Experiments under microgravity conditions were carried out to study "condensed" (liquid and crystalline) states of a colloidal plasma (ions, electrons, and charged microspheres). Systems with ϳ10 6 microspheres were produced. The observed systems represent new forms of matterquasineutral, self-organized plasmas-the properties of which are largely unexplored. In contrast to laboratory measurements, the systems under microgravity are clearly three dimensional (as expected); they exhibit stable vortex flows, sometimes adjacent to crystalline regions, and a central "void," free of microspheres.

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Experiments were carried out to investigate a three-dimensional (3D) plasma crystal. A method of determining the positions of each individual microparticle has been developed. A crystal volume of about 2x10(4) particles in 19 horizontal planes was analyzed. Direct imaging and the 3D pair correlation function show that "domains" of fcc and hcp lattices coexist in the crystal. Other structures, in particular, the theoretically predicted bcc lattice, were not observed.

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.

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 lowpressure radio-frequency discharge. It is shown that the oscillations become strongly nonlinear and secondary harmonics are generated as the amplitude increases. The theory of anharmonic oscillations provides a good qualitative description of the data and gives estimates for the first two anharmonic terms in an expansion of the sheath potential around the particle equilibrium. PACS number(s): 52.25.Zb, 52.25. Gj, Typeset using REVT E X * Permanent address: