Strongly Correlated Dusty Plasmas

When dust grains are suspended in a background neutral plasma discharge, the grains become highly negatively charged. If the temperature is not too large, the correlation parameter g;(Ze)²/kTA_gbetween dust grains can be much larger than unity because Z~10^4 (here A_g is the average inter-grain spacing), even though «1 for the plasma. In fact, in preliminary work at least two experiments have observed the formation of Coulomb crystals of dust grains suspended in a neutral plasma discharge.

In theoretical treatments of these dusty plasma systems, it has previously been assumed that the dust grains interact via a linear Debye-shielded interaction potential of the form =Ze/r*exp(-r/_d), where _d is the Debye length. As far as we know, this has not been analyzed when the interaction potential is small, but not when Z=10^4. As far as we know, this has not been analyzed previously.

For example, consider the force between two dust grains. In addition to the usual linear Debye-shielded force, one grain will polarize the Debye sphere surrounding the other grain, creating an attractive force which can reduce or possibly even reverse the overall repulsive force. That is, the Debye sphere around a dust grain acts like a spherical conductor, and a charged dust grain outside this sphere will polarize it, just as a charge outside a conducting sphere produces an image charge in the sphere.

This effect arises from the nonlinear terms in the Boltzmann distribution of the plasma shielding the grains. Such terms are neglected in the usual linear analysis of Debye shielding, but they will be important if e*(_d)/kT1. This inequality can be written as Z*()³1. In laboratory experiments in argon discharges. the background ions are weakly correlated with ~10¯², but since Z is very large the above inequality can be easily satisfied, so nonlinear Debye shielding can have important consequences for the inter-grain interaction.

We propose to study these effects in order to determine the correct form for the interaction between dust grains in such discharges. Preliminary calculations show that the intergrain force is strongly reduced by the nonlinear effects, in agreement with the simple intuitive model described above. Other effects such as nonthermal distributions and depletion of background electrons by the dust grains will also be studied in the context of their effect on the intergrain interactions.