What is a plasma?
In physics, a plasma is an ionized gas, meaning that one or more of the gas atom’s electrons are stripped off. Hence, a plasma consists of electrically charged particles (electrons, ions), making it electrically conducting. However, the number of positive and negative charges balances on average so that it is electrically neutral on average (quasi-neutral). A partially ionized plasmas, contains neutral particles (atoms, molecules) besides electrons and ions, while a fully ionized plasma only consists of the latter two.
Creating a plasma requires energy, which may be delivered by high temperatures, electric fields or radiation. Naturally ocurring plasmas include the sun, its solar wind, the interstellar medium, earth’s ionosphere and lightnings. Artificially produced plasmas are widely used in technological applications such as surface treatments (functional coatings, silicon wafer production), light production (floodlights, displays, fluorescent lamps), decontamination (drinking water, medical equipment) and many more. Finally, the goal of producing energy by nuclear fusion employs magnetically confined high temperature plasmas.
What is a dusty plasma?
Dusty plasmas are plasmas containing additional solid dust particles. The dust particles typically have sizes varying from nanometer to millimeters and may consist of any material.
What happens to the dust in the plasma?
Due to the freely moving electrons and ions in the plasma, the dust particles can acquire a high electric charge by collecting these particles. In most situations, all dust particles charge up with the same polarity (in technical plasmas usually negative). As a consequence, the dust particles strongly repel each other. The resulting electrical forces lead to a wide variety of dynamical phenomena and with a complex interaction with the surrounding plasma.
Where can dusty plasmas be found?
Besides laboratory experiments, dusty plasmas occur in manifold situations in space since almost the entire universe is in the plasma state:
- interstellar clouds contain dust and are hotbeds of star formation. The formations of stars and planets starts with the agglomeration of dust particles.
- planetary rings are dusty plasmas, the most famous example of course being Saturn: Already in 1981, Voyager 2 observed dust dynamics in the B ring (more recent: Cassini mission).
- comets have a dust tail and a plasma tail. (The recently operating Rosetta spacecraft has 3 devices for the research on dust, which was one of the mission’s main goals).
- electrically charged aerosols in the earth’s ionosphere create noctilucent clouds at altitudes of about 85km (usually visible in june and july, especially at high latitudes)
Furthermore, in many of the technological plasmas applications, dust particles may grow (sometimes uncontrollable) as a by-product of the plasma reactions, where they can be either harmful or useful:
- dust growth in processing plasmas for computer chips (silicon wafers) disturbs the plasma parameters and dust particles crashing into the wafers destroy the fine wafer structure
- dust particles can act as crystallization seeds for the production of polymorphous solar cells (the cheap, but not so efficient ones, like in your pocket calculator)
- dust particles growing in fusion plasmas can be a serious impurity and may have a strong (harmful) influence on the hot plasma required for fusion
Why is it called „dust“?
„Dusty Plasmas“ is not a registered trademark. You may also find the expressions
Complex Plasma
Colloidal Plasma
(Fine) Powder Plasma
and others, which more or less mean the same thing as „Dusty Plasmas“. This diversity is due to the topic’s young age: When the field of dusty plasmas emerged in the late eighties and early nineties, it attracted researchers of different scientific background (e.g. astrophysics and technological plasma physics), who brought their own interpretation of what the essence of the field was.