June 20, 2024
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NIF Takes a Quantum Leap into Elusive Metallic Hydrogen

By Charlie Osolin

Second in a series of articles describing current National Ignition Facility (NIF) Discovery Science experimental campaigns.

Part 1. Using NIF to Study the Sluggish Pace of Star Formation”

In 1935, not long after quantum mechanics emerged as the go-to theory of the subatomic world, Nobel Prize-winning physicist Eugene Wigner and his colleague Hillard Huntington predicted that squeezing hydrogen atoms at high pressures and low temperatures would create a novel metallic material with intriguing quantum properties.

“This was one of the oldest applications of quantum mechanics to the properties of materials,” says Raymond Jeanloz, professor of earth and planetary science and astronomy at the University of California, Berkeley. “It’s ironic that almost 90 years later, we still have not been able to accomplish what was theoretically predicted.

Depiction of the current known phase diagram of hydrogen, with temperature shown in degrees Kelvin and pressure shown in gigapascals (500 GPa equals 5 million Earth atmospheres). The solid phase lines are a combination of static compression studies of solid hydrogen using diamond anvil cells and laser-driven dynamic compression studies of fluid deuterium, an isotope of hydrogen, such as those conducted at NIF. The dashed lines represent extrapolations of the combined results. Credit: AIP Publishing (click image to expand)

“We don’t view this as a deficiency,” he says. “We view this as motivation to do more experiments.”

And more experiments are in fact now underway in NIF’s Discovery Science program, as researchers continue to explore new extremes of pressure and temperature available only at NIF. Their goal is to characterize hydrogen’s optical properties and equation of state (EOS)— and perhaps, finally, create the metallic state of hydrogen predicted decades ago.

Achieving metallic hydrogen is considered one of the three most important challenges in modern physics, just behind the quest to harness the power of nuclear fusion energy. Technologies using metallic hydrogen could be transformative for construction and energy production, make electric vehicles more efficient, and possibly provide the fuel for future spaceships.

The main reason scientists haven’t been able to fulfill the 1935 prediction is that Wigner and Huntington, not knowing hydrogen’s compressibility, badly underestimated the amount of pressure it would take to cause the transformation. They thought subjecting crystalline (frozen) hydrogen to about 250,000 times Earth’s atmospheric pressure would do the trick. But many subsequent experiments using diamond anvil cells (DACs), including some reaching more than 4 megabars (4 million atmospheres)—16 times the Wigner-Huntington prediction—have still come up short (DACs are small mechanical presses that gradually produce ultrahigh pressures).

Meeting the Challenges
Researchers have dubbed metallic hydrogen the Holy Grail of materials science and one of the three most important and interesting challenges in modern physics. Number one on that list is achieving controlled, net-energy-producing nuclear fusion, something LLNL took a big step toward accomplishing in December 2022 by using NIF’s 192 powerful lasers to fuse hydrogen isotopes and create a self-sustaining thermonuclear burn leading to ignition. Number two is high-temperature and room-temperature superconductivity.

That’s where NIF, the world’s largest and highest-energy laser system, could make a difference, Jeanloz says. He’s the principal investigator for NIF’s current metallic hydrogen campaign, which has members from LLNL, the Laboratory for Laser Energetics (LLE) at the University of Rochester, the University of Illinois Chicago (UIC), and the French Alternative Energies and Atomic Energy Commission (CEA).

“With NIF, we’re at more than 30 times higher pressures than what the 1935 theory predicted would be needed,” Jeanloz says. In addition, the campaign’s first experiment in March “was the lowest-temperature experiment done at NIF by far,” thanks to the development of a custom-made cryogenically cooled target by the NIF/LLE target fabrication team.

In NIF inertial confinement fusion (ICF) experiments aimed at achieving fusion ignition, a thin layer of the hydrogen isotopes deuterium and tritium (DT) is cooled to about 18 Kelvin (-420° Fahrenheit). Because hydrogen has a lower melting point than DT—about 14 Kelvin—“we’ve been looking at 15 to 20 percent lower temperature (in the target) than has previously been achieved at NIF,” Jeanloz says. “That’s quite a substantial drop down in terms of changing the quantum properties of materials that can be accessed in NIF experiments.”

NIF’s ability to compress materials at much lower temperatures than was previously possible, Jeanloz adds, “will have a wide range of possible applications, not just for basic science, but for understanding aspects of thermonuclear fusion.” Characterizing the properties of hydrogen under high pressure is a key element of NIF’s primary mission—supporting the National Nuclear Security Administration’s science-based Stockpile Stewardship Program to ensure the safety and reliability of the nation’s nuclear deterrent in the absence of underground nuclear testing.

An Essential Element

Hydrogen, consisting of one proton and one electron, is the simplest and most abundant chemical element in the universe; it’s also one of the most essential to life as we know it.

A monatomic (single) hydrogen atom (left) and a hydrogen molecule (click on image to expand)

In its normal molecular state—a bonded pair of two hydrogen atoms (H2)—hydrogen combines with oxygen to form life-sustaining water; and the hydrogen fusion reactions in the sun warm the planet and make life possible.

Under high pressure or temperature, molecular hydrogen can turn into a shiny metallic liquid, similar to mercury, that’s thought to generate the protective magnetic fields in the cores of Jupiter, Saturn, and many planets outside our solar system—some of which might just sustain life (see “NIF Reveals How Hydrogen Becomes Metallic in Giant Planets”).

And in 1968, Cornell University physicist Neil Ashcroft predicted that under the right combination of pressure and temperature, strong quantum effects would enable monatomic crystalline hydrogen to display a variety of unique properties, including room-temperature superconductivity—the ability to conduct electricity without resistance—and superfluidity—the ability to flow without resistance.

One of the remarkable features of molecular hydrogen is that its thermodynamic properties depend on the arrangement of the nuclear “spins” in the molecule. These are called “spin isomers.” When the spin of the bonded protons that make up the nuclei point in the same direction, they form the “ortho” state of hydrogen, which tends to dominate the mixture at higher temperatures. As the temperature drops and approaches absolute zero, the proton spins point in opposite directions and the lower-energy “para” state takes over. Some researchers speculate that using pure parahydrogen in a NIF target might lower the sample’s temperature by a factor of two or more, boosting the chances of creating monatomic metallic hydrogen. Credit: Wikimedia Commons (click on image to expand)

Metallic hydrogen that’s metastable—able to retain its properties at normal temperature and pressure—could enable the production of super-lightweight structural materials. Some theorists also think it might be used as rocket fuel for space exploration, with nearly four times the propellant power per kilogram of liquid hydrogen.

And achieving room-temperature superconductivity would be a game-changer for energy production, distribution, and storage, reducing energy demand and greenhouse gas emissions. In transportation, it could allow magnetic levitation of high-speed trains and make electric vehicles more efficient. High-temperature superconducting material might also enable construction of a compact, economical magnetic fusion power plant (most superconductors now operate at below 77 Kelvin, or -321° Fahrenheit, which severely limits their usability).

But despite decades of research, monatomic metallic hydrogen remains elusive.

Computer simulations and experiments show that below 2,000 Kelvin, increasing the pressure on hydrogen can trigger a sudden transition from an insulator to a reflective, electricity-conducting metal. But to reach the phase predicted by Wigner and Huntington, “you need to show that hydrogen remains reflecting and conducting at zero temperature,” says LLNL physicist Jon Eggert, a member of the Discovery Science team. “That would require extrapolation to demonstrate, since we can never truly reach zero temperature.

“It would be really exciting if we could show that atomic hydrogen at room temperature is actually a superconducting, superfluid liquid,” Eggert says. “But showing this is going to be really hard.”

A Challenging Target

Developing a target capable of attaining the lowest cryogenic temperatures yet seen in a NIF experiment was a four-year project led by LLNL physicist Jim Sater, who has worked on innovative methods of improving cryogenic DT fuel-layer performance for NIF ICF experiments since 1984. The Discovery Science team recruited him to design the target for the hydrogen experiments.

“We were pushing the capabilities a bit,” Sater says. “There are always external heat sources impinging on the target. It’s one thing to get it that cold, but it’s another challenge to keep it cold” for the eight seconds from the opening of the cryogenic target positioner’s protective shrouds to the laser shot. “The room-temperature environment in the Target Chamber is radiating energy onto your target, and that’s a substantial extra heat load.”

A cryogenically cooled target is inserted into the NIF Target Chamber by a cryogenic target positioner arm. The partially open protective shroud retracts a few seconds before shot time. (click on image to expand)

The target fabrication team met the challenge by modifying a target used in earlier hydrogen EOS experiments. “We took a little bit more care in reducing its response to radiative heat load—to reducing the emissivity,” Sater says. “We made some of the parts more reflective and tried to minimize the size of some of the places that would absorb radiative heat.”

Why Such Low Temperatures?

The March experiment’s goal was to test the new platform by slowly ramping up the pressure on a sample of crystalline hydrogen to at least 1 megabar while holding the temperature below about 2,000 Kelvin.

Keeping the initial target temperature as low as possible is key to creating the frozen hydrogen needed for the experiments, says LLNL plasma physicist Peter Celliers.

“We’re trying to configure both the target and the laser compression pulse to keep the sample from getting too hot,” says Celliers, the Lab point of contact for the campaign and lead on experiments and data analysis and interpretation. “What is thought to happen at these compression conditions is eventually, if it’s hot enough, it’ll simply melt.

“We’ve already been investigating this liquid-metallization transition (in NIF’s 2017 experiment on deuterium),” Celliers says. “If the temperature is high enough and the pressure is high enough, the molecules have dissociated—the bond has fallen apart. You have this soup of protons or (in the 2017 case) deuterons, moving around independently of the delocalized electrons, and that stuff is a metallic fluid. It’s a really interesting transition.

“If you can maintain the solid state,” he says, “you should get a similar situation where the electronic bonds between the molecules have broken. It has to rearrange itself into a new thing. And that new thing is the Wigner-Huntington transition.

“Essentially, the protons sit on a bunch of lattice points in a solid. The particles are in a fixed spatial relationship relative to their neighbors. And this also becomes metallic because the electrons become delocalized.” Achieving metastability in this state would be the key to using the material in real-world applications.

Image of States of Hydrogen Under Pressure
Representation of the gaseous and solid states of hydrogen under different pressures at room temperature (300 Kelvin): (a) gaseous molecular state at 1 atmosphere; (b) phase I, with hcp (hexagonal tightly packed) structure at 55,000 atmospheres; (c) phase IV, with mixed molecular and atomic state at 2.3 million atmospheres; (d) purely atomic and metallic state at more than 4.4 million atmospheres. Credit: AIP Publishing

“So the real challenge from our point of view,” adds Celliers, “is to say, ‘OK, what does it take to get there?’ And what we seem to know so far is that whatever that is, it’s really hard to do.”

In the March experiment, the researchers used 176 of NIF’s 192 laser beams to create an x-ray bath in the hohlraum that compresses the frozen hydrogen sample between a thin copper plate and a transparent lithium-fluoride “window.” To generate maximum compression at the lowest possible temperature, they employed a laser pulse with a reduced-power initial “picket” followed by a longer ramp-compression pulse.

To avoid a sudden shock that would raise the sample’s temperature, the laser pulse reverberated between the copper plate and the window multiple times to gradually reach megabar pressures. The researchers have to take care that a shock wave doesn’t form and overheat the window; that could make it opaque and block the sightline of the VISAR (velocity interferometer system for any reflector) diagnostic that determines the pressure–density conditions achieved in the experiment.

Exceeding Expectations

The March test of the new platform, as measured by the VISAR and streaked optical pyrometry (SOP) diagnostics, produced “absolutely spectacular” data, Jeanloz says. “First and foremost, this was a campaign of target development, and that development was absolutely an astounding success. It didn’t just meet expectations; it far exceeded expectations.”

According to Celliers, the experiment reached an estimated pressure of 7 to 8 megabars and temperature in the compressed hydrogen below 1,000 Kelvin.

“We’re at the very beginning of embarking on the campaign to really dig into this,” Celliers says. “We’re hoping eventually to get to a pressure well over 10 megabars, but we weren’t trying to hit for the fences on this first attempt.

“(1,000 Kelvin) is a very low temperature,” he adds. “But the real question was, is it low enough to be in the solid state and reach the Wigner-Huntington transition?

”And what the experiment leads to is a question mark—maybe, and maybe not. We’d like to eventually get to high enough pressure to really see it. So a future experiment might tell us some more.”

Further experiments in the campaign plan to build on the success of the initial test by increasing the hydrogen sample’s density and cooling it to even lower temperatures. Celliers says the researchers will take advantage of NIF’s new High-Fidelity Pulse Shaping capability, which has enabled a 10-fold improvement in pulse-shaping precision, by modifying the pulse shape in the campaign’s next experiment, now scheduled for November.

The work was partially funded by the Laboratory Directed Research and Development (LDRD) program.

Joining Jeanloz, Eggert, Celliers, and Sater on the campaign are Marius Millot, Dayne Fratanduono, Trevor Hutchinson, Yong-Jae Kim, Travis Briggs, and Montu Sharma from LLNL; Ryan Rygg, Rip Collins, and Arnold Schwemmlein from LLE; Stéphanie Brygoo and Paul Loubeyre from CEA; and Russell Hemley from UIC.

Next Up: “Putting the Squeeze on Helium”

More Information:

“Gently Compressing Materials to Record Levels,” Science & Technology Review, September, 2019

“Discovery Science Strengthens NIF’s Mission,” Nif & Photon Science News, November, 2018

“NIF Reveals How Hydrogen Becomes Metallic in Giant Planets,” NIF & Photon Science News, August, 2018

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