Plasma Physics in Zero Gravity
In basic research, it is important to understand the properties of matter in detail and to create completely new forms of matter. This often results in practical applications. In addition to the earthly laboratory, weightlessness and microgravity offer unimagined possibilities. Here we present the complex plasmas that are explored and manipulated in parabolic flights and on space stations.
Scientific experiments have been carried out on the International Space Station ISS for almost 20 years now. In addition to medical and biological studies, researchers there investigate issues of materials science and physics. One of the first experiments was the PKE Nefedov laboratory for studying dusty plasmas in zero gravity. PKE stands for Plasma Crystal Experiment, which we will explain in detail.
The conditions on the ISS are unique and unthinkable on Earth. Only here can the most diverse systems be studied in almost ideal weightlessness over a longer period of time. Weightlessness is not understood to mean the absence of gravity, as is generally assumed, but rather that gravity alone acts on a body (apart from apparent forces such as centrifugal force). The best example is the free-falling elevator in the earth's gravitational field in a vacuum, where weightlessness prevails. This was Albert Einstein's thought experiment that helped him develop the general theory of relativity. Such experiments are used in drop towers such as those at the Center for Applied Space Technology and Microgravity (ZARM) in Bremen to create weightlessness. The ISS also "falls" around the earth at 28,000 kilometers per hour, so that it is in a state of weightlessness. This is often also referred to as microgravity, since there is no absolute weightlessness on the ISS, but accelerations in the range of 10-5 times the gravitational acceleration g exist. 1 g corresponds to an acceleration of 9.81 meters per second squared that a body experiences due to gravity near the surface of the earth towards the center of the earth.
Why is it now of interest to carry out physical experiments in weightlessness? There are various reasons for this: First of all, gravity has a disruptive effect on certain types of research. For example, liquids are strongly influenced by it. Apart from small droplets, they form flat surfaces on earth, while in weightlessness liquids appear as spheres due to surface tension. Sedimentation, buoyancy and thermal convection, i.e. the rise of the warmer and lighter liquid and the sinking of the cooler and heavier liquid, which do not occur in weightlessness, can impair some investigations or precision measurements on earth. For example, thermophysical properties of melts, such as density or viscosity, knowledge of which is necessary for the simulation of new materials can be determined much more precisely in microgravity. The Electromagnetic Levitator experiment, EML for short, is currently on the ISS under the direction of the Institute for Materials Physics in Space of the German Aerospace Center (DLR) in Cologne.
Another interesting area for research under microgravity conditions is the physics of "soft matter". This means micro- to millimeter-sized building blocks embedded in a homogeneous background (liquid or gas). and can be easily moved against each other. These include colloids, which are solid particles in a liquid, or foams, which are small air bubbles in a liquid. Here, the sedimentation and the buoyancy of the particles in colloids or the breakdown of foams due to the liquid running out under gravity prevent precise studies of stable systems over a longer period of time. These experiments can contribute to the basic understanding of the physics of these and similar systems and are therefore used for basic research; however, they can also be of interest for technical applications. Finally, granular media and quantum systems such as the Bose-Einstein condensates (see p. 24) also represent important research topics under microgravity.