Dusty Plasma Physics


Experiments with Complex Plasmas

Collaborators: Alexander Piel, (U. Kiel, Germany), Mark Koepke

Complex plasma is defined as plasma with embedded charged microparticles. Such plasma is found in astrophysical (interplanetary medium, dust rings, comets) and industrial (manufacturing of amorphous photovoltaic cells, etching of semiconductor microchips) situations. Interest in this field is growing rapidly worldwide with more than 500 articles on this subject every year. Complex plasmas have unusual roperties. Because the microparticles in a complex plasma attain high electric charge (10.000 electrons per particle for a particle of 10µm diameter), the system of microparticles can become strongly coupled, which leads to the formation of Coulomb solids, so-called plasma crystals [Thomas et. al., Phys. Rev. Lett. 73, 652 (1994)], and Coulomb fluids. Our experiments investigate waves in complex plasmas. Experimental research in complex-plasma waves lags the theoretical research and is being lead by a small number of groups, most notably the group at University of Iowa. Our project represents a new collaboration between plasma groups at Kiel and West Virginia in complex-plasma waves. The nonlinearity of the dynamics is being explored using techniques previously exploited by the Kiel and WVU plasma groups in past plasma experiments.

Waves in complex plasmas are studied in the small device MATILDA at Kiel University, which is operated with DC plasma generation. Dust particles (e.g. 10 m monodisperse polymer particles) are injected simply by striking a small, open reservoir to form a dust cloud within the plasma. The particles are confined by the ambipolar electric field of the plasma. For such small particles, the gravitational force is negligible, but ion drag forces need consideration which, under certain conditions, may cause the formation of particle free regions [Goree et. al., Phys. Rev. E 59, 7055 (1999)].

The Kiel group is among the leading experimental groups in the field of complex plasmas, having made important contributions to the understanding of the phase transition of the plasma crystal [Melzer et. al., Phys. Rev. E 53, 2757 (1996); Schweigert et. al., Phys. Rev. Lett. 80, 5345 (1998)], to the quantification of interparticle forces, and to the excitation of low-frequency oscillations and waves with focused laser radiation [Piel et. al., Plasma Phys. Contr. Fus. 41, A453 (1999)].

Dust can affect or create new wave types in plasma. For example, the dust-grain dynamics were crucial for the dust-acoustic waves [Rao et. al.., Planet. Space Sci. 38, 543 (1990); D'Angelo, Planet. Space Sci. 38, 1143 (1990)]. Experiments on the dust acoustic wave [Barkan et. al.., Phys. Plasmas 2, 3563 (1995); Thompson et. al.., Phys. Plasmas 4, 2331 (1997)] have verified theoretical predictions that the presence of the dust significantly modifies the frequency and growth rate of the ordinary ion-acoustic wave. These experiments also demonstrate that the relative number of free electrons, i.e., those not attached to the dust grains, affect the wave characteristics. The project is applicable to industrial, laboratory, and space dusty plasmas, where small differences in free-electron concentration lead to large deviations in the product, behavior, and evolution, respectively

Previously established techniques for diagnosing the plasma parameters and the equilibrium of dusty plasmas are being used, specifically, probe measurements and video microscopy [Thompson et. al.., 1997], respectively. Wave-phase selective analysis of the video data is an example of a new technique that has been developed in the project to analyze dust-wave propagation.

Reference

1.  I Pilch, A Piel, T Trottenberg,and M E Koepke, Dynamics of small dust clouds trapped in a magnetized anodic plasma, 2007 Phys. Plasmas 14 123704

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The listed article appeared in Physics of Plasmas and may also be found here.