Physics of charged dust particles in a plasma

Electrical charge accumulation on the surface of an insulating particle is the basic mechanism by which particle matter interacts with plasma.  In the space environment, this charge accumulates via photoionization, secondary electron emission due to impacts with energetic particles, and collisions with the background thermal plasma.  For the laboratory experiments on dusty plasmas, the principle charging mechanism will be the flux of charged particles from the plasma to dust particles residing on a plasma-exposed surface.

Particle Suspension

Assume that a spherical, macroscopic particle (grain) resides on a conducting surface exposed to a plasma. As charges pass through the sheath to the surface, they collect on the surface of the grain; in most experiments, due to the mobility of the electrons, the grain becomes negatively charged. Once the surface charge of the dust grain, Q_D, is large enough, the electric force (F_E), due to the sheath electric field, can exceed the combination of the gravitational (m_Dg) and the adhesive (F_ad) forces that bind the grain to the surface.

Once released from the surface, the dust particle is accelerated through the sheath and passes into the main plasma. In laboratory conditions it is the balance between gravitational forces, electrical forces (due to electric fields within the plasma), and various neutral particle and ion drag forces that controls the transport of individual dust particles in the plasma.  

Effect of a dust grain on the plasma:

Once the charged dust particles are present in the plasma, they can carry a large fraction of the negative charge of the plasma. As a result, the condition for quasi-neutrality of the plasma must be rewritten to include the effect of the dust particles.

Here, n_iis the ion density, n_eis the electron density, and the third term represents the sum of the charges carried by each of the dust particles in the plasma.  

In studying the basic physics of dusty plasmas, this third term carries very interesting implications.  For regions of a dusty plasma in which there are few dust particles, there can be a large excess of positive charges.  The presence of the dust particles in the plasma allows the formation of electric fields within the plasma, alters the local plasma potential profile, modifies the transport of particles in the plasma, modifies certain types of ion plasma waves, and introduces new dust plasma wave modes.

Dusty plasma applications:

Because of the ubiquitous presence of particulates in plasmas, there has been a rapid rise of interest in dusty plasmas.

Plasma-material interactions -

• In plasma etching of microchips and other semiconductor devices, micron-sized debris is a common side-effect.  
• This debris can (and in many cases does) redeposit on the devices causing irreparable damage.  
• Such debris may also be formed at the edges of large fusion experiments due to energetic particle impacts with the plasma facing wall.

Space plasmas -

• Spacecraft interactions - Charged dust grains are common in low earth orbit and in the interplanetary medium.  The presence of this charged material can cause both physical damage and electrical problems for spacecraft.  
• Solar system formation - Charged dust particles in space plasmas may help to explain the formation of planetary rings, comet tails and nebulae.  Because of the variety of arenas in which a dusty plasma may play a role, it is important to understand the physical properties of this plasma system.

Laboratory plasmas -

• In controlled laboratory settings, many of the phenomena of interest to both the industrial and space plasma communities can be explored.
  • Collective behavior - Many experiments, including those at Auburn, are interested in measuring the properties of collective (wave) modes in dusty plasma.  Much of this work is used to compare against theoretical predictions.
  • Plasma (Coulomb) Crystals - Under the right conditions, the dust particles can form crystal-like 2-d and 3-d structures.