A world of “microgravity,” where gravity is extremely low compared to that of the earth, is found on the Space Shuttle and the International Space Station. In this strange space, we can observe phenomena different from those on the ground. Since sedimentation, which are caused by differences in density, does not occur in this space, we cannot observe the phenomenon of “heat convection” that occurs in a pot of boiling water on earth. Salad dressing will not separate into water and oil in this space. A stack of blocks of tofu (soybean curds) will not collapse under its own weight. “Containerless levitation,” where a liquid can be held without a container such as a cup, is another typical example of microgravity environment phenomenon.
Development of levitation technology
Gravity, albeit “micro-gravity,” and other forces are at work in the Space Station. Thus, even if we do nothing, samples floating in the station will move. To heat samples by heating laser beams or precisely measure the samples’ temperature, we need special equipment to control sample position without touching them.
Space agencies across the world have competed to develop levitation equipment (levitator). From the late 1980s to the early 1990s, NASA developed a levitator using acoustic waves and conducted experiments on the Space Shuttle, and Germany conducted experiments on the Shuttle with an electromagnetic levitator. Japan also developed acoustic-wave levitators. They were used for experiments on the Space Shuttle in 1992, during the first flight of Japanese astronaut Mouri.
Unfortunately, in those early levitation experiments, we were unable to obtain sufficient results taking advantages of containerless levitation because of insufficient skill that allowed us only to float samples. It was difficult to float, hold with stability, and melt samples even in a microgravity environment. Space agencies around the world later proceeded with R&D on levitators, e.g., producing ground models, and have made remarkable advances in levitation technology. A German-made electromagnetic-levitation furnace installed on the Space Shuttle in 1997 succeeded in stably floating and melting metal samples and produced much precious data.
Electrostatic levitation technology
For the levitator available to the International Space Station, JAXA selected the electrostatic levitation method and is now engaged in its R&D. This method uses Coulomb's force (the force of attraction arising between plus and minus, and the force of repulsion between plus and plus or between minus and minus) generated by electrodes located above and below a charged test sample. This method has some excellent features. For example, disturbance on test-sample is small compared with methods using acoustic waves or electromagnetic force, and it is possible to float any materials once we can charge them. Its development has stagnated, however, because of some technological issues, such as the need for fast feedback control to adjust the position of samples. JAXA is proceeding with research on this method based on basic technology achieved by the Jet Propulsion Laboratory (JPL), known for NASA’s planetary exploration.
Fig. 1 shows the position-control mechanism for the electrostatic levitation method. The position of the test sample is measured by projecting the sample shadow to a position sensor. In response to the difference between its measured position and planned position, a computer regulates the voltage to maintain stable levitation. In 1998, we conducted a six-minutes microgravity experiment with this method onboard a sounding rocket. In this experiment, we levitated and successfully melted a ceramic test sample. A problem remained, however, because it became difficult to stabilize the test sample’s position as the sample’s charge decreased when it melted. Based on this lesson, we are trying to improve the position-control technique through study on the ground. At the same time, we are trying to expand research fields using this levitation technique.
The electrostatic levitation furnace manufactured at JAXA for ground research is shown in the main figure. This furnace floats a test sample between electrodes (25mm diameter) with an interval of 10mm. On the ground, around 15,000 volts must be imposed between the two electrodes to generate enough force to offset gravity. Using this furnace, we can levitate a test sample up to tens mg weight and about 2 mm diameter. The test sample is heated by a carbon-dioxide gas laser and its temperature is measured by pyrometers. Position-control accuracy has been improved to keep the test sample under 50m allowance with the capability to heat it over 3,500 deg. C.