMAS
ISSN journal
1070-664X → ACNP
Volume
8
Issue
9
Year of publication
2001
Pages
3884 - 3896
Database
ISI
SICI code
1070-664X(200109)8:9<3884:SAWIAM>2.0.ZU;2-6
Abstract
Experiments are performed in the Large Plasma Device (LaPD) [Gekelman , Rev . Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angel es 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 fi eld-aligned electron current drawn to a disk antenna with a radius on the o rder of the electron skin-depth, delta = c/omega (pe). The resulting shear waves have a nonzero parallel electric field and propagate both parallel an d perpendicular to the background magnetic field. In this experiment, the w ave is launched in a region where its frequency, omega equals one-half the local ion-cyclotron frequency, omega (ci) and the local Alfven speed, upsil on (A), is approximately equal to the electron thermal speed, <(<upsilon>)o ver bar>(e). The wave propagates along a slowly decreasing background field to where omega = omega (ci) and upsilon (A) approximate to <(<upsilon>)ove r bar>(e)/2. The wave thus propagates from a region where Landau damping is significant to where ion-cyclotron damping dominates. Detailed two dimensi onal measurements of the wave magnetic field morphology are presented. The measured wavelength decreases in accord with WKB solutions of a modified wa ve 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 fi eld amplitude twenty times larger at the ion-cyclotron resonance point than observed. (C) 2001 American Institute of Physics.