An Ultracold Plasma Models the Universe’s Most Extreme Places


The plasma made by the physicists in Hamburg is a good candidate for such tests because it was, in a way, more extreme than any before. Because it was really dense, the electric couplings—the interactions between charged particles within it—were very strong. Making a strongly interacting plasma has always been both a wishlist item and a technical challenge for ultracold plasma physicists, says Steven Rolston, a pioneer in the field and a scientist at the University of Maryland who was not involved with the study. “Plasmas actually 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, their electric potential energy can build up and make them wiggle, overpowering the interactions that couple them together.

Because of how hard it is to engineer them in labs and reach them in space, strongly coupled plasmas represent mostly unexplored terrain for physicists. They are a state of matter that scientists don’t fully grasp yet and want to explore more.

Part of the success of the new experiment, according to Juliette Simonet, co-leader of the Hamburg team, comes from bringing together ultracold and ultrafast physics experts. This resulted in the one-two punch of using extremely cold and controlled atoms as the base of the experiment and an extremely fast laser as the main tool for manipulating them. “It’s a big collaboration between the two research fields,” she says.

The machine her team built also allowed the researchers to directly track what the electrons did after they broke off from their atoms. In past experiments, physicists only inferred what may be happening 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 just an instant before they cooled back down in response to the pull of the ions. “This is beyond anything that has been seen so far,” Simonet says about this detailed observation.

According to Killian, such details have so far also eluded physicists’ theories. “A lot of the standard theories that people use in plasmas that describe the way energy is transported or mass is transported through the system don’t work in this [interaction] regime,” he notes.

To ensure that they understood what they were seeing, the Hamburg team turned to computer calculations. Because their plasma was very small, Mario Grossman, a graduate student in the group and a coauthor on the study, says they could calculate how every plasma particle interacted with every other one. It was like asking a computer to describe the noise in a crowded room by gathering minute details of conversations between every two people.

For their 8,000-particle system, he had to wait for up to 22 days for a computer to produce results. Encouragingly, simulated plasma particles did almost exactly what researchers saw real particles do in their experiment. This simulation approach, however, would be impractical for any larger, naturally occurring plasma.

“Most of the theory really has been kind of brute force—‘Let me just put it on a really big computer and calculate interactions’—which scales poorly,” Rolston agrees. He points out that there may not be computers powerful enough to simultaneously handle every single particle interaction in big plasmas. A more sophisticated theory would zoom out, forget about the nitty-gritty particle details, and predict plasma behavior based on its properties as a whole.

This kind of theory would help both ultracold physicists and researchers who study 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 evolution of—or 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 this coupling, so then you can really contribute to what’s going on in a white dwarf.”

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