Laboratory Investigation of the Dynamics of Shear Flows in a Plasma Boundary Layer

Abstract

The laboratory experiments presented in this dissertation investigate a regime of instabilities that occur when a highly localized, radial electric field oriented perpendicular to a uniform background magnetic field gives rise to an azimuthal velocity shear profile at the boundary between two interpenetrating plasmas. This investigation is motivated by theoretical predictions which state that plasmas are unstable to transverse and parallel inhomogeneous sheared flows over a very broad frequency range. Shear driven instabilities are commonly observed in the near-Earth space environment when boundary layers, such as the magnetopause and the plasma sheet boundary layer, are compressed by intense solar storms. When the shear scale length is much less than the ion gyro-radius, but greater than the electron gyro-radius, the electrons are magnetized in the shear layer, but the ions are effectively un-magnetized. The resulting shear driven instability, the electron-ion hybrid instability, is investigated in a new interpenetrating plasma configuration in the Auburn Linear EXperiment for Instability Studied (ALEXIS) in the absence of a magnetic field aligned current. In order to truly understand the dynamics at magnetospheric boundary layers, the EIH instability is studied in the presence of a density gradient located at the boundary layer between two plasmas. Theoretical models are used to show that the EIH instability in a uniform density plasma cannot be supported by the parameters that are accessible to ALEXIS. As plasma boundary layers begin to relax from a compressed state, the ratio of the ion gyro-radius to the shear scale length decreases, and observations of broadband electrostatic noise, which extend from well below the ion cyclotron frequency to the electron plasma frequency, have been reported. By decreasing the magnetic field strength in the ALEXIS device, a continuous variation of the ratio of the ion gyro-radius to the shear scale length is observed. As a result, a transition of the shear flow driven instability regime is also observed, which is reminiscent of the satellite observations of broadband electrostatic noise. For the first time, a laboratory experiment has reproduced the actual space observation of broadband emission, which is a characteristic signature of boundary layer crossings by a satellite.