Making waves in a complex plasma

By injecting micrometer-sized solid particles into a gas discharge, researchers can create dusty, or complex, plasma (see the article by Robert Merlino and John Goree, Physics Today, July 2004, page 32 ). Physicists can use the low-temperature plasma as a model to study generic processes in condensed matter. Gravity, however, affects those processes. To limit gravity’s effect, Surabhi Jaiswal (now at Auburn University), Mikhail Pustylnik (German Aerospace Center), and colleagues took advantage of the microgravity conditions of the Plasma Kristall-4 facility on board the International Space Station (ISS). For the experiment, a cylindrical glass chamber filled with neon gas and a cloud of plastic spheres was subjected to a DC discharge. Ion currents streaming in the plasma excited dust density waves. The composite shown here, of several images taken at different times, captures the wave pattern before, during, and after a reversal of the DC discharge polarity: Blue corresponds to the earliest image; red, the latest one.

The researchers found that the head of the microparticle cloud had a phase velocity that was a factor of two greater than that of the tail. To explore the origin of that variation, Jaiswal, Pustylnik, and colleagues compared their observations with a model that could account for the influence of an additional, changing local electric field between the head and the tail. The closest agreement was obtained in an analysis that included a local electric field that varied by a factor of two. Because the complex plasma facility on board the ISS can change the electric field in dusty plasma, researchers will be able to investigate how to potentially manipulate the dusty density waves and plasma in future microgravity experiments. The analogy between dusty plasmas and condensed-matter systems could become closer and more enlightening. (S. Jaiswal et al., Phys. Plasmas 25, 083705, 2018. )