J. Goree

Charging of particles in a plasma

Several models that predict the charge of particles in a plasma are reviewed. The simplest is based on orbit-limited probe theory. This basic model can be improved by adding several effects: charge reduction at high dust densities, electron emission, ion trapping and fluctuations. The charge is reduced at high dust densities, when a significant fraction of the charge in the plasma resides on the particles, depleting the plasma. Electron emission due to electron impact or ultraviolet exposure can cause a particle to have a positive charge, which has useful implications for plasma processing, since particles are confined in a discharge only if they have a negative charge. Ion trapping occurs due to ion-neutral collisions within the attractive Debye sphere of a negatively charged particle. Trapped ions reduce the net electric force on a particle. A particles charge fluctuates because the currents collected from the plasma consist of discrete charges arriving at the particle at random intervals. The root mean square fractional fluctuation level varies as 0.5(N)- 12 / where (N)=(Q)/e is the mean number of electron charges on the particle.

Dusty Plasmas in the Laboratory, Industry, and Space

Charged microparticles are an annoyance in the plasmas of fusion energy schemes and semiconductor manufacturing. But in laboratory plasmas and in space, they can be uniquely informative.

Superdiffusion and Non-Gaussian Statistics in a Driven-Dissipative 2D Dusty Plasma

Anomalous diffusion and non-Gaussian statistics are detected experimentally in a two-dimensional driven-dissipative system. A single-layer dusty plasma suspension with a Yukawa interaction and frictional dissipation is heated with laser radiation pressure to yield a structure with liquid ordering. Analyzing the time series for mean-square displacement, superdiffusion is detected at a low but statistically significant level over a wide range of temperatures. The probability distribution function fits a Tsallis distribution, yielding q, a measure of nonextensivity for non-Gaussian statistics.

Killing of S. mutans Bacteria Using a Plasma Needle at Atmospheric Pressure

Streptococcus mutans (S. mutans) bacteria were killed using a low-power millimeter-size atmospheric-pressure glow-discharge plasma or plasma needle. The plasma was applied to a culture of S. mutans that was plated onto the surface of an agar nutrient in a Petri dish. S. mutans is the most important microorganism for causing dental caries. A spatially resolved biological diagnostic of the plasma is introduced, where the spatial pattern of bacterial colonies in the sample was imaged after plasma treatment and incubation. For low-power conditions that would be attractive for dentistry, images from this biological diagnostic reveal that S. mutans was killed within a solid circle with a 5-mm diameter, demonstrating that site-specific treatment is possible. For other conditions, which are of interest for understanding plasma transport, images show that bacteria were killed with a ring-shaped spatial pattern. This ring pattern coincides with a similar ring in the spatial distribution of energetic electrons, as revealed by Abel-inverted images of the glow. The presence of the radicals OH and O was verified using optical-emission spectroscopy

Fluctuations of the charge on a dust grain in a plasma

A dust grain in a plasma acquires an electric charge by collecting electron and ion currents. These currents consist of discrete charges, causing the charge to fluctuate around an equilibrium value /spl lang/Q/spl rang/. Electrons and ions are collected at random intervals and in a random sequence, with probabilities that depend on the grains potential. We developed a model for these probabilities and implemented it in a numerical simulation of the collection of individual ions and electrons, yielding a time series Q(t) for the grains charge. Electron emission from the grain is not included, although it could be added easily to our method. We obtained the power spectrum and the RMS fluctuation level, as well as the distribution function of the charge. Most of the power in the spectrum lies at frequencies much lower than 1//spl tau/, the inverse charging time. The RMS fractional fluctuation level varies as 0.5 /spl verbar//spl lang/N/spl rang//spl verbar//sup /spl minus/1/2/, where /spl lang/N/spl rang/=/spl lang/Q/spl rang//e is the average number of electron charges on the grain. This inverse square-root scaling means that fluctuations are most important for small grains. We also show that very small grains can experience fluctuations to neutral and positive polarities, even in the absence of electron emission. >

Experimental observation of very low-frequency macroscopic modes in a dusty plasma

Images of a cloud of grains in a dusty plasma reveal a pair of very low‐frequency modes, termed here the filamentary and great void modes. The plasma was a radio‐frequency discharge formed between parallel‐plate graphite electrodes. A cloud of 100 nm carbon particles was produced by accretion of carbon atoms produced by sputtering the graphite. The cloud was illuminated with a laser sheet and imaged with a video camera. The great void mode was a spoke‐shaped region of the cloud that was free of dust and rotated azimuthally in the discharge. The filamentary mode had the appearance of turbulent striations, with a smaller amplitude than the great void. The filamentary mode sometimes appeared as a distinctive vortex, curling in the poloidal direction. Both modes had a very low frequency, on the order of 10 Hz. Two possible causes of the modes are discussed. The low phase velocity of the modes may be consistent with a dust‐acoustic wave. Alternatively, the great void may be an ionization wave that moved the du...

Collisional plasma sheath model

The effects of ion collisionality on the plasma sheath are revealed by a two‐fluid model. In contrast to previous work, the ion–neutral collision cross section is modeled using a power law dependence on ion energy. Exact numerical solutions of the model are used to determine the collisional dependence of the sheath width and the ion impact energy at the wall. Approximate analytical solutions appropriate for the collisionless and collisionally dominated regimes are derived. These approximate solutions are used to find the amount of collisionality at the center of the transition regime separating the collisionless and collisional regimes. For the constant ion mean‐free‐path case, the center of the transition regime for the sheath width is at a sheath width of five mean‐free paths. The center of the transition regime for the ion impact energy is at a sheath width of about one‐half of a mean‐free path.

Shear Flows and Shear Viscosity in a Two-Dimensional Yukawa System (Dusty Plasma)

The shear viscosity of a two-dimensional liquid-state dusty plasma was measured experimentally. A monolayer of highly charged polymer microspheres, with a Yukawa interaction, was suspended in a plasma sheath. Two counterpropagating Ar+ laser beams pushed the particles, causing shear-induced melting of the monolayer and a shear flow in a planar Couette configuration. By fitting the particle velocity profiles in the shear flow to a Navier-Stokes model, the kinematic viscosity was calculated; it was of order 1 mm(2) s(-1), depending on the monolayers parameters and shear stress applied.

Radiation pressure and gas drag forces on a melamine-formaldehyde microsphere in a dusty plasma

Measurements are reported for the radiation pressure and gas drag forces acting on a single melamine-formaldehyde microsphere. The radiation pressure force coefficient q, which would be unity if all incident photons were absorbed, has the value q=0.94±0.11. For argon, the Epstein gas drag force coefficient δ, which would be unity if impinging molecules underwent specular reflection, has the value δ=1.26±0.13 as measured with our single-particle laser acceleration method, or δ=1.44±0.19 as measured using the vertical resonance method.

Model of energetic electron transport in magnetron discharges

A particle model of energetic electron transport in sputtering magnetron discharges is presented. The model assumes time‐independent magnetic and electric fields and supposes that scattering by neutral atoms is the dominant transport mechanism. Without scattering, we find that some orbits are confined indefinitely. Using the differential cross sections for elastic, excitation, and ionization collisions in argon, we perform a Monte Carlo simulation of the electrons emitted by ion bombardment of a planar magnetron cathode to predict the spatial distribution of ionization. We find good agreement with experimental measurements of the radial profile of ion flux to the cathode and of the axial profile of optical emission.