Dusty Plasmas have been the focus of intense research in the past decade. "Dusty Plasmas" are normal plasmas, consisting of ions and free electrons, which contain micron-sized particles. The dust grains become electrically charged due to interaction with the background plasma, causing them to act as a third charged plasma species. The dust grains can then significantly alter the behavior of the background plasma.
Dusty plasma research has been driven by concerns in space, and here on earth. Dusty plasmas occur frequently in space, and are thought to play and important role in star and planet formation. Most research in space plasmas has focused on Saturn's rings, and in the tails of comets. In these two regions, there is a high density of dust, and dusty plasma interaction may be responsible for much of the structure observed by spacecraft missions. The industrial community has also encouraged the study of dusty plasma. Plasmas are used to produce microchips, thin film coatings and hardened metals. In these cases, dust may form from materials in the plasma reactor, creating a source of contamination. Understanding of how dust is trapped, and how it interacts with the background plasma, could improve manufacturing yields. The study of dusty plasma has been advanced because of these two reasons.
Research at the University of Iowa
Research by the group has focused mainly on waves and instabilities in dusty plasmas. Specifically, the following wave modes have been investigated experimentally (a theoretical overview can be provided by D'Angelo 1990, and the listed review references listed below):
The Dusty Plasma Device
For two of the experiments described below, a typical Q-machine was slightly modified by use of the Dusty Plasma Device. The device consists of a rotating drum that continuously recycles dust through the plasma column, in a motion similar to a clothes dryer. The dust used is either .01 micron nominal Aluminum Oxide power, or 0.4 micron nominal Kaolin dust. For the majority of experiments described here, the Kaolin dust was used to produce the observed results.
Electron Dust-Ion Cyclotron Wave
The Electron Dust-Ion Cyclotron (EDIC) instability is a modification of the usual Electron Ion Cyclotron (EIC) instability in normal plasmas. The EIC instability is produced in Q-machine plasma by drawing an electron current along the axis of the plasma column. A small disk is placed at one end of the plasma column, which is biased slightly above the space potential of the plasma. The produced electron drift is sufficient to excite electrostatic waves with a frequency slightly above the ion gyrofrequency, which propagate radially outward, nearly perpendicular to the magnetic field.
In order to study the effect of dust on the instability, measurements of the wave amplitude were taken, and compared to identical plasma conditions where the dust dispenser was turned on. This amplitude could then be used as an indication of the effect of dust on the plasma. In the experiments, it was found that the EDIC instability could be excited simply by injecting dust into the plasma, in agreement with theoretical predictions.
Dust-Ion Acoustic Waves
In a similar experiment, a grid was used to launch ion acoustic waves into a dusty plasma column. The grid was biased several volts negative relative to the space potential, and a sinusoidal pulse of ~20-80 kHz was applied. The disturbance created then traveled down the plasma column as an ion-acoustic wave. By altering the amount of dust (and the amount of negative charge attached to the dust grains), it was found that wave damping decreased and the phase velocity increased as the dust density was increased by injecting more dust into the plasma column.
The second experiment involving ion acoustic phenomena used a small anode to draw a current through the entire cross section of the plasma column. In a normal Q-machine plasma this will lead to low frequency potential relaxation (PRI) oscillations. It was found that when dust was injected into the plasma column, the PRI oscillations (f ~ 1.5 kHz) were quenched while somewhat higher frequency (f ~ 3-5 kHz) oscillations were generated. These oscillations were identified as the dust-modified ion-acoustic (DIA) waves. (A similar effect has been observed in a negative ion plasma.) The frequency of the oscillation also depended on the amount of negative charge attached to the dust. Reasonable theoretical agreement supports the identification of these waves as DIA oscillations.
To observe the low frequency dust acoustic mode it was necessary to develop a method for trapping dust grains within a plasma for long periods of time. To this end, these experiments were performed in a double plasma device, where the discharge plasma was used to form an anode double layer. This double layer had strong enough electric fields to suspend dust against gravity, allowing dust grains to participate in dynamic wave motion.
If the discharge current was sufficient, acoustic waves could be visibly seen propigating through the suspended dust cloud. The waves were easily visible through the use of light scattering from a bright lamp placed behind the dust cloud. Once the waves were observed a way to deterimine their dispersion properties was devised. Using a programmable high voltage power supply, the current provided to the anode was pulsed at frequences from 5-30 Hz. With this known frequency, video images (examples shown below) were analyzed to provide measurements of wavelength. The obtained dispersion relation could be plotted and measured directly.
More information about the various topics introduced here can be found on our Publications page, or in a recent paper and the references contained therein:
R.L. Merlino, A. Barkan, C. Thompson, and N. D'Angelo. "Laboratory Studies of Waves and Instabilities in Dusty Plasmas", Phys. Plasmas, 5, 1607 (1998).
Note: This paper is provided free of charge in PDF format by the journal Physics of Plasmas. (follow the instructions provided)
One of the problems encountered in the previous experiment was the interaction of suspended dust clouds with electrostatic probes. Once a probe was inserted into a dust cloud, the probe would often completely disrupt the dust cloud. This experiment attempted to perform another diagnostic using a moving object in a dust cloud.
This experiment used a new experimental device (IQ-3) as a discharge chamber. An anode, filled with dust-laden steel wool, was biased positive to form a double layer structure similar to that in the dispersion experiment. From this, video images were taken, which were broken into the three classes "still", "moderate speed" and "high speed". Sample images are shown below:
"Still"
"Moderate"
"High"
The formation of a cavity in the still and moderate speed cases was not expected. A reasonable radius for this cavity was obtained by a simple screening model, given that only the ions and electrons provide electrostatic screening in the dusty plasma.