Shear Alfvén Wave Perpendicular Propagation from the Kinetic to the Inertial Regime |
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We report on observations of shear Alfve´n waves radiated from a source of small transverse size, and the subsequent radial confinement of wave magnetic field energy within a cylindrical plasma. The radius of confinement lies between the kinetic regime of the bulk plasma and the inertial regime at the plasma edge; this radius is found to be a function of wave frequency. Numerical calculations using kinetic theory predict a zero in the perpendicular group velocity at a radius which varies in accord with the observations. An analytic expression for the perpendicular group velocity (valid for small perpendicular wave numbers) is given in the vicinity of the zero crossing. |
Shear Alfvén waves in a magnetic beach and the roles of electron and ion damping |
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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 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, d=c/wpe. 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, omegaci and the local Alfvén speed, vA, is approximately equal to the electron thermal speed, ve. The wave propagates along a slowly decreasing background field to where w=wci and vA=ve/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 (36 References).
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Drift-Alfvén wave mediated particle transport in an elongated density depression |
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Cross-field particle transport due to drift-Alfvén waves is measured in an elongated density depression within an otherwise uniform, magnetized helium plasma column. The depression is formed by drawing an electron current to a biased copper plate with cross-field dimensions of 28 by 0.24 ion sound-gyroradii rs=cs /wci. The process of density depletion and replenishment via particle flux repeats in a quasiperiodic fashion for the duration of the current collection. The mode structure of the wave density fluctuations in the plane perpendicular to the background magnetic field is revealed using a two-probe correlation technique. The particle flux as a function of frequency is measured using a linear array of Langmuir probes and the only significant transport occurs for waves with frequencies between 15%–25% of the ion cyclotron frequency (measured in the laboratory frame) and with perpendicular wavelengths k^rs=0.7. The frequency-integrated particle flux is in rough agreement with observed increases in density in the center of the depletion as a function of time. The experiments are carried out in the Large Plasma Device (LAPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 1991] at the Basic Plasma Science Facility, located at the University of California, Los Angeles. |
Visualizing shear Alfvén wave currents near the ion-cyclotron frequency |
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We present measurements of the three-dimensional structure of shear Alfven wave currents near the ion-cyclotron frequency. The waves are launched by modulating an electron current filament with transverse size on the order of the electron collisionless skin depth. The component of the current parallel to the background magnetic field is carried by the electrons, while the cross-field current comprises both the ion-polarization and E X B currents (8 References).
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Imaging complex three dimensional Alfven wave currents |
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Alfven waves exist in plasmas in which the ions are magnetized. They are found in the solar wind, the earth's magnetosphere, the solar corona, and possibly in intergalactic space. They play a role in Tokamak edge turbulence. Toroidal Alfven eigenmodes could effect the confinement of future fusion devices. With the advent of quiescent plasma sources, these waves are now studied in great detail in controlled laboratory experiments. We present measurements of three-dimensional current systems associated with these waves, and illustrate a visualization technique necessary to quickly picture them (4 References).
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Laboratory experiments on shear Alfven waves and their relationship to space plasmas |
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Alfven waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Shear Alfven waves radiated from sources with cross-field scale size of the order of the electron inertial length, delta=c/omega_pe, have properties which differ considerably from planar magnetohydrodynamic waves. Currents of cross-field size, delta, are common in space plasmas. A series of experiments in the Large Plasma Device at the University of California, Los Angeles is presented which illustrates that waves generated by small-scale fluctuating currents have a parallel electric field and radiate across magnetic field lines. Data and theory are presented for varying plasma collisionality and ratio of Alfven speed to electron thermal velocity. Waves generated by two sources are observed to constructively interfere to produce large magnetic fields in spatial regions away from source field lines. The shear Alfven wave is important in the Earth's auroral regions and may play a role in auroral dynamics (27 References). |
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The expansion of a dense (initially, nlpp/n0>>1) laser-produced plasma into an ambient magnetized plasma (n0=2x10^12/cm^-3) capable of supporting Alfven waves has been studied. The interaction results in the production of shear Alfven waves as well as large density perturbations (Deltan/n0~0.3) associated with the moving dense plasma. The waves propagate away from the target and are observed to become plasma-column resonances. Spatial patterns of the wave magnetic fields are measured and are used to estimate the coupling efficiency of the laser energy and the kinetic energy of the dense plasma into wave energy (20 References). |