Mar 17, 2021 8:00 AM

An Ultracold Plasma Models the Universe’s Most Extreme Places

The super cool, dense particle swarm gives physicists a way to study the insides of stars—without ever leaving the lab.

Though plasma is the most common state of matter in the universe, it doesn’t get as much love as its gaseous, solid, and liquid counterparts. For most of us, plasmas seem like something exotic and reserved for space: hot concoctions of positively and negatively charged particles. A team of physicists from the University of Hamburg would beg to differ. In a new study published in Nature Communications in January, they created an unprecedentedly dense plasma out of a few thousand extremely cold atoms by hitting them with laser light for just one quadrillionth of a second.

A plasma is like a gas that got energetic enough for its atoms to slightly fall apart. All the atoms within a plasma have lost some of their electrons. They became positively charged ions, but are still hanging out with the electrons that escaped them. Such mixtures of ions and electrons are created in extreme environments. The interior of the sun is a plasma, as are the insides of white dwarf stars and possibly gas giant planets, and lightning bolts. In every case, thermal or electric energy turns a gas of well-behaved, neutral atoms into a fiery plasma. A plasma like the one created by the Hamburg team could soon serve as an emulator for these astronomical systems that are difficult to study otherwise.

Physicists have been creating ultracold plasmas since the late 1990s, but the new plasma stands out because it is about million times denser than any of its ultracold predecessors. “It's a completely new regime,” says Philipp Wessels-Staarmann, a physicist at University of Hamburg and co-leader of the team. “Nothing that was possible [in the lab] before.”

The physicists used rubidium atoms that had been cooled to temperatures about a millionth of a degree Kelvin above absolute zero as a starting point for their plasma—an extremely cold temperature instead of the extremely hot one inside the sun. They used lasers and magnets to trap the atoms in place and lower their energies so much that they stayed virtually still instead of jiggling in place like room temperature atoms do. Then they hit them with an energetic burst of light that lasted for only a millionth of a billionth of a second and tore off all of their electrons. Instantaneously, the system became a dense plasma. In a machine small enough to fit into a room, they now had an analogue of a star’s interior smaller than the diameter of a human hair. Actual stars cannot be directly experimented on, but this system was fully at their disposal.

Video: UHH/Mario Großmann

But how can an ultracold plasma tell us anything about super hot stars? Thomas Killian, an ultracold plasma physicist from Rice University who was not part of the study, says that, for example, in order to study how heat or mass are transported from one place to another inside a white dwarf, physicists can use ultracold plasma laboratory experiments as a benchmark. “If we can measure transport rates in these experiments, it really tests the exact same theories,” he notes.

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.”

As his team continues to experiment on their plasma, their ability to add complexity to its structure and make precise measurements will further the fundamental understanding of this state of matter. They are enthusiastic about pushing their plasma to interact even more in upcoming studies. They aim to tweak the burst of light they use to make it—it will be crucial to be able to tell their electrons exactly what to do after leaving their atoms.

Even though this machine provides an insight into a plasma more extreme than any that have been created before, the team feels like they are just getting started. “It’s a nice simulator,” Simonet says of their machine. “Let’s say that we just checked that it works.”

Update 3-19-2021 3:32 pm: This story was updated to reflect that while it has been theorized that plasma can be found in the interior of gas giant planets, this has not been proven.