Laboratory Investigations of Current Sheets at the Electron Skin Depth Scale

Details Laboratory Investigations of Current Sheets at the Electron Skin Depth Scale Laboratory Investigations of Current Sheets at the Electron Skin Depth Scale

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsm23b0425v&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SM23B-0425

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2159 Plasma Waves And Turbulence, 2463 Plasma Convection (2760), 2471 Plasma Waves And Instabilities (2772), 2721 Field-Aligned Currents And Current Systems (2409)

Scientific paper

Laboratory Investigations of Current Sheets at the Electron Skin Depth Scale. Theoretical investigations, in situ spacecraft and rocket missions, and laboratory studies form an essential triad for understanding the variety of current sheet phenomena found in space plasmas. In the Large Plasma Device (LAPD) at UCLA, the formation dynamics, equilibrium state, and wave-mediated disruptions of current sheets can be studied with great spatial and temporal resolution using a variety of probes as well as non-invasive laser induced fluorescence and other optical diagnostics. The LAPD is aptly suited for studying current sheets flowing in a magnetized background plasma which is capable of supporting Alfvén waves. The cylindrical device is 20m long and one meter in diameter with a solenoidal magnetic field as high as 3000 Gauss. For the parameters in this experiment, the plasma column is ten shear Alfvén wavelengths along the field and 100 electron inertial lengths (δe) (or 200 ρi) in the perpendicular direction. An electron current sheet is created in the plasma by placing a thin copper plate in the plasma column at one end of the device and pulsing this plate positive with respect to the chamber wall. The current sheet extends for the length of the device and has an initial cross-field size of roughly 45 δe by 0.5δe. A parallel flow of ions is observed with similar dimensions and moves in the same direction as the electrons in the current sheet with a velocity of 0.2 times the ion sound speed. A much weaker sheared perpendicular flow is also measured. Cross-sections of the ion flow are measured at several axial locations over a distance of six meters. Second, as the ion flow increases in magnitude, a much broader (8ρi) density depletion (n=0.25nO) develops around the flow. The gradient scale length of the depletion shortens until the spontaneous growth of drift waves occurs. This disrupts the electron current and ion flow, and leads to cross-field transport of plasma and a relaxation of the density gradient. The process of steepening and disruption repeats during the bias pulse. Detailed two-dimensional correlation measurements reveal the density and magnetic field propagation of the waves and statistics on the wave fluctuations.

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