July 31, 2024
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Putting the Squeeze on Helium

By Charlie Osolin

Third 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

Part 2: “NIF Takes a Quantum Leap into Elusive Metallic Hydrogen

Scientists have two primary tools to study how the properties of materials change under extreme pressures and temperatures: a “static” mechanical device called a diamond anvil cell (DAC), and a “dynamic” system using high-energy lasers, like Lawrence Livermore National Laboratory (LLNL)’s National Ignition Facility (NIF).

Both have their advantages and disadvantagesbut what if the two tools were combined in the same experiment?

“Static and dynamic compression factors can be multiplied by launching shock waves in the diamond anvil cell,” says Paul Loubeyre, a senior fellow and research scientist at the French Alternative Energies and Atomic Energy Commission (CEA). “Very dense states can thus be obtained that are inaccessible for static and dynamic compression alone.

“Launching a shock in a DAC enables us to reach a very dense state and limit the temperature rise of the compression,” adds Loubeyre, who is the principal investigator of a current NIF Discovery Science campaign aimed at understanding the properties of helium when it’s subjected to millions of atmospheres of pressure. “That is especially true for compressible systems like hydrogen, helium, water, and methane, which are the constituents of planetary interiors.”

In the campaign, which already has set records for helium compression and density, a team of researchers is developing an experimental platform to “precompress” a sample of helium inside a DAC—a small press that can squeeze material under very high pressure—and then blasting it with NIF’s lasers to compress it even further. The goal is to learn the roles of density and temperature in converting liquid helium from an insulator to a metallic substance able to conduct electricity.

The research could shed light on the structure and evolution of gas giant planets like Jupiter and Saturn and on the atmospheres of most white dwarf stars—stars that have exhausted their nuclear fuel and collapsed into hot, super-dense mixtures of carbon and oxygen. It could also be relevant to NIF inertial confinement fusion (ICF) experiments that support the National Nuclear Security Administration’s science-based Stockpile Stewardship Program.

The experiments will be useful, Loubeyre says, “to answer whether helium is conductive under the deep planetary conditions of Jupiter or more massive planets.  The data will also have important significance to understand white-dwarf cooling (the atmospheres of most white dwarfs contain a high percentage of helium and hydrogen). An understanding of white-dwarf cooling is greatly influenced by the knowledge of the insulator-metal transition in warm dense helium.”

Precompression at OMEGA

The helium campaign builds on more than 15 years of precompression experiments on the OMEGA Laser, a 60-beam facility at the University of Rochester. “The use of pre-compressed targets at OMEGA has already enabled the collection of interesting data on dense hydrogen, helium, and their mixtures,” Loubeyre says.

For example, the OMEGA experiments validated the existence of helium rain inside planets like Jupiter and Saturn; found experimental evidence for “superionic” water ice inside giant planets like Uranus and Neptune; and enabled the measurement of the phase separation of the hydrogen-helium mixture under Jupiter planetary interior conditions.

The researchers, however, say that to fully probe the transition of helium to a conducting state requires precompressing the sample to 20 gigapascals (GPa), or about 200,000 times Earth’s atmospheric pressure. “To achieve these initial pressures,” they say, “a diamond window of at least 650-micron thickness must be used (compared to 200-micron windows used in the OMEGA shots). To couple high laser pressure through such a thick diamond requires energy that can only be delivered by NIF,” the world’s highest-energy and most precise and reproducible laser system.

“It’s an experimental challenge,” says LLNL physicist Yong-Jae Kim, the LLNL point of contact for the campaign. “There were efforts to push the measurements to higher densities at OMEGA, but they were limited by the laser energy.

“NIF is needed to achieve a breakthrough compression state,” he says. “We need to combine a diamond anvil cell with cryogenic cooling and NIF ramp compression to disentangle the temperature and density contributions in the insulator-to-conducting transition in dense fluid helium.”

Bringing DAC Expertise

 Kim came to LLNL in 2019 after serving as a postdoctoral researcher at the Korea Research Institute of Standards and Science (KRISS) in South Korea, where he learned the craft of fabricating DACs—mechanical presses that squeeze a tiny sample between the points of two gem-quality diamonds similar to those used in jewelry (see “The Pressure’s On: Diamond Anvil Cells Reimagined”).

Yong-Jae Kim Tests a Diamond Anvil Cell Sample
At LLNL’s HED Science Center Technology Facility, physicist Yong-Jae Kim obtains an interference fringe spectrum and ring pattern from a diamond anvil cell sample. Credit: Julie Russell

The version of DACs used at OMEGA and NIF use thin diamond anvils, sometimes called diamond “windows,” so the shock or pressure wave from the lasers gets through to the sample.

Kim worked on the DACs used in OMEGA experiments, including one that obtained high-precision thermodynamic data on warm dense nitrogen at extreme conditions. The team conducted shock experiments on precompressed molecular nitrogen fluid at up to 800 GPa (8 million atmospheres) of pressure.

Problems: Data and Debris

But using NIF to apply more pressure on a helium sample than was possible at OMEGA posed its own challenges.

The early shots in the campaign in 2021 were unable to obtain good data from the principal diagnostic, the velocity interferometer system for any reflector, or VISAR, due to warping and low reflectivity on the mirror in the VISAR cone. In addition, experimental modeling and a risk assessment by NIF’s Debris and Shrapnel Working Group raised concerns that the extreme energy from the NIF lasers could send unacceptable amounts of debris and shrapnel from the DAC into the Target Chamber.

Experimental Setup for the NIF Discovery Science Helium Campaign
Illustration of the experimental setup for the NIF Discovery Science helium campaign. An experiment on Dec. 28, 2023, created a record shock pressure of 1 terapascal (almost 10 million atmospheres) on a helium sample. The results are measured by the velocity interferometer system for any reflector, or VISAR.

“That’s a really serious problem,” says Discovery Science team member Jon Eggert, who has been working with DACs since he was a graduate student at Harvard University in the early 1990s and helped to design the first precompression experiments at OMEGA. “As far as safety in the lab, the diamond cells are not dangerous,” he says. “These pressures are huge, but the diamond cell has such a tiny amount of material that the energy stored in the high pressure is extremely small.

“But Debris and Shrapnel were worried,” he says, “because they were calculating that when we shot this laser so hard, it would send debris straight up and we would be hitting an important diagnostic on the north pole of the Target Chamber.”

The researchers worked on improving the VISAR cone mirror and redesigning the DACs to try to strengthen or eliminate potential fracture points. They also adjusted the shape of the laser pulses to reduce the risk of debris damaging NIF’s optics. The most recent experiment, on Dec. 28 of last year, finally was able to record high-quality VISAR and streaked optical pyrometer data; debris and shrapnel, however, continues to be an issue.

In that experiment, the DAC, with its 650-micron-thick diamond windows, precompressed the helium sample to 11 GPa—a record for laser-driven shock experiments. Then 24 NIF beams delivered 182 kilojoules of ultraviolet light to the target in a 13-terawatt peak power pulse lasting 15 nanoseconds. The resulting compression of the sample reached a record 1 terapascal, or almost 10 million atmospheres, at a temperature of 7,000 Kelvin, and achieved a helium density of 3.5 grams per cubic centimeter, also a record.

“The December experiment was very successful in observing a beautiful reflecting shock in helium,” Loubeyre says. “The helium was highly reflecting, hence conducting. A very valuable equation-of-state data point has been measured.

“Unfortunately,” he adds, “the recovered DAC shows a fractured diamond seat implying issues for debris and shrapnel prevention, so the campaign is stopped at present.

“Two more shots are pending,” he says. “A new design of the DAC target has to be proposed and a debris and shrapnel simulation has to validate it. Then these shots should be planned, hopefully in 2025.”

Precompressing Hydrogen

Meanwhile, a related Discovery Science campaign is focused on the properties of hydrogen at extreme pressures and temperatures approaching absolute zero, where scientists have predicted it will become a unique atomic metal with exotic properties—possibly including the ability to conduct electricity without resistance at room temperature (see “NIF Takes a Quantum Leap into Elusive Metallic Hydrogen”).

DACs have been used for years to compress hydrogen samples to high densities. But until now, the cells have been unable to reach the necessary pressure—estimated at up to 500 GPa—to force that transition. Researchers believe the approach used in the helium campaign could eventually be applied to hydrogen as a means of characterizing the quantum regime of hydrogen over a broad range of pressures and temperatures, allowing creation of higher-density phases of hydrogen than could be achieved by either tool alone.

“While the helium campaign aims to determine when density effects become predominant for the transition to the conducting state,” says Loubeyre, “the hydrogen campaign seeks to observe the pure density metallization effect.”

Precompressing low-temperature hydrogen in a DAC and then hitting it with NIF’s lasers could enhance the quantum effects relevant to the deep interiors of gas giant planets and white dwarf stars—and potentially enable the first undisputed demonstration of atomic metallic hydrogen.

“If at some stage it becomes possible to launch a shock compression in a hydrogen sample under 100 GPa of pressure in the DAC,” Loubeyre says, “that could be another approach to obtain monatomic metal hydrogen.”

“The DAC platform at NIF opens up a whole new range of studies,” adds LLNL physicist Marius Millot, a member of both Discovery Science teams, “and it is exciting to see all of the progress being made, even with the significant challenges posed by the extreme complexity of fielding a diamond anvil cell in the NIF chamber."

Milot is principal investigator of a Laboratory Directed Research and Development (LDRD) project (19-ERD-031) that was key to the development of the new platform.

“This is a wonderful achievement for the team and Yong-Jae Kim," Millot says, "who is one of our rising stars and deserves a lot of credit for his excellent and hard work on improving this technique and continuing to innovate.”

Joining Loubeyre, Kim, Eggert, and Millot on the helium campaign are Ryan Rygg, the co-principal investigator, and Rip Collins from the Laboratory for Laser Energetics at the University of Rochester; Peter Celliers and Dayne Fratanduono from LLNL; Stéphanie Brygoo and Florent Occelli from CEA; and Raymond Jeanloz from UC Berkeley.

More Information:

Experiments Validate Possibility of Helium Rain in Jupiter, Saturn,” NIF & Photon Science News, June 2, 2021

“Evidence of hydrogen−helium immiscibility at Jupiter-interior conditions,” Nature, May 26, 2021

“Record EOS Measurement Pressures Shed Light on Stellar Evolution,” NIF & Photon Science News, Aug. 5, 2020

The Pressure’s On: Diamond Anvil Cells Reimagined,” Science & Technology Review, July/August, 2019

“Dynamic compression provides new insight into understanding and predicting crystal growth,” LLNL News Release, June 4, 2019

 “Experiments Verify ‘Mind-Boggling’ Behavior of Water Ice,” NIF & Photon Science News, Feb. 14, 2018

NIF achieves record ramp-compression pressures,” LLNL News Release, Dec. 9, 2011

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