Plasmas are denoted frequently as fourth aggregate state and consist of ionized gases, in which gas atoms are split into positively charged ions and electrons. Plasmas containing colloids are called complex or dusty plasmas. In 1994, scientists of the Max-Planck Institute for Extraterrestrial Physics proved that such complex plasmas can self-organize under certain conditions spontaneously to a crystalline-like state, the so called plasma crystal. Plasma crystals were an up to then unknown state in a complex/dusty plasma, and can be used to study material characteristics in phase transitions from gas to liquid and solid states. Such three-dimensional plasma crystals can only be produced under microgravity, since, on earth, gravity squeezes the crystals together.
Plasma Crystal Experiments to Control Microparticles Using an Adaptive Electrode
In the year 2001 the plasma crystal experiment on the International Space Station (ISS) was realized under the leadership of the German, Kayser Threde, GmbH. Figure 1 shows a schematic experimental set-up, which is similar to the experiment on ISS. The control and the manipulation of the microparticles in the investigated low-temperature plasmas, are achieved here by means of a so-called adaptive electrode. This adaptive electrode is composed of several separate, electronically controllable electrode segments. This allows local modifications of the plasma boundary zone.
Figure 1. Schematic diagram of a possible plasma crystal experiment chamber.
How Microgravity Research about Complex Plasmas Might Improve Some Industry Applications
Apart from basic research in fundamental and plasma physics, application-orientated questions like particle coating, the production of nanoporous materials or the optimization of plasma processes in semiconductor industries, can also be examined with the experimental set-up in principle. It is expected that knowledge about complex plasmas obtained in microgravity research will contribute to the optimization of industrial terrestrial plasma processes.
Potential Industry Applications and Processes for Complex Plasmas
To be mentioned as relevant application fields, among others, is the coating of pharmaceutical drugs and surface refinement in semiconductor technology (Stuffler 2001). Also, the formation of nanoscale carbon structures (nanotubes or diamond films) by electrical arc discharge plasma synthesis, has already been investigated in microgravity experiments by NASA. Furthermore, complex plasmas are relevant for processes in which a particle formation is to be prevented, if possible, as, for example, within plasma etching processes for microchip production. Here, a contamination of the sensitive circuits with particles must be absolutely avoided.
Knock-On Benefits of Developing the Adaptive Electrode
Development potential concerning the experimental set-up can be determined in the advancement of the adaptive electrode. The objectives pursued here are the integration of a larger number of manipulation channels with reduced surfaces and a miniaturized electrode structure, as well as the availability of dynamic and automated methods for an appropriate manipulation of the plasma. A further stimulation for the plasma research in microgravity is expected with the implementation of the International Microgravity Plasma Facility (IMPF), whose employment on ISS is scheduled for the year 2005/2006.