Dusty Plasma Research under Microgravity Conditions on the ISS

Dusty plasma is a four-component mixture of rarefied neutral gas, electrons, positive ions, and micron-size particles of solid matter. The solid particles, or “dust,” gain large electric charges by absorbing more light electrons than positive ions. A dusty plasma is analogous to a charged colloid, except that there is a rarefied gas instead of an aqueous solvent, so that buoyancy is negligible and the dust particles fall rapidly to the bottom of the experimental chamber in the presence of gravity. Microgravity conditions completely change an experiment, making it possible to fill a three-dimensional volume with the dust particles in a way that is impossible in terrestrial laboratories. Like a charged colloid, a dusty plasma can exhibit crystalline or liquid-like behavior on a macroscopic scale, using microscopic video imaging as a diagnostic, but the dust motion is underdamped, unlike the overdamped motion in a colloid. On the International Space Station (ISS), the European Space Agency operates a dusty plasma user facility called PK-4. The PK-4 scientific team, led by Germans and Russians, includes scientists in the US and other countries. Physics topics such as nonlinear waves and nonequilibrium statistical mechanics are studied by US scientists. Some early results will be presented along with plans for future experiments.