T. Antonova

Study of the 3D plasma cluster environment by emission spectroscopy

Three-dimensional (3D) plasma clusters were formed inside a quasi-neutral plasma of very small size (38?mm3) obtained by applying a radio frequency (rf) to a small electrode at the edge of a main plasma. In order to find the density of such a plasma, spectroscopic analysis at three wavelengths was performed. The emission structure of the small plasma as well as of the whole discharge was obtained with a resolution of 0.5?mm. The optical thickness of the plasma allowed us to apply the steady-state corona model for the calculation of the plasma density. The density was estimated to be 2.8?1016?m?3 in the small plasma, one order higher than in the main plasma volume.

Complex-plasma manipulation by radiofrequency biasing

This paper presents an experimental study on the nature, the dimensions and the timescale of the perturbation introduced by radiofrequency (rf) biasing of areas adjacent to the plasma. The analysis of the rf sheath, and of the charging of particles in it, has disclosed a levitation force on particles, which is substantially different from the dc one often used in complex plasmas. Experimentally, the rf heavily loaded sheath presents characteristics completely different from the normal case v rf ≤ V dc . Regions of extra ionization and complex electrostatic structures arise. These have been visualized by nanoparticles grown in the plasma. A variety of equilibrium positions for a controlled number of microparticles (injected) can be achieved by fine balancing of do and rf on a pixel with the neighbouring sheath kept under control In certain situations gravity is completely compensated, allowing the study of three-dimensional clusters. The motion of clusters from 4 to about 100 particles is simultaneously monitored by a three-dimensional visualization based on two laser lights modulated in intensity. This method enables the study of time-varying effects, such as transitions and vibrations, as well as the study of static structures and lattice defects. At pressures below 40 Pa in large clusters a poloidal motion appears.

Collective effects in complex plasma

The aim of this chapter is to demonstrate that a complex plasma requires formulation of a new ground state with the balance of fluxes as an inevitable feature. The flux variation has a strong effect on the propagation of collective modes leading not only to the mode damping but also to an instability which leads to formation of structures in complex plasmas.

Agglomeration of mesoscopic particles in plasma

We disclose the basic mechanism of agglomeration of nano-sized particles. While for weakly coupled, mono-dispersed particles the electrostatic agglomeration has always been found to be unlikely, in strongly coupled complex (dusty) plasmas the occupation of positive states for small particles is relevant, leading to electrostatic attraction between differently charged particles. The occupation of positively charged states is further enhanced by dispersed distribution of size. The smaller particles are trapped by the larger, the accretion of which gives a positive feedback on the probability of positively charged small grains and then further accretion. Experiments on growth of carbon particles from sputtered graphite in RF and dc Argon plasma confirm the general theoretical prediction when the energy of the ions corresponds to plasma boundaries.

Frequency dependence of microparticle charge in a radio frequency discharge with Margenau electron velocity distribution

rf discharges are widely used in complex plasma experiments. In this paper, we theoretically investigate the dependence of the particle floating potential on the discharge frequency, assuming the model Margenau expression for the electron velocity distribution function. In doing so we use the orbital motion limited cross section to calculate the electron flux to the particle and collision enhanced collection approximation for the ion flux to the particle. The floating potential is then obtained from the flux balance condition. It is shown that for typical plasma conditions in laboratory rf discharges, normalized floating potential grows with increase of the discharge frequency in collisionless regime and decreases in weakly collisional regime. However, variations in the floating potential are usually small when plasma parameters do not depend on the rf frequency.

Energy relaxation and vibrations in small 3D plasma clusters

The time evolution of three-dimensional (3D) plasma clusters containing 17 and 63 particles has been analyzed. Using a radiofrequency (rf) spot electrode, we were able to get almost un-stressed 3D clusters under gravity conditions on Earth. Fast 3D diagnostics of the particle positions allowed us to study the cluster structure and dynamics in detail. In particular, we were able to follow the evolution of the systems through rearrangement and particle evaporation to their final equilibrium state with minimum energy. The vibrations of the larger (63 particles) cluster were compatible with theoretical estimates for a liquid drop with surface tension. This indicates that macroscopic properties, normally associated with systems in the cooperative regime, provide an adequate description even for small (discrete) clusters.

Microparticles deep in the plasma sheath: Coulomb “explosion”

A cloud of microparticles was trapped deep in the sheath of a radio-frequency (rf) discharge, very close to the lower (grounded) electrode of the plasma chamber. This was achieved by employing a specifically designed rf-driven segment integrated in the lower electrode, which provided an additional confinement compressing the cloud to a very high density. After switching the rf-driven segment off, the cloud “exploded” due to mutual interparticle repulsion. By combining a simple theoretical model with different numerical simulation methods, some basic properties of complex plasmas in this highly non-equilibrium regime were determined.

Parameters of a collisional radio-frequency sheath and dust characteristics resulting from the microparticle levitation

The screening length, the time-average electric field, and the particle charge as well as the local vertical gradients of these quantities are determined experimentally within a sheath of a capacitively coupled rf, 13.56 MHz, discharge at enhanced argon gas pressures of 30, 55, and 100 Pa. The parameters are derived directly from comparative measurements of levitation positions of the particles of different sizes and variations in the levitation heights caused by formation of new dust layers. The electrostatic effect of the horizontally extended dust layers on the sheath electric field is investigated.

Agglomeration of dust

The agglomeration of the matter in plasma, from the atomic level up to millimetre size particles, is here considered. In general we identify a continuous growth, due to deposition, and two agglomeration steps, the first at the level of tens of nanometres and the second above the micron. The agglomeration of nano‐particles is attributed to electrostatic forces in presence of charge polarity fluctuations. Here we present a model based on discrete currents. With increasing grain size the positive charge permanence decreases, tending to zero. This effect is only important in the range of nanometre for dust of highly dispersed size. When the inter‐particle distance is of the order of the screening length another agglomeration mechanism dominates. It is based on attractive forces, shadow forces or dipole‐dipole interaction, overcoming the electrostatic repulsion. In bright plasma radiation pressure also plays a role.

Instabilities in a complex DC plasma

Summary form only given. The PK-4 setup uses a DC discharge to create a plasma inside a glass tube in Argon or Neon at a pressure of around 1 mbar. Plastic micro-spheres injected into the plasma collect several thousand electrons and interact with each other over a screened (Debye-Huckel) potential. This tunable interaction allows to study matter on an atomistic level in nearly every state, from crystalline (‘plasma crystal’) to super-critical.1,2,3