Making plasmas in the deepest of deep freezes

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Making plasmas in the deepest of deep freezes
Using lasers to make never-before-seen states of matter

Rice University physicist Tom Killian is 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,” said Killian, associate professor of physics and astronomy. “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.”

Plasma is a state of matter, just like the three states of matter we’re familiar with on Earth — gas, liquid and solid. But plasma is by far the most abundant, accounting for about 99 percent of the visible matter in the universe. The problem, for physicists studying strongly interacting plasmas, is that they naturally occur only in very dense and energetic environments like a white dwarf star — where it isn’t practical to set up a laboratory.

Fortunately, 21st Century technology makes it possible to hold and photograph ultracold plasmas, which are beginning to give up some of the secrets of their dense, hot, energetic cousins.

Killian was invited by the editors of Science magazine to summarize the state of the emergent discipline in a May 4 review article.

He 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 never-performed experiments within reach.

“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.

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 both free-flowing electrons and ions — electrically charged atoms.

In Killian’s Dell Butcher Hall laboratory, plasmas are both created and cooled by lasers. The lab is dominated by two enormous tables that are covered with lenses and equipment that focus nine different laser beams into the same small space — a sealed chamber where a few hundred million vaporized strontium atoms are trapped between beams of laser light. Inside the trap, lasers form an “optical molasses” that slows the atoms until they are almost at a complete stop. The ultracold strontium is then hit with a high-powered pulsed laser that strips away an electron from each atom. The resulting electron-ion plasma — which exists only for about one-thousandth of a second — is then photographed. By slightly varying the conditions of the plasma, and by photographing it at various points throughout its short lifespan, Killian and his colleagues open a window on a bizarre place where matter behaves in fundamentally different ways than are normally observable.

Researchers have already made liquid-like systems that resemble the interiors of gas giant planets like Jupiter. Now, with access to the same technology, 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 that is 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, he said. In a neutron star, for example, a teaspoon of matter has a mass around 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.

To make their short-lived plasmas solid, Killian’s team, which includes postdoctoral researcher Hong Gao and graduate students Priya Gupta, Sampad Laha, Clayton Simien and Jose Castro, will have to further cool their plasmas. That, in turn, requires a new apparatus that will focus even more laser beams on the target.

“One question people ask is 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.”