Plasma with Tiny Solid Particles: Dirty or Dusty Plasma

In normal life, people think that matter of the visible universe exists either in solid, liquid or gas phase. Sometimes we do not think beyond these states of matter. Many people still are not aware of the fourth state of matter, which is called plasma. This is an ionized gas consists of electrons and positively charged ions, and capable of exhibits the collective motion in the presence of external forces. It is assumed that 99 % of the visible universe is in the plasma state. One can experience the plasma in daily life as sun, neon lamp, tube light, plasma TV, lightning, etc. What about the remaining 1 % in the visible matters of the universe? Scientists have discovered that the remaining 1 % in the visible matters of the universe is nothing but the tiny solid dust particles. Now, there are many open questions in our mind to know the role of these solid particles in the plasma. Since plasma is an admixture of electrons and ions, the solid particles get negatively charged due to the collection of higher electron current than the ion current. These negatively charged particles could play two different roles in plasma. In the first case, well separated charged dust only modifies the characteristics of ambient plasma, which is called “Dirty Plasma”. In the second case, charged dust experience long-range columbic interaction and exhibits the collective response to external forces, which is named as “Dusty Plasma”.

Natural dusty plasma: Noctilucent cloud

Study of dust-plasma interaction is also necessary to understand the growth of dust impurities and transport of dust impurities to improve the quality of products in plasma processing semiconductor industries. In the future, a better life is expected with clean and cheap energy sources. Nuclear fusion is one of the clean energy sources. One of the challenges in the fusion reactor is dust impurities from wall erosion. Therefore, studies on dust dynamics in a magnetized plasma is required to resolve such issues.

In a laboratory, it is easy to create a 2D or 3D dusty plasma. Since the dust particles are negatively charged and having mass, a strong electric field is required to compensate for the gravitational force. Plasma is used to create such strong electric field regions as cathode sheath, double layer, and the diffused edge of the plasma. To diagnose the confined dust particles or dusty plasma, particles are illuminated with laser light and scattered light from particles are captured by a high frame rate CCD camera. One can study the dynamics of dusty plasma at kinetic level after tracing the motion of particles using the captured images from the dusty plasma medium.

Dusty Plasma setup in a DC discharge configuration

The dusty plasma has some remarkable features. Firstly, it is easy to record the dynamics of the massive particles and hence to get the characteristics of the medium at the microscopic level. Therefore, one can consider the dusty plasma as a fluid or liquid to study the flow characteristics at the microscopic level. Excitation of the waves and vortex motion in dusty plasma are examples which represent it’s fluid or liquid state characteristics.

Laboratory dusty plasma: Waves motion in a dusty plasma. White dots represent the dust particles.
Laboratory dusty plasma: Vortex motion in a dusty plasma. White dots represent the dust particles.

Secondly, a large amount of charges on dust increases the average potential energy of the dust components compared to its average kinetic energy, which leads the medium to strongly coupled state. The coupling strength determines its liquid-like or solid-like characteristics. Hence, dusty plasma provides a platform to understand the underlying physics at microscopic levels for various multidisciplinary fields (e.g. solid state physics, fluid physics, and life sciences).

Dusty plasma crystal (solid-like characteristics)

Dr. Mangilal Choudhary

Dr. Mangilal Choudhary is a post-doc fellow at I-Physics Institute, JLUGiessen (35392), Germany


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