One of the most frequently-used physics laboratories on the International Space Station recently completed its final set of experiments. The Plasma Kristall Experiment (PK-3 Plus) lab, a Russian-German cooperation in operation since January 2006, has provided new insight into an unusual type of matter called plasma crystals. Though the experiment runs came to a close on June 14, the research continues to open an exciting world of potential technological spin-offs in medicine, agriculture and general science.
The unique environment of microgravity allows physicists to study how these crystals form inside dusty plasmas--a type of matter with unique properties found everywhere--in ways not possible on Earth.
Plasmas are one of the four states of matter, along with liquid, solid and gas. Complex or "dusty" plasmas get their name from the presence of small solid particles mixed into the plasma's charged gases. These particles can dramatically change the behavior of a plasma, and sometimes the particles even form crystalline structures. Dusty plasmas are found near artificial satellites, occur in Earth's upper atmosphere, in interstellar clouds and can be produced in lab settings.
Dusty plasmas are favored by physics researchers because they are relatively easy to control and provide a unique view of physics at the single-particle level. This form of matter can illuminate basic kinetic theory, including how colloids mix, how liquids and solids move and how waves propagate. But in many cases they're difficult to study in pure form because Earth's gravity affects the way dust particles settle and how they crystallize. That's not the case aboard the space station, however.
The PK-3 Plus investigation was designed to create dusty plasmas containing argon or neon gas as well as small, micron-size particles. A radio-frequency discharge device ionizes the gas molecules so they form a plasma, and the particles are injected into it. A laser lights them up and a camera records what happens as the particles move through the plasma and organize themselves in crystal structures. Basic experiments are designed to test a wide range of particle sizes and different gas types, and researchers have found a plethora of interesting new phenomena.
As researchers have found through a wide array of other investigations on the space station, including the study of dusty plasma physics, the exact transfer from basic research to applications on Earth is unpredictable and often works in unexpected ways. This shows the importance of funding basic research without imposing application-oriented restrictions.
Results from PK-3 Plus could eventually be used in agriculture, medicine and hygiene, according to Prof. Gregor E. Morfill, director at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, where the experimental apparatus was built. Because it is a gaseous state, plasma can disinfect surfaces quickly and efficiently, even neutralizing drug-resistant bacteria like MRSA in seconds. It has been demonstrated in clinical trials (more than 3,500 applications) that plasma helps disinfect chronic wounds and speeds up wound healing. Other studies have shown that in concert with chemotherapy, it efficiently fights cancer, boosting the cancer cell inactivation by up to 500 percent compared to chemotherapy alone. Plasma can even jump-start plant growth.
"In my case, the technical challenges of the space research came first, and provided the knowhow for the medical spin-off. In other words, without the [space station] research there would have been no engagement in plasma medicine on my part," said Morfill.
The research has benefits to basic science, too. In one example, researchers used the PK-3 Plus high-resolution camera to examine the exact point at which matter changes its phase from liquid to solid. Other experiments have tested how radio-frequency waves cause particles in a dusty plasma to move. Scientists saw particles at the top of the plasma rotating clockwise, while particles at the bottom of the plasma rotated counterclockwise.
Still another study measured the speed of sound through a dusty plasma, and found it was about 28 millimeters per second, much slower than the speed of sound in the air (about 340 meters per second). Measuring the speed of sound through any medium, such as water, air or a complex plasma, helps scientists understand the nature of the medium in greater detail.
The speed of sound result happened by chance, according to Dr. Hubertus Thomas, a scientist at the Max Planck Institute for Extraterrestrial Physics and principal investigator for PK-3 Plus.
"A single big particle was accelerated (and we don't know how!) into a huge cloud of 'small' particles, moved through the cloud like a projectile and formed a Mach-cone behind the particle," Thomas said. "Mach cones have been observed already under gravity conditions in 2-D plasma crystals, but this was the first observation in a 3-D structure and under microgravity. Fortunately, the particle was decelerating during its path, which allowed us to calculate the sound speed in our complex plasma."
Astronauts and cosmonauts operated the PK-3 Plus equipment during 20 separate missions, each lasting about five days. Scientists on the ground obtained test run results in near real-time, while video was downlinked from the station during the experimental runs. The lab also contained four hard disks for video storage, which were swapped when full and returned to Earth with station crew rotations. The PK-3 Plus equipment was designed by German researchers and delivered aboard a Russian Progress cargo spacecraft.
All told, collaborators on the PK-3 Plus investigation and its predecessor, PKE-Nefedov, which was named for the project's Russian lead scientist who passed away, have published more than 70 scientific papers and given at least 100 presentations at scientific conferences.
"Now that we have more than seven years [of research], I believe it is time to start a new era of complex plasma research with our next lab, PK-4, which is planned to be launched in October next year," Thomas said. "The resources (microparticles and gas) are depleting, the plasma chamber is getting more and more dirty, and the computer hardware and software produces some problems, which so far we could solve. The lab is just degrading, so we have to allow him/her to retire."
Along with illuminating the nature of matter, dusty plasmas have practical applications in space, on Earth and even on other planets. In plasma processing, for instance, removal of microscopic particles grown in the reactive processing plasma is crucial for preventing contamination of computer chips. A deep understanding of how gases and dusty plasmas interact is critical for improving plasma technology. Understanding this interaction could also help scientists create powders containing specific ingredients.
"New horizons are produced by the [plasma medicine] research practically on a monthly basis," Morfill said. "Exactly which of the fields will become commercialized first is difficult to tell at this early stage. All applications (and new ones not yet discovered) have enormous potential and touch on major worldwide problems."
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