Milenko Zuzic

The plasma condensation: Liquid and crystalline plasmas

Colloidal plasmas may “condense” under certain conditions into liquid and crystalline states, while retaining their essential plasma properties. This “plasma condensation” therefore leads to new states of matter: “liquid plasmas” and “plasma crystals.” The experimental discovery was first reported in 1994, and since then many researchers have begun to investigate the properties of condensed plasma states. In this paper we describe some of the basic physics required to understand colloidal plasmas and discuss experiments conducted to investigate the details of the interaction between the plasma particles (in particular, the interaction potential), the melting phase transition, and the thermodynamics of this new state of matter.

Wave propagation and damping in plasma crystals

Wave propagation through plasma crystals provides an interesting experimental approach towards understanding strongly coupled plasmas as well as certain solid state properties of crystals. In order to build up our basic understanding of conditions and processes, oscillation modes of one‐ and two‐particle systems are investigated first, and the damping process is identified by studying the amplitude variations of the oscillations as a function of the excitation frequency. Next, single layer plasma crystals are investigated using the same techniques. An induced spontaneous transition from the solid to the gas phase is observed, and its possible origin is discussed.

NonliNEAR vertical oscillations of a particle in a sheath of a rf discharge.

A new simple method to measure the spatial distribution of the electric field in the plasma sheath is proposed. The method is based on the experimental investigation of vertical oscillations of a single particle in the sheath of a low-pressure radio-frequency discharge. It is shown that the oscillations become strongly nonlinear as the amplitude increases. The theory of anharmonic oscillations provides a good quantitative description of the data and gives estimates for the first two anharmonic terms in an expansion of the sheath potential around the particle equilibrium.

Plasma crystals and liquid plasmas

General properties of strongly coupled colloidal plasmas are briefly summarised, and their properties of being able to “condense” into a self-organised liquid and crystalline form is discussed. Both laboratory and microgravity aspects of the research into this new form of matter are described and the theoretical constraints are compared with available measurements. Finally, the phase transition solid-liquid-gaseous is investigated using measurements of the “normalised interaction cross section,” ∑σp, derived from “molecular diffusion of the colloid component.

Detection of stochastic waves in plasma monolayer crystals from video images

The motion of dust particles confined in plasma monolayer crystals is analyzed from video images, under conditions dominated by dust-neutral collisions. In these crystals, dust-neutral collisions will act as a random driving force, exciting phonons with a stochastic nature. The phonons are investigated using standard statistical tools, including both single- and multiparticle correlation functions. Single-particle correlations as obtained from the velocity autocorrelation function yield oscillations in a very narrow frequency band. Similar behaviors have previously been reported for strongly coupled one-component plasmas, and for trapped Brownian particles. Spatial correlations in the crystal lattice are studied from multiparticle correlation functions, suggesting an average wavelength slightly larger than the dimension of the crystal. Throughout the crystal, the dust velocity amplitude and polarization vary significantly, with the main variation in the radial direction out of the crystal center. This sug...

Lattice Waves in Plasma Crystals

“Plasma crystals” have rapidly become an established means of investigating certain “solid state” properties. Their possible existence was first suggested by Ikezi on theoretical grounds, experimentally the first announcements came 8 to 9 years later. They consist of charged monodisperse microspheres (typical sizes are a few microns) embedded in a plasma, which in most investigations is generated by radio-frequency discharges and recently also in DC glow discharges 8 and combustion plasmas. The microspheres become self-organized by mutual Coulomb interaction into a regular crystalline structure — which is their minimum energy state — provided the Coulomb coupling parameter and the spatial density exceed certain thresholds [see e.g.,]. Since the original discovery there have been important fundamental investigations on processes such as the solid/liquid phase transition, dislocations and stimulated sublimation. For a review of our work on plasma crystals see [16]. The unique properties of plasma crystals, i.e., their fast response times (seconds), the small damping, the easy experimental control, the wide accessible range of parameters and the detailed imaging, possible even at high time resolution, make them ideal objects for studying dynamical effects of interest in crystal physics, colloidal physics, strongly coupled plasma physics and monolayer physics. Here we report on one such dynamical aspect of crystal physics — the propagation and damping of lattice waves. In a recent paper, Pieper and Goree examined the dispersion of compressional waves propagating through a colloidal (or dusty) plasma in the strong coupling regime. Over a wide range of coupling strengths where = Coulomb potential / kinetic energy) they found that the measured dispersion relation could be fitted rather well to the theory of damped dust acoustic waves (DAW, see [18]) but not dust lattice waves (DLW). Surprisingly, DLW were never seen! In many ways plasma crystals can be expected to behave much like natural crystals — the heavy, charged, microspheres correspond to the ions and the plasma in the Debye-sphere surrounding the microspheres may be viewed analogous to the electron cloud surrounding the ion. Thus plasma crystals should propagate “acoustic” lattice waves as well as the equivalent of the “optical branch” — the latter occurring at infrared–microwave frequencies, however.