Fundamental Physics: Physics of plasmas and solid/liquid dust particles

Particle agglomeration

49th ESA Parabolic Flight Campaign

4 November 2008

A. Piel ⁽¹⁾, A. Melzer ⁽²⁾, O. Arp ⁽¹⁾, B. Buttenschön ⁽¹⁾, D. Caliebe ⁽¹⁾, K. Menzel ⁽¹⁾, K. Bluhm ⁽¹⁾, M. Poser ⁽¹⁾, T. Bockwoldt ⁽¹⁾

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Plasma is the state of matter, which exists at the highest temperatures. Surprisingly, in this regime of most violent atomic motion, where atoms are split into ions and electrons, regular crystalline structures of embedded micro-particles can form. This apparently paradoxical world of “complex plasmas” is the subject of our investigations in the laboratory and under microgravity conditions. The embedded nano- or micrometer sized particles become highly electrically charged and can strongly couple via Coulomb interaction. Instead, ordinary plasmas, which consists of singly charged atoms, are weakly coupled. Naturally occurring strongly coupled plasmas are found in “white dwarf” stars or in the core of giant planets. On the other hand, plasmas containing sub-micron sized particles can be found as ingredient or waste-product in industrial process plasmas, for example in the manufacturing of computer chips or innovative material surfaces, for example in microcrystalline solar cells.
In the laboratory, clouds of easily observable micrometer sized particles are compressed, due to their weight, to a flat nearly two-dimensional object. Under microgravity conditions we can study the dynamics and formation of volume-filling complex plasmas, such as the “void” phenomenon – the appearance of a dust-free area in the center of weightless complex plasmas. We observe the motion of particles by video microscopes, measure spatially resolved plasma densities and potentials and, thus, learn about the forces which determine the particle locations, excite density waves and open the void. The major topics of this year's parabolic flight campaign are as follows:
Observation of dust density waves inside the complex plasmas. These waves emerge under certain conditions spontaneous or can be excited externally by an electrostatic probe or intense laser radiation.
Test of a new stereoscopic diagnostic, which will be used to analyze the dynamics of the dust particles in three dimensions.
Test of a new dust accelerator device, which will be used to study the interaction of fast moving particles with a stationary dust could.

The experiment consists of two racks. The main rack contains the plasma chamber, vacuum and gas-handling system, particle illumination and manipulation lasers, optical diagnostics, i.e a high speed camera, the stereoscopic diagnostic and a plasma glow diagnostic. The second rack houses various control and data recording computers.
A plasma is generated in a low pressure argon atmosphere by applying radio-frequency power at 13.56 MHz to the electrodes. Micrometer-sized plastic particles (3.4 µm or 6.8 µm diameter) are injected into the plasma by two dust dispensers. Under microgravity conditions the particles fill generally the whole plasma between the electrodes except for a central particle-free void region.
The standard optical diagnostic is a high-speed camera, which is mounted together with a vertical illumination laser sheet on a translation stage. By moving the camera together with the laser sheet, we can observe thin vertical slices of the dust cloud at different positions inside the plasma chamber. This 2D diagnostic is complemented by a new stereoscopic diagnostic, consisting of three movable high-speed cameras, which observe a small volume of the dust cloud from different angles. This diagnostic will yield the full three-dimensional information of the particle dynamics in this volume.
Apart from the optical diagnostics, we perform measurements of the plasma properties with a so-called Langmuir probe. The probe can be positioned in a two-dimensional cross section of the chamber and measures the density and temperature distribution of the electrons as well as the electric field within the plasma, which determines the forces acting on the dust particles.
Aiming the additional movable beam of the high power manipulation laser at a certain position in the plasma, a radiation pressure is exerted and accelerates the particles. From the resulting movement the local magnitude of forces is determinable. A random scan of the laser agitates the complex plasma, increases its temperature and makes the melting process of particles structures visible to the experimenters.
Besides the particle acceleration by means of radiation pressure, we use a mechanical dust accelerator to generate fast particles. The accelerator uses a quickly spinning cogwheel to produce a shower of fast dust particles. The interaction of these accelerated particles and a stationary dust cloud will be studied.

This campaign is not only of scientific interest, but it is also intended as a technical test of the new setup, which we intend to use in a series of additional campaigns funded by the German aerospace agency DLR. A significant amount of experimental time was spent to check the correct function of the experiment under microgravity conditions and to identify problems and subsystems which need to be improved.
From the technical point of view, the experiment worked as expected under microgravity conditions during the 49th ESA parabolic flight campaign. No major technical problems were encountered. We were able to complete all scheduled measurements and recorded several hundred gigabytes of data.
However, a first examination of the data showed that the dust cloud inside the plasma was much stronger affected by residual gravity, as compared to the old experiment in previous parabolic flight campaigns. This resulted in more or less strongly disturbed dust clouds during many parabolas. After the campaign we found that this problem was related to electric asymmetries in the discharge due to the modified electrode setup.
Further, the mechanical stability of the new stereoscopy setup turned out to be not as good as expected, which led to unwanted vibrational disturbances in the video data.
Nevertheless, we were successful in obtaining high-quality data of the particle dynamics under microgravity conditions.
The new dust accelerator device behaved significantly different under microgravity conditions than during laboratory tests. It was possible to inject large amounts of particles into the plasma, but the particle velocities were not as fast as desired. Nevertheless, the measurements showed interesting flow patterns of the injected particles and gave us useful hints for improvements.
All the mentioned problems have been tackled and fixed in the meantime. All systems proved their correct function during the 14th DLR PFC in September 2009.
Despite the mentioned problems, the analysis of the recorded data revealed several interesting new results, which have been presented and published in numerous contributions to conferences and in peer-reviewed journals (see below).
One example is the simultaneous observation of the plasma glow and the dust density wave. For the first time, we were able to detect a significant effect of the dust density wave on the plasma glow. We found a periodic modulation of the plasma glow with an unexpected phase relation to the dust density wave.
A second interesting result was obtained by a new high speed camera for the observation of dust density waves. It allowed us to improve our data quality significantly. By moving this camera along its optical axis we recorded adjacent parallel slices of the dust cloud to obtain information on the three-dimensional shape of the complicated wave pattern in the dust cloud. Under certain experimental conditions it was even possible to use the resulting data to make a complete three-dimensional reconstruction of the wave field.

Olivier Minster (e-mail: olivier.minster@esa.int)