D. Samsonov

Particle growth in a sputtering discharge

Submicron to micron size particles are produced in the gas phase of sputtering discharges. These particles can contaminate thin films grown by sputter deposition. On the other hand, particle production in a discharge can be desirable when used for manufacturing fine powders. Here, experimental results are presented demonstrating particle production in an argon discharge, using a variety of target materials. The rate of particle growth varied widely, depending on the target material. Particles grown to 300 nm–5 μm, usually have one of two different shapes. Compact particles with a nearly spherical shape were produced by sputtering graphite, titanium, tungsten, and stainless steel targets, while filamentary-shaped fractal particles formed when sputtering aluminum and copper targets. Particle growth was also observed for a target made of an insulating material.

Line ratio imaging of a gas discharge

The intensity ratio of two atomic or ionic spectral lines in a plasma is a function of electron temperature. By imaging two spectral lines, and computing the line ratio, pixel by pixel, an image of the electron temperature is produced. This is demonstrated using a dusty plasma consisting of submicron particles suspended in an argon RF glow discharge.

Mutual interactions of magnetized particles in complex plasmas

Various mutual dust-dust interactions in complex plasmas, including the forces due to induced magnetic and electric moments of the grains are discussed. It is shown that the dipole short-range forces can be responsible for the formation of field-aligned chains. Such chains may incorporate a few tens of individual particles, as frequently observed in experiments.

Large-scale steady-state structure of a 2D plasma crystal

An analytical model, based on a simple physical analogy, and a non-linear analytic/numerical model of the large-scale structure (averaged over the size of a single cell) of a two-dimensional (2D) lattice have been developed. In the first model, a physical analogy between the lattice layer steady state and the stressed state of a rotating solid body was used to derive the model equations and the model is shown to be in good agreement with the results of a 2D simulation. The non-linear model is derived from the force balance between external and internal forces in the continuum limit and compares favourably with an experimental example.

Shock Waves and Solitons in Complex (Dusty) Plasmas

Shock waves and solitons were obtained in 2D complex plasmas i.e. plasmas mixed with micron‐sized particles. These particles acquire large (1000 – 50000e) negative charges and strongly interact with each other. The particle cloud can form ordered structures and exist in a solid, liquid, or gaseous state. A monolayer hexagonal lattice was formed from monodisperse plastic micro‐spheres in the sheath of an rf discharge and excited with an electrostatic pulse. It was found that weak pulses produced solitons, which did not change the phase state of the lattice. Stronger excitation created shock waves. The lattice melted behind the shock front and later recrystallized. Two shock regimes were distinguished. One had a stable, thin, well defined front, the other had an unstable front. The shocks were analyzed by tracking individual particles with a video camera and calculating their velocity, number density, kinetic temperature, and defect density. Molecular dynamics simulation reproduced the experimental results....

Instabilities driven by ion drag

When ions stream past a charged dust particle, their trajectories are deflected, resulting in the ion drag force, Fi. This force scales as the square of the particle charge, and therefore as the square of the particle diameter, in contrast to the Coulomb force QE which scales linearly with particle diameter. When the particle has a negative charge, the two forces point in opposite directions. The net force, which is the sum of these two forces, will then reverse direction at a critical particle size. In a series of experiments, it is shown that this effect leads to an instability. Particles are grown in an rf sputtering plasma, so that their diameter and therefore charge increase with time. The dust cloud is confined by the electrostatic force QE, due to global electric fields in the discharge. As the charge grows, the confinement equilibrium abruptly becomes unstable to very-low-frequency modes, in which the dust number density is modulated. The instability begins as the ‘filamentary mode,’ and then deve...

Structural Properties and Melting of 2D‐Plasma Crystals

Melting of a monolayer plasma crystal was induced by an electric pulse. We investigated, how structural parameters like defect fraction and correlation lengths as well as dynamical properties like the particle kinetic energy changed during the recrystallisation. As an indication of the phase transition, the change of the Lindemann parameter and the Coulomb coupling parameter were considered.

Vertical wave packets in a two-dimensional strongly coupled complex (dusty) plasma

Summary form only given. Propagation of vertical wave packets was observed experimentally in a hexagonal monolayer complex plasma. The complex plasma was formed of 8.9 /spl mu/m charged monodisperse plastic microspheres suspended a few millimeters above a flat electrode in a capacitively coupled radio-frequency discharge. Vertical oscillations were excited by a short electrostatic pulse applied to a wire stretched below the lattice. The lattice was illuminated by a thin laser sheet and observed with a top view high speed video camera. A new method of vertical motion diagnostics based on particle visible brightness was used. Wave packets propagating away from the excitation source were observed. They kept their width L=18 mm (equal to the modulating wavelength) constant. It was found that the phase velocity of the vertical waves exceeded its group velocity by a factor 70 and had the opposite direction as expected for an optical-like dispersion relation (backward wave). The complex plasma was in a strongly coupled (crystallized) state and had the following properties: the vertical wave group velocity V/sub gr/=4 mm/s, the vertical wave phase velocity V/sub ph/=-290 mm/s, and the compressional (dust-lattice) wave velocity C/sub DL/=35 mm/s. The theory describing the lattice motion is based on three-dimensional equations of motion and uses a long wave-length weak dispersion weak inhomogeneity approximation. It takes into account interaction of each particle with each other and is not limited by the nearest neighbour approximation. It was found that the constant wave packet with is provided by a balance of dispersion and weak lattice inhomogeneity. While the dispersion causes the wave packet to spread, lattice inhomogeneity focuses it. Neutral gas damping can also counteract packet spreading. The theory describes propagation of well formed wave packets (not their initial stages after the excitation) and shows a good agreement with the experiment. A new method of plasma diagnostics was developed that is based on the ratio between vertical and dust-lattice waves speeds. This ratio is very sensitive to the screening parameter /spl kappa/ in a very useful range of /spl kappa//spl les/2. It was found that only a two-dimensional lattice model can provide a quantitative description of the vertical waves, while a linear chain model gives only a qualitative agreement.

Vertical oscillations of magnetized particles in complex plasmas

Summary form only given. In this article, we treat grain levitation in an external magnetic field and consider vertical vibrations of the magnetized particles around an equilibrium state taking into account magnetic field gradients. For a single magnetized particle a novel type of vertical vibrations in discharge plasmas is found. These vibrations can be stable or unstable depending on the distribution of the magnetic field inside the particle cloud. The vertical resonance frequency is independent of particle mass, but is completely specified by the magnetic field profile inside the complex plasma and magnetic properties of the grain material. A numerical estimate of the resonance frequency for typical complex plasma parameters in the magnetic experiments, gives magnitudes /spl sim/50-100 s/sup -1/. Such values are easily measured in experiments and the latter can provide a tool for determining the complex plasma parameters. In a one-dimensional particle string the magnetic force causes a new low-frequency oscillatory mode, which is characterized by inverse optic-mode-like dispersion when the wavelength far exceeds the inter-grain distance. The characteristics of the mode are specified by the gradients of the external magnetic field and thus can be effectively controlled in experimental conditions. This opens new opportunities for the investigation of particle behavior at the kinetic level as well as for stimulating phase transitions in the system, and for the study of self-organized structures in the experiments.

Interactions of magnetized dust particles in complex plasmas

Summary form only given, as follows. Recently, multilayer complex plasma structures were studied in an external magnetic field In these experiments, spherical micrometer sized paramagnetic particles are levitated in the sheath region. The levitation increased with the strength of the magnetic field Moreover, some particles attracted each other and formed, elongated structures - grain chains, oriented vertically, parallel to the field lines of the external magnetic field. These features cannot be explained on the basis of pure electrostatic forces. Short-range dipole interactions between magnetized grains have to be invoked. Hence there is a need for a comprehensive treatment of various mutual dust-dust interactions, including the forces due to induced magnetic and electric moments of the grains. Which of the forces prevail is determined not only by the parameters of the dust particles, but is dependent on the discharge plasma conditions and magnitudes of the sheath electric and magnetic fields as well. Here, various mutual dust-dust interactions in complex plasmas, including the forces due to induced magnetic and electric moments of the grains are discussed. It is shown that the dipole short-range forces can be responsible for the formation of the field-aligned chains. Such chains may incorporate a few tens of individual particles, as frequently observed in experiments.