Dusty Plasma Dynamics in the NRL Space Physics Simulation Chamber Laboratory

Introduction: Dusty plasmas have become a topic of great interest because they provide an excellent tool for exploring many of the fundamental assumptions used in plasma physics. A dusty plasma consists of electrons, ions, neutral gas, and charged microparticles ("dust") with diameters ranging from a few nanometers to a few micrometers. They exist naturally in space, being found in the low Earth orbit region, planetary rings, comet tails, and in planetary nebulae. From its early beginnings with observations of astrophysical phenomena, this area of plasma physics research has grown to encompass industrial plasma, space plasma, and basic plasma issues ranging from strongly coupled systems, to transport, to waves and instabilities. In the laboratory, experiments have evolved from observations of the behavior of the microparticles in the plasma to direct manipulation of the microparticles and use of the microparticles themselves for plasma diagnosis. However, most dusty plasma experiments have been performed in relatively small setups, most often with dust cloud scale sizes of the order of 2-3 cm.

DUPLEX: The Naval Research Laboratory (NRL) DUPLEX—the DUsty PLasma EXperiment—device was developed to investigate fundamental issues in the physics of large-scale dusty plasmas in an environment far from the chamber boundaries. The chamber, shown in Fig. 8 with plasma, is unique among laboratory dusty plasma devices. It is 80 cm in diameter and 80 cm in height and is constructed from 0.5-in. thick optically transparent polycarbonate plastic, providing a 360° view of the experimental region. The top and bottom endcaps of the device are also transparent. Alumina microparticles with an average particle diameter of <d> ~ 1.2 ± 0.5 mm are typically used in these experiments.

FIGURE 8 The NRL DUPLEX device with argon dc glow discharge plasma.

The large-scale nature of the experiment makes it ideal for investigation of the interaction of dusty plasmas with materials of various shapes, sizes, and electrical characteristics, and allows for the study of phenomena that have not been previously observed in dusty plasma experiments. Individual charged dust clouds with diameter as large as 70 cm and heights as large as 50 cm have been observed in DUPLEX. The clouds often have complex spatial structures with apparent void regions and considerable internal transport.

Argon dc glow discharge plasmas are created in DUPLEX. A 10-cm diameter disk anode biased in the range 300 to 1000 V is suspended vertically above a grounded 75-cm diameter cathode. The separation between the anode and cathode can be adjusted to a maximum of 75 cm, but typically the separation is maintained between 15 and 20 cm. Experiments are performed at pressures ranging from 50 to 300 millitorr.

One of the primary diagnostic techniques used to observe the dust particles is Particle Image Velocimetry (PIV). This technique was pioneered for dusty plasmas by Dr. Edward Thomas, Jr. and the dusty plasma group at Auburn University. It involves illuminating suspended microparticles with a pair of 30-ns laser pulses that are expanded using cylindrical lenses into vertically or horizontally oriented light sheets, separated in time. The laser pulses are synchronized to the frame grabbing rate of a CCD camera, ensuring that each pulse appears on a single video frame. The displacement of the particles can then be calculated by viewing their relative positions in subsequent video frames. From the displacement and the known time interval, two-dimensional velocity vectors can be computed in the plane of illumination.

Dust Cloud Structuring: Dusty plasmas in DUPLEX are observed to have complex structure. Figure 9 is an example. This image shows a structured cloud suspended above the cathode. The neutral pressure is 250 millitorr and the anode bias voltage is 1000 V. The cloud in this image has a horizontal extent of ~5 cm and a vertical extent of ~3 cm. The "banding" observed in the cloud is not due to reflections or shadows in the experiment, but are real regions that have a significantly lowered—possibly zero—dust density as compared to adjacent regions. The motion of particles within the clouds, as measured using the PIV diagnostic and shown in Fig. 10, suggests that the particles are generally constrained within each band. This observation, combined with the upward flow of the particles suggests that the exchange of particles between the different bands probably occurs at the boundaries of the clouds.

FIGURE 9
Example of complex structuring observed in DUPLEX dusty plasma clouds with 1-mm alumina microparticles.

FIGURE 10
Particle Imaging Velocimetry measurement of the charged microparticle velocities in the dust cloud shown in Fig. 9.

Dusty plasmas of the size and nature observed in the DUPLEX device can be generated because there are none of the usual sheath effects near the walls that are present in most smaller experiments. This allows the dust clouds to expand to the size where an equilibrium is achieved between the mutual repulsion of the negatively charged microparticles and an inward pressure force from the plasma. The dust cloud internal structure suggests a possible underlying electrostatic potential structure in the region where the particles are suspended. Because these highly structured clouds form above the dust sources, it is possible that the insulation provided by the unlevitated dust on the cathode alters the local potential profile directly above the source. Future work on this experiment will focus on the nature of the formation of such void structures by using electrodes to control the potential structures in the plasma.

Acknowledgments: The experimental assistance of Dr. E. Thomas, Jr., C. Compton, and B. Christy of Auburn University is gratefully acknowledged.

[Sponsored by ONR]