February 18, 2010, 12:00 pm - 1:00 pm

February 18, 2010, 12:00 pm - 1:00 pm

Structure of a Dense Expanding Plasma within a Background Magnetized Plasma

Andrew Collette (UCLA)

The behavior of expanding dense plasmas has long been a topic of interest in space plasma research, particularly in the case of expansion within a magnetized background plasma. Expansion perpendicular to B causes a wide range of effects, including a localized reduction of the background field ("diamagnetic cavity"), as well as visible periodic structures on the expanding plasma surface, which have been observed in a number of experiments at NRL and elsewhere. At the UCLA Large Plasma Device (LaPD), a new series of experiments studies these phenomena via a laser-produced plasma (10^15 cm^-3 initial density, 4cm maximum diameter) immersed in a large magnetized background plasma (10^12 cm^-3 at 600-1200G, 60 cm x 16 meter cylindrical plasma column). The expanding plasma is diagnosed with a novel high-resolution 2D probe platform, which combines small-scale (1mm) magnetic and Langmuir probes with high-accuracy vacuum ceramic positioning motors. Repeating the experiment at 1 Hz over a period of many days allows the direct, detailed measurement of the plasma structure in 2D "cutplanes", in both magnetic field and floating potential. In addition to measurement of the plasma structure, two-probe correlation analysis also reveals large-scale (L ~ plasma radius) periodic electrostatic structures at the surface of the cavity. Similar to features observed photographically, they are thought to be the saturated result of instabilities which grow on the density and temperature gradients at the edge of the expanding plasma. We present the first detailed 2D probe measurements of these structures and their evolution over time.