Scientists from Lawrence Livermore National Laboratory (LLNL) used giant lasers to flash-freeze water into its exotic superionic phase and record x-ray diffraction patterns to identify its atomic structure for the first time – all in just a few billionths of a second. The findings were reported May 8 in Nature.
Scientists first predicted in 1988 that water would transition to an exotic state of matter characterized by the coexistence of a solid lattice of oxygen and liquid-like hydrogen – a phase called superionic ice – when subjected to the extreme pressures and temperatures that exist in the interiors of water-rich giant planets like Uranus and Neptune. These predictions remained in place until 2018, when a team led by scientists from LLNL presented the first experimental evidence for this strange state of water.
Now the LLNL scientists describe new results. Using laser-driven shockwaves and in-situ x-ray diffraction, they observed the nucleation of a crystalline lattice of oxygens in a few billionths of a second, revealing for the first time the microscopic structure of superionic ice.
“We wanted to determine the atomic structure of superionic water,” said LLNL physicist Federica Coppari, co-lead author of the paper. “But given the extreme conditions at which this elusive state of matter is predicted to be stable, compressing water to such pressures and temperatures and simultaneously taking snapshots of the atomic structure was an extremely difficult task, which required an innovative experimental design.”
The researchers performed a series of experiments at the Omega Laser Facility at the University of Rochester’s Laboratory for Laser Energetics (LLE). They used six giant laser beams to generate a sequence of shockwaves of progressively increasing intensity to compress a thin layer of initially liquid water to extreme pressures (100-400 gigapascals [GPa], or 1-4 million times Earth’s atmospheric pressure) and temperatures (3,000-5,000 degrees Fahrenheit).
“We designed the experiments to compress the water so that it would freeze into solid ice, but it was not certain that the ice crystals would actually form and grow in the few billionths of a second that we can hold the pressure-temperature conditions,” said LLNL physicist and co-lead author Marius Millot.
To document the crystallization and identify the atomic structure, the team blasted a tiny iron foil with 16 additional laser pulses to create a hot plasma, which generated a flash of x rays precisely timed to illuminate the compressed water sample once brought into the predicted stability domain of superionic ice.
“The x-ray diffraction patterns we measured are an unambiguous signature for dense ice crystals forming during the ultrafast shockwave compression demonstrating that nucleation of solid ice from liquid water is fast enough to be observed in the nanosecond timescale of the experiment,” Coppari said.
“In the previous work we could only measure macroscopic properties such as internal energy and temperature,” Millot added. “Therefore, we designed a new and different experiment to document the atomic structure. Finding direct evidence for the existence of crystalline lattice of oxygen brings the last missing piece to the puzzle regarding the existence of superionic water ice. This gives additional strength to the evidence for the existence of superionic ice we collected last year.”
Analyzing how the x-ray diffraction patterns varied for the different experiments probing increased pressure and temperature conditions, the team identified a phase transition to a previously unknown face-centered-cubic (f.c.c.) atomic structure for dense water ice.
“Water is known to have many different crystalline structures known as ice Ih, II, III, up to XVII,” Coppari said. “So, we propose to call the new f.c.c. solid form ‘ice XVIII.’ Computer simulations have proposed a number of different possible crystalline structures for superionic ice. Our study provides a critical test to numerical methods.”
The team’s data has profound implications for the interior structure of ice giant planets. Since superionic ice is ultimately a solid, the idea of these planets having a uniform rapidly convecting fluid layer no longer holds.
“Because water ice at Uranus’ and Neptune’s interior conditions has a crystalline lattice, we argue that superionic ice should not flow like a liquid such as the fluid iron outer core of the Earth,” Millot said. “Rather, it's probably better to picture that superionic ice would flow similarly to the Earth’s mantle, which is made of solid rock, yet flows and supports large-scale convective motions on the very long geological timescales. This can dramatically affect our understanding of the internal structure and the evolution of the icy giant planets, as well as all their numerous extrasolar cousins.”
Co-authors include Antonio Correa Barrios, Sebastien Hamel, Damian Swift and Jon Eggert from LLNL and Ryan Rygg, previously at LLNL and now at the University of Rochester. The work was partially funded by the Laboratory Directed Research and Development (LDRD) program.
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The first proton-acceleration experiments using NIF’s Advanced Radiographic Capability (ARC) short-pulse laser have produced protons with energies about 10 times higher than previous experience would have predicted (see “A Powerful New Source of High-Energy Protons”).
Beams of high-energy protons can be precisely targeted and are able to quickly heat materials before they can expand. Ultrafast heating of matter will enable opacity and equation-of-state measurements at unprecedented energy densities and could open the door to new ways of studying extreme states of matter, such as stellar and planetary interiors. Proton acceleration also promises to enable a variety of other applications in high energy density (HED) and inertial confinement fusion (ICF) research.
In a Physics of Plasmas paper published online on April 24, an international team of researchers reported that the maximum proton energies created in the February 2018 experiments—from 14 to 18 MeV (million electron volts)—are “indicative of (an)…electron acceleration mechanism that sustains acceleration over long (multi-picosecond) time-scales and allows for proton energies to be achieved far beyond what the well-established scalings of proton acceleration (at ARC-level intensities) would predict.
“Coupled with the NIF,” the researchers said, “developing ARC laser-driven ion acceleration capabilities will enable multiple exciting applications. For example, the NIF can deliver 1.8 MJ (million joules) of laser light to drive an experiment and with an energetic proton beam, we could begin to diagnose electromagnetic fields in these experiments by using proton radiography.”
LLNL engineering physicist Derek Mariscal, lead author of the paper, said the surprise results at ARC’s quasi-relativistic, or “modest” laser intensities—about a quintillion (1018) watts per square centimeter—“forced us to try to understand the source of these particles, and we ultimately found that a different mechanism for accelerating particles to MeV electrons was necessary to explain the results.
“While we haven’t completely explained this mechanism,” he said, “we’ve been able to start discounting mechanisms that have been identified in previous short-pulse work to start honing in on how we could get such unexpected electron and subsequent proton energies.
“These results are really encouraging not only for ARC-driven proton beams,” he added, “but for particle acceleration in what’s referred to as the quasi-relativistic laser regime.”
ARC is a petawatt (quadrillion watt)-class short-pulse laser created by splitting two of NIF’s 192 long-pulse beams into four rectangular beamlets. Using a 2018 Nobel Prize-winning process called chirped-pulse amplification, the beamlets are stretched in time to reduce their peak intensity, then amplified at intensities below the optics damage threshold in the laser amplifiers, and finally compressed to picosecond (trillionth of a second) pulse lengths and highest peak power in large compressor vessels, as shown in this video:
In the experiments, which are supported by LLNL’s Laboratory Directed Research and Development (LDRD) and NIF’s Discovery Science programs, two ARC shots were fired onto 1.5×1.5-millimeter-square, 33-micron-thick titanium foils. About 2.6 kilojoules of energy was delivered in a 9.6-picosecond pulse and 1.1 kJ was fired in a 1.6-ps pulse. A Target Normal Sheath Acceleration (TNSA) field, first observed on LLNL’s Nova petawatt laser two decades ago, accelerated high-energy protons and ions from the contamination layer of proton-rich hydrocarbons and water coating the target’s surface.
“We plan to take this platform in several directions,” Mariscal said. “One of the most obvious directions is for probing electromagnetic field structures generated during experiments driven by the NIF long-pulse beams, which has been a standard use for these proton beams since their discovery here at LLNL around 20 years ago on the Nova Petawatt laser.
“In addition to using proton beams as a diagnostic tool,” he said, “we plan to continue to use these beams to create high-energy-density conditions. Since we’re able to generate around 50 joules of proton beam energy, if we can deposit it over a 10-picosecond time-scale we can generate plasmas at near solid density with temperatures over 100 eV, which is a truly exotic state of matter known as Hot Dense Matter.”
The researchers are also exploring new target designs that could enhance ARC’s laser intensity to achieve even higher proton energy, enabling probes of ICF experiments. And by varying the length of ARC pulses, they hope to create shaped short pulses using ARC laser beams.
“Pulse shaping with nanosecond pulses allows for driving precision shocks in materials for studying material equations of state, but we plan to use this idea at the sub-picosecond level to manipulate particle acceleration physics,” Mariscal said. “We’ve tried this scheme on the OMEGA EP laser at the University of Rochester’s Laboratory for Laser Energetics and saw greatly enhanced laser coupling to high-energy particles over single short pulses.”
Additional NIF shots will use a 10-picosecond ARC beam to drive a beam of protons intended to rapidly heat a solid sample to more than 50 eV. Concurrently, a higher-intensity one-picosecond ARC beam will be used to generate a second proton beam that will probe the electromagnetic field structures of the heating experiment. “That will ultimately help us to understand how particles are being accelerated to MeV energies with a 10-picosecond pulse,” Mariscal said.
Mariscal credited the “fantastic” suite of diagnostics at the ARC Diagnostics Table and modeling support from the NIF ARC Laser Team with enabling the researchers to learn “some very interesting fundamental short-pulse-driven particle acceleration physics in this new regime provided by ARC.
“We’re given a new level of confidence in our interpretations due to the high-quality characterization of delivered ARC laser pulses,” he said. “This allows our physics team to accurately model the laser conditions of the experiment and maximize our understanding from the limited overall number of ARC laser experiments.”
Joining Mariscal on the paper were LLNL colleagues Tammy Ma, Scott Wilks, Andreas Kemp, G. Jackson Williams, Pierre Michel, Hui Chen, Prav Patel, Bruce Remington, Mark Bowers, Lawrence Pelz, Mark R. Hermann, Warren Hsing, David Martinez, Ron Sigurdsson, Matt Prantil, Alan Conder, Janice Lawson, Matt Hamamoto, Pascal Di Nicola, Clay Widmayer, Doug Homoelle, Roger Lowe-Webb, Sandrine Herriot, Wade Williams, David Alessi, Dan Kalantar, Rich Zacharias, Constantin Haefner, Nathaniel Thompson, Thomas Zobrist, Dawn Lord, Nicholas Hash, Arthur Pak, Nuno Lemos, and Max Tabak, along with collaborators from the University of California at San Diego, General Atomics, the University of Oxford and the Central Laser Facility at the STFC Rutherford Appleton Laboratory in the UK, the Institute of Laser Engineering at Osaka University in Japan, and Los Alamos National Laboratory.
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