Settling Arguments About Hydrogen With 168 Giant Lasers

With gentle pulses from gigantic lasers, scientists at Lawrence Livermore National Laboratory in California transformed hydrogen into droplets of shiny liquid metal.

Their research, reported Thursday in the journal Science, could improve understanding of giant gas planets like Jupiter and Saturn whose interiors are believed to be awash with liquid metallic hydrogen.

The findings also could help settle some fractious debates over the physics of the lightest and most abundant element in the universe.

At the temperatures and pressures found at the surface of Earth, hydrogen atoms pair up in molecules and bounce around as a colorless gas.

At ultracold temperatures, below -423 degrees Fahrenheit, hydrogen condenses into a liquid. It also turns into a liquid at higher temperatures when squeezed under immense pressure. The molecules remain intact, and this state of liquid hydrogen is an insulator — a poor conductor of electricity.

Under even higher pressures, the molecules break apart into individual atoms, and the electrons in the atoms are then able to flow freely and readily conduct electricity — the definition of a metal.

An experiment at the Livermore lab in the 1990s was the first to convincingly create metallic hydrogen using a gas gun to send monstrous pressure waves through samples of hydrogen. But those tests did not reveal all of the details about how the transition occurred.

In recent years, researchers have figured out additional ways to make liquid metallic hydrogen. Isaac Silvera, a professor of physics at Harvard, and his colleagues used two interlocking pieces of diamond to compress a smidgen of hydrogen and then heated it with laser pulses. At Sandia National Laboratories in Albuquerque, New Mexico, intense bursts of magnetic fields were used to compress samples of deuterium, a heavier form of hydrogen.

The Sandia scientists reported that the metallic transition occurred at 44 million pounds per square inch, or about 3 million times the atmospheric pressure at ground level on Earth. That was considerably higher than many had expected.

“There’s been a kind of confusion of predictions,” said Peter Celliers, a physicist at the Livermore lab who is the lead author of the new paper.

Experiments on matter at ultrahigh pressure are difficult to perform, often with conflicting results, which “made for a picture that has looked to date fairly muddled,” Celliers said. “We think it’s starting to clear up with this new data set.”

The new research is a collaboration by American, French and British scientists with experiments performed at Livermore’s National Ignition Facility. The mammoth apparatus, which made a cameo in the movie “Star Trek: Into Darkness,” is housed in a building 10 stories high and three football fields long. With 192 gargantuan lasers aimed at a BB-size target, it was built to generate bursts of fusion, although it has fallen short of its original objectives.

The current experiment used 168 of the lasers at a lower setting to set off a series of reverberating shock waves through ultracool liquid deuterium. That compressed the deuterium to a pressure that was 6 million times greater than atmospheric pressure while keeping temperatures to what the physicists regarded as cool: below 3,000 degrees Fahrenheit. (A more indiscriminate laser blast would heat the gas to hundreds of thousand of degrees.)

They found that the metallic hydrogen transition occurred at less crushing pressures: only 2 million times atmospheric pressure.

In addition, the scientists argued that turbulence in the larger Sandia samples masked this transition, and that the temperatures at which hydrogen becomes metal should be revised downward, where they would lie along the same curve as the new Livermore data. “Overall, we think there is a picture emerging that is converging on the truth,” Celliers said.

Liquid metallic hydrogen does not naturally occur on Earth — except possibly at the core. But at Jupiter, the solar system’s largest planet, most of the hydrogen could be flowing as a liquid metal and generating the planet’s powerful magnetic fields. Understanding the properties of metallic hydrogen could help scientists decipher data from NASA’s Juno mission, in orbit around Jupiter.

The data could also sift out which theoretical models work in describing the properties of hydrogen under extreme conditions and which should be discarded.

David M. Ceperley, a physicist at the University of Illinois at Urbana-Champaign, and his collaborators worked on herculean computer calculations, each data point the result of about 100 hours on a supercomputer with 10,000 computer cores, to model the transition of liquid hydrogen from insulator to metal.

He said the Livermore results agree within about 10 percent of what their findings predicted in 2009, within the uncertainties of the experiment’s measurements.

“For us, they’re great,” Ceperley said. “They not only end up right on top of our calculations,” but also help reconcile the disparate Harvard and Sandia findings.

The Sandia scientists, however, remain confident in their data and did not agree with how the new paper reinterpreted their findings.

“We have a different interpretation of what they’re seeing.” said Michael P. Desjarlais, a Sandia scientist who worked on that experiment. “Their temperatures are actually higher than they believe. Then their results would actually be quite consistent with ours.” Both the Sandia and Livermore experiments cannot measure temperatures directly and must infer them from computer simulations of the shock waves.

The researchers at both labs are preparing to run additional experiments, compressing hydrogen instead of deuterium. The lighter hydrogen should turn metallic at lower pressures.

Even as scientists get a better idea of liquid metallic hydrogen, the ultimate feat of high-pressure alchemy would be to transform hydrogen into a solid metallic form at yet higher pressures.

In January 2017, Silvera reported that he and Ranga P. Dias, a postdoctoral researcher in his laboratory, had indeed achieved that feat inside a diamond vise. But that sample disintegrated when they attempted additional measurements, and they have not yet succeeded at making a second speck of solid metallic hydrogen.

Silvera said a series of mishaps have slowed follow-up attempts, and many scientists remain skeptical. Silvera said his laboratory is preparing a couple of additional diamond vises. “It will happen soon,” he said.