DUSTY PLASMAS UNDER MICROGRAVITY CONDITIONS

J Goree; I Hahn

When atoms in a gas become ionized, the result is called a plasma. It almost always contains both negative electrons and positive ions. A background of neutral gas atoms that did not become ionized are often present as well. These three constituents of most plasmas (electrons, ions and gas atoms) exhibit a rich variety of collective behavior, especially because charges on the electrons and ions lead to electric and magnetic fields that can push other electrons and ions about. The physics of plasmas, as rich as it is, becomes even richer when a fourth constituent is added: micron-size particles of solid matter. For historical reasons, they are called dust particles, and when mixed into plasma one is said to have a "dusty plasma." super(1) In analogy to colloidal suspensions and other complex fluids, some authors use the term "complex plasma" instead of dusty plasma. The dust particles are typically polymer microspheres with a diameter of several microns. Experimenters use video microscopy to track their motion. Physical processes in dusty plasmas are dominated by a large negative electric charge that accumulates on dust particles, due to the collection of surrounding electrons. A charge as big as 10,000 electrons can accumulate on a single 5-micron sphere. Because they have such a large negative charge, dust particles interact strongly with one another, leading to a rich variety of collective interactions. For the area of plasma physics, dusty plasmas exhibit many kinds of waves, instabilities, and anomalous transport coefficients. These have always been important topics in plasma physics. For statistical physics, dusty plasmas serve as nonequilibrium systems that are attractive because experimenters can record data at the most fundamental physical level: the positions and velocities of individual particles. super(2) For condensed matter physics, dusty plasmas serve as a model system for studying melting, solidification, and liquid-phase physics topics such as relaxation and viscoelasticity. Beyond fundamental physics, dusty plasmas are also of interest in astronomy, space exploration, nuclear fusion, and industry. Astronomers were the first to study dust plasmas because they occur naturally in the rings of Saturn, comet tails, and nebula in interstellar space. In space exploration, dusty plasma effects can be exploited in dust mitigation strategies. Dust particles immersed in plasma are a vexing contamination problem in nuclear fusion reactors and semiconductor fabrication plasmas. Microgravity conditions are valuable for dusty plasma experiments because gravity causes sedimentation and it obscures weak forces. These are discussed next. Sedimentation is rapid because no buoyancy offsets it; this is so because the surrounding medium for the dust particles is a rarefied gas with a negligible mass density. Thus, when introduced into a plasma the dust particles simply fall to the bottom of the vacuum chamber. The only thing that can stop them is a large vertical electric field. Such fields are used in laboratories in 1g laboratory conditions, allowing experimenters to suspend a thin horizontal layer of dust particles near the bottom of the vacuum chamber. To study physics of a 3D suspension larger than a centimeter, however, requires eliminating the effects of gravity. Weak forces acting on dust particles are obscured by gravity. There are several such forces. As one example, hydrodynamic flow of ions driven by electric fields leads to wake-fields downstream of the dust particles that are the subject of much theoretical research in dusty plasmas. These wakefields apply weak forces to dust particles. Another example is the force of gas atoms impinging randomly on dust particles - this Brownian motion force leads to fluctuations that are interesting in statistical physics, but difficult to detect in the presence of gravity. Because gravity is such a disturbing force in dusty plasmas, they have been the subject of many microgravity experiments. Some of these have been performed in the U.S. and Europe using parabolic aircraft flights, super(3) which allow observing phenomena like waves, as in Fig. 2. However, many other phenomena, for example crystal formation and melting, require microgravity conditions with a longer duration and smaller acceleration than can be achieved in parabolic flights. For this reason, German and Russian groups super(4) have operated experiments on ISS since the first expedition in 2001. We will describe a proposed ISS facility for fundamental physics experiments. This NASA facility will support multiple users. It will have a modular design so that it can be used for additional purposes such as experimental simulations of astrophysical conditions.

DUSTY PLASMAS UNDER MICROGRAVITY CONDITIONS

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