If we reduce the gravity to app. 10-4g, monodispers microparticles (polymer spheres of micron size) injected into a low temperature plasma arrange totally different than in a ground based laboratory setup with normal gravity. Here, the dominating forces are the electro-magnetic interaction that repells the particles from each other, but also attracts the particles towards the center of the plasma discharge; the drag force of the streaming ions; and the thermophoretic force that try to push the particles out of the discharge. In all experiments under microgravity conditions we observed an equilibrium that with most experiment parameters creates a particle-free zone in the center of the discharge, the so-called 'void'. It is surrounded by a region where plasma and particles are mixed.
Charged micro-particles inside the discharge gather around
the 'void' (particle-free zone) in the center of the plasma.
Since we try to study the plasma-particle interaction, a particle-free zone is not really wanted. But it provides a deeper insight into the relative relation between the forces mentioned above because at the boundary between plasma and complex plasma we have an equilibrium of these forces.
In the beginning the main activity of micro-g experiments was on sounding rockets (TEXUS) with 6 minutes and on parabolic flights with 25 seconds of µ-g time. Now we focus on experiments onboard the International Space Station where many hours of measurements are available. Since the crystallization of a complex plasma takes typically some minutes, the advantage of nearly unlimited micro-gravity time in space offers us a unique progress in complex plasma research.
The PKE-Nefedov experiment is made of a small (0.5 liters) plasma chamber where a high frequency (HF) voltage drives a gas discharge in Argon at a few mbar pressure. Monodisperse (equal-sized) microparticles with diameters of 3.4 or 6.8 µm are injected into the plasma. The particles are illuminated by a Laser sheet and observed with two CCD cameras with different magnification. The camera images are recorded on video tape recorders.
Main adjustable experiment parameters are gas pressure and HF power. The plasma parameters are monitored with several diagnostic sensors. The vacuum needed for the plasma chamber operation is provided by a connection to open space. The whole apparatus is sealed inside a cylindrical aluminum container to meet the strict safety demands of the space station. A laptop computer serves as operation terminal for the cosmonauts, controlling the experiment and also the video tape recorders to record the video data. First intention was to control the experiment from the ground (''tele-science'') but the failure of a relais satellite made it necessary to fly the computer also into space. Now the cosmonauts perform the experiments onboard or run pre-programmed measurement sequences uplinked to the ISS. Scientists from MPE and IHED follow the experiments in the Russian ground control center in Moscow.
The first permanent ISS crew (''Expedition One'') during
training with the Plasmakristall hardware.
Two identical PKE units, consisting of an experiment container and a telescience unit, were manufactured and underwent the mandatory space station quality tests. One unit, the training model, is used for cosmonaut training in Moscow. The other one, the flight model, was launched from Baikonur to the ISS with a Russian Progress cargo ship. The experiment container and the telescience unit are stored on the Russian segment of the ISS. The first and the third permanent ISS crew were trained several times in Garching and in Moscow to operate PKE Nefedov. The first crew (''Expedition One'': Gidzenko, Krikalev und Shepherd) performed the first planned experiment session in March 2001 very successfully. When returning back to Earth onboard the Space Shuttle, they brought also the first video tapes with experiment data from PKE Nefedov.
|© Max-Planck-Institut für extraterrestrische Physik|