Progress in the study of dusty plasmas

While the study of dust–plasma interactions is by no means new, early progress in the field was slow and uneven. It received a major boost in the early 1980s with the Voyager spacecraft observations of peculiar features in the Saturnian ring system (e.g. the `radial spokes') which could not be explained by gravitation alone and led to the development of the gravito-electrodynamic theory of dust dynamics. This theory scored another major success more recently in providing the only possible explanation of collimated high-speed beams of fine dust particles observed to sporadically emanate from Jupiter by the Ulysses and Galileo spacecrafts.

These dynamical studies were complimented in the early 1990s by the study of collective processes in dusty plasmas. Not only has this led to the discovery of a whole slew of new wave modes and instabilities with wide ranging consequences for the space environment, it also spurred laboratory studies leading to the observation of several of them, including the very low frequency dust acoustic mode, which can be made strikingly visual by laser light scattering off the dust.

The most fascinating new development in dusty plasmas, which occurred about 7 years ago, was the crystallization of dusty plasmas in several laboratories. In these so-called `plasma crystals', micrometre-sized dust, which are either externally introduced or internally grown in the plasma, acquire large negative charges and form Coulomb lattices as was theoretically anticipated for some time. This entirely new material, whose crystalline structure is so strikingly observed by laser light scattering, could be a valuable tool for studying physical processes in condensed matter, such as melting, annealing and lattice defects. Recognizing the crucial role of gravity on the crystal structure, microgravity experiments have already been performed in aircraft, sounding rockets, the Mir Space Station, and most recently in the International Space Station, leading to interesting new phenomena and insights.

Dust–plasma interactions are also important in the industrial laboratory. The nuisances and hazards related to dust charging in mills, granaries and mines have been known for a long time. At the same time dust charging has been effectively used in electrostatic precipitation, separation and spraying as well as in the sterilization and electrophoresis of biological materials. Also, plasma processing is now used in the semiconductor manufacturing industry. The condensation and transport of fine dust particles in these high-density, low-temperature plasmas leading to product yield loss, is a significant problem facing this multibillion dollar industry, and is expected to become more important as feature sizes decrease in the future. The role of dust–plasma interactions in magnetic fusion devices, where dust could grow in the edge of the fusion plasma discharge itself and be subsequently transported elsewhere, is also beginning to receive attention, lately.

I expect this rapid growth will continue well into the foreseeable future. While the richness and complexity of the field, with numerous unresolved and challenging problems, will continue to stimulate the field, several forthcoming space missions (e.g. Cassini and Rosetta) will provide important observational boosts. The fascinating high-resolution Hubble Space Telescope photographs of dusty cosmic regions, such as those where new stellar and planetary formation are taking place in plain sight (the so-called cosmic nurseries), is only now beginning to underscore the importance of dusty plasma phenomena in these vital astrophysical regions. Finally, the proposed International Microgravity Plasma Facility, dedicated to dusty plasma studies in the International Space Station within the next few years, will provide a major boost to this fascinating field.