Deep Freezing Plasmas
By Jade Boyd
There are fewer than a dozen laboratories in the world working on ultracold neutral plasmas, but the field is growing quickly because technology is bringing previously unperformed experiments within reach.
Forget solids, liquids and gases. Plasmas are, by far, the most abundant state of matter, accounting for about 99 percent of the visible matter in the universe. But you’re not likely to encounter plasma here on Earth. Strongly interacting plasmas naturally occur only in very dense and energetic environments where it isn’t possible to set up a laboratory, such as a white dwarf star.
Rice University physicist Tom Killian remains unfazed. He’s one of a growing group of researchers worldwide who are unlocking some of the mysteries of plasmas by doing something nature never does — freezing them to less than a degree above absolute zero.
“Our plasmas behave differently because they’re cold," Killian said. “The particles inside them slow down to the point that they feel one another and interact with their neighbors much more strongly than standard plasmas, and we have the technology to take pictures of them while they do it." He hopes to make his cold plasmas give up some of the secrets of their dense, hot, energetic cousins.
The field sprang into existence only recently, when technology advanced to the point where we could make exotic states of nature that were previously limited to the realm of theory," Killian said. There are fewer than a dozen laboratories in the world working on ultracold neutral plasmas, but the field is growing quickly because technology is bringing previously unperformed experiments within reach.
Ultracold plasmas are something of a conundrum. To start with, matter in a plasma state doesn’t exist as discrete atoms. Instead, plasma is a kind of atomic soup that contains about equal numbers of free-flowing electrons and ions. Plasmas have some of the properties of a gas but differ from gases in that they are good conductors of electricity and are affected by magnetic fields.
In Killian’s laboratory, plasmas are created and cooled by lasers. They exist only for about one-thousandth of a second, but that’s long enough to be photographed. By slightly varying the conditions of the plasma and by photographing it at various points throughout its short lifespan, Killian and his colleagues are opening a window on a bizarre place where matter behaves in fundamentally different ways than are normally observable.
Researchers already have made liquidlike systems that resemble the interiors of gas giant planets like Jupiter. Now, several research groups around the world, including Killian’s, are racing to become the first to create a solid neutral plasma — a bizarre state of matter believed to exist in the crust of superdense neutron stars.
“The concept of a solid plasma is counterintuitive," Killian said. “How can you have this flowing mix of ions and electrons in a solid form?" In nature, the answer lies in the density of the material. In a neutron star, for example, a teaspoon of matter has a mass of about 100 million metric tons. So a plasma there becomes solid due to the crushing density of its surroundings. In the lab, Killian hopes to get the same effect by making the plasma ultracold.
“People ask what applications there are," Killian said. “It’s a natural question, and though there are some indications of ways we might use ultracold neutral plasmas — to improve electron microscopy, for example — researchers in this field are primarily inspired by a desire to explore new realms of nature that no one has ever seen before."