Dynamics near the critical point
Special features emerge at the critical point, such as an indistinguishable liquid and vapor phases, or divergence of compressibility, thermal-expansion coefficient and specific heat. It is also said that thermal diffusion worsens at the critical point. The D-1 mission conducted in Germany in 1985 discovered, however, that heat was transported tremendously fast. Later, experiments using the Space Shuttle and sounding rockets were conducted primarily in Europe. In 1990, Prof. Akira Onuki et al., Kyoto University, proposed a model of this anomalous transport. The phenomenon was named the “piston effect” by Zappoli et al. of Centre National d'Etudes Spatiales (CNES) in France in 1990. The model by Onuki et al. explains that, even when temperature change is very small, rapid thermal expansion generates a compressional wave like a shock wave that performs the rapid thermal transport. There have been no experiments, however, that observed directly the compressional wave’s propagation. Our WG intends to verify the model by measuring the compressional wave’s propagation experimentally. On the ground, however, density fluctuation around the critical point causes a density gradient and, as a result, only some regions come close to the critical point. Microgravity is expected to obtain a region being large enough to observe the compressional wave near the critical point.
The WG has been developing a experimental apparatus for future microgravity experiments with a sounding rocket. Fig. 3 shows a part of the developing apparatus, the sample cell component. The cell component can carry up to three sample cells. The cells will be enclosed by a three-fold thermal shield, allowing us to control temperature to a level of mK. The thermal shield also constitutes the body structure to improve vibration resistance.
Apart from the ISS, the only other ways to conduct microgravity experiments in Japan are drop-tower test facilities and aircraft. As it is expensive to develop experimental facilities onboard the ISS, new development is difficult. Although there are new themes of space experiment with scientific significance, it is expected to remain difficult to conduct Japanese original experiments. We believe that the solution is international cooperation.
Europe has been researching dusty plasmas for a long time. The Max Planck Institute for Extraterrestrial Physics (MPE) in Germany and the Institute for High Energy Densities (IHED) in Russia developed and loaded an apparatus named PKE Nefedov on the Russian Service Module in the ISS, and completed experiments with it. In December 2005, PK-3 Plus, the successor to PKE Nefedov, was also installed on the Russian Service Module and the experiment is planned to continue to around 2008. We are now discussing an international collaboration with MPE, IHED, the European Space Agency (ESA) and the German Aerospace Center (DLR) for Japan to join the PK-3 Plus project. We also seek a chance to use the European sounding rocket, TEXUS, for a short-duration microgravity experiment of dynamics near the critical point. For this experiment, we will proceed with cooperation with ESA and CNES. With regard to solid helium, microgravity also acts effectively because solid helium is as fragile as being deformed by gravity. This is a promising space-experiment theme for physics of crystal growth involving the quantum effect.