Dusty plasma

A dusty plasma is a plasma containing nanometer or micrometer-sized particles suspended in it. Dust particles may be charged and the plasma and particles behave as a plasma [1] [2], following electromagnetic laws for particle up to about 10 nm (or 100 nm if large charges are present). Dust particles may acrete into larger particles resulting in "grain plasmas".

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Dusty plasmas are encountered in:

• Industrial processing plasmas
• Space plasmas

Dusty plasmas are interesting because presence of particles significantly alters the charged particle equilibrium leading to different phenomena. It is field of current research. Electrostatic coupling between the grains can vary over a wide range so that the states of the dusty plasma can change from weakly coupled (gaseous) to crystalline. Such plasmas are of interest as a non-Hamiltonian system of interacting particles and as a means to study generic fundamental physics of self-organization, pattern formation, phase transitions, and scaling.

Characteristics

The temperature of dust in a plasma may be quite different from its environment. For example:

The electric potential of dust particles is typically 1–10 V (positive or negative). The potential is usually negative because the electrons are more mobile than the ions. The physics is essentially that of a Langmuir probe that draws no net current, including formation of a Debye sheath with a thickness of a few times the Debye length. If the electrons charging the dust grains are relativistic, then the dust may charge to several kilovolts [3]. Field emission, which tends to reduce the negative potential, can be important due to the small size of the particles. The photoelectric effect and the impact of positive ions may actually result in a positive potential of the dust particles.

Dynamics

The motion of solid particles in a plasma follows the momentum equation for ions and electrons:

where m, q are the mass and charge of the particle, g is the gravitation acceleration, mv_cv is due to viscosity, and f respresents all other forces including radiation pressure. q (E + v x B) is the Lorentz force, where E is the electric field, v is the velocity and B is the magnetic field.

Then depending in the size of the particle, there are four categories:

1. Very small particles, where q (E + v × B) dominates over mg.
2. Small grains, where q/m ≈ √G, and plasma still plays a major role in the dynamics.
3. Large grains, where the electromagnetic term is negligible, and the particles are referred to as grains. Their motion is determined by gravity and viscosity, and the equation of motion becomes mv_cv = mg.
4. Large solid bodies. In centimeter and meter-sized bodies, viscosity may cause significant perturbations that can change an orbit. In kilometer-sized (or more) bodies, gravity and inertia dominate the motion.

References

• Dusty Plasmas: Physics, Chemistry and Technological Impacts in Plasma Processing , John Wiley & Sons Ltd.
• Merlino, Robert L., "Experimental Investigations of Dusty Plasmas" (2005) (PDF preprint); highlights some of the history of laboratory experiments in dusty plasmas,