Ultra-cold plasma models the most extreme places in the universe
The plasma made by physicists in Hamburg is a good candidate for such tests because it was, in a way, more extreme than ever. Because it was really dense, the electrical couplings – the interactions between the charged particles within it – were very strong. Making a highly interactive plasma has always been both a wishlist item and a technical challenge for ultra-cold plasma physicists, says Steven Rolston, a pioneer in the field and a University of Maryland scientist who did not participate in the study. “In fact, plasmas don’t like to be strongly coupled,” he says. Once the atoms in the plasma become charged ions, he says, if there is enough time left, their electrical potential energy can build up and cause them to quiver, overpowering the interactions that couple them.
Due to the difficulty of designing them in laboratories and reaching them in space, strongly coupled plasmas represent largely unexplored terrain for physicists. This is a state of matter that scientists do not yet fully grasp and want to explore further.
Part of the success of the new experiment, according to Juliette Simonet, co-head of the Hamburg team, comes from bringing together experts in ultra-cold and ultra-fast physics. This resulted in the use of extremely cold, controlled atoms as the basis of the experiment and an extremely fast laser as the primary tool to manipulate them. “It’s a great collaboration between the two research areas,” she says.
The machine his team built also allowed researchers to directly track what electrons did after they broke loose from their atoms. In previous experiments, physicists only deduced what could happen to them by measuring other aspects of the plasma. Here, they determined that the laser pulse caused the temperature of the electrons to skyrocket to over 8,000 degrees Fahrenheit for a moment before they cooled in response to the attraction of the ions. “It is beyond anything that has been seen so far,” Simonet says of this detailed sighting.
Such details have so far escaped the theories of physicists, Killian says. “Many of the standard theories used in plasmas describing how energy is transported or mass is transported through the system do not work in this area. [interaction] diet, ”he notes.
To make sure they understood what they were seeing, the Hamburg team turned to computer calculations. Because their plasma was very small, Mario Grossman, a graduate student of the group and co-author of the study, said they could calculate how each plasma particle interacted with the others. It was like asking a computer to describe the noise in a crowded room, pulling together minute details of conversations between two people.
For their 8,000-particle system, he had to wait up to 22 days for a computer to produce results. Encouragingly, the simulated plasma particles did almost exactly what the researchers saw real particles do in their experiment. This simulation approach, however, would not be practical for any larger, natural plasma.
“Most of the theory has really been some sort of brute force – ‘Let me just put it on a really big computer and calculate the interactions’ – that doesn’t work out well,” Rolston admits. He points out that there may not be computers powerful enough to simultaneously handle every particle interaction in large plasmas. A more sophisticated theory would zoom out, forget about particle details, and predict the behavior of plasma based on its properties as a whole.
This type of theory would help both ultra-cold physicists and researchers studying celestial bodies. It could predict when strongly coupled plasmas can develop ripples or sustain electrical currents. These predictions could be tested in laboratory experiments on Earth and offer insight into the evolution – even mergers between – white dwarves in space. “We have an initially super-coupled plasma,” says Wessels-Staarmann. “The interesting thing would be to really maintain that coupling, so that you can really contribute to what’s going on with a white dwarf.”