Laboratory Investigation of Boundary Layer Processes Due to Strong Spatial Inhomogeneity*

Boundary layers are often found in laboratory plasmas, such as at the edges of laser-produced plasmas, and in natural settings, such as the magnetosphere. These are regions in which plasma characteristics undergo rapid transition and are usually the sites of wave activity. An investigation of boundary layer processes has been conducted in the NRL Space Physics Simulation Chamber (SPSC). A method has been developed to create a boundary layer with controllable density gradients and transverse electric fields with arbitrary amplitude and scale length as small as one-fourth of an ion gyroradius. This method involves interpenetrating plasmas with different diameters, whose plasma potentials can be controlled. Under these conditions, waves in the lower hybrid frequency range have been observed and their characteristics documented. Theoretical work indicates that the observed mode characteristics are similar to those expected for the Electron-Ion Hybrid Instability. As an illustration, we apply our results to space plasmas and the dynamics of the plasma sheet boundary layer (PSBL). The PSBL is key in the transfer of solar wind mass, energy, and momentum into the magnetosphere and ultimately into the auroral regions. During times of solar activity, the PSBL can become compressed to widths on the order of an ion gyroradius and can contain localized transverse electric fields similar to those generated in the SPSC experiments. At these times, thin layers of energetic electrons and broadband wave activity have been observed in the PSBL. The details of the response of a plasma to such conditions and the observable signatures are the focus of this study. *Work supported by ONR.