Imagine trying to bake a cake in an oven with uneven heating. Chances are you’ll get a cake with the top undercooked and the bottom overcooked, or vice versa. The same principle applies to NIF hohlraums and targets; if the x-ray energy generated in the hohlraum by NIF’s lasers is uneven, the target capsule’s implosion won’t be symmetrical and implosion performance will suffer.
Recent inertial confinement fusion (ICF) experiments on NIF have shown just such energy imbalances between the inner and outer cones of the laser beams caused by the growth of a “bubble” of hohlraum wall material (hohlraums are made of gold or depleted uranium). The bubble, a result of the higher-intensity outer-cone beams hitting the hohlraum wall, absorbs energy from the inner-cone beams and causes an oblate, or “pancaked,” implosion that limits implosion performance.
To deal with this problem, LLNL researchers have designed a new shaped hohlraum called the “I-Raum” that shows promise of enhancing the energy yield from NIF implosions by equalizing the energy deposited by the laser beams on the walls of the hohlraum. The innovative design, aimed at controlling and maintaining implosion symmetry for as long as possible, was described in a Physics of Plasmas paper published online on Jan. 22.
The researchers said the absorption of the inner cone beams by the “bubble” reduces the laser energy reaching the hohlraum equator during the later stages of the laser pulse. The new hohlraum is designed to reduce the bubble’s impact by adding a recessed pocket at the location where the outer cones hit the hohlraum wall.
“This recessed pocket displaces the bubble radially outward,” they said, “reducing the inward penetration of the bubble at all times throughout the implosion and increasing the time for inner beam propagation by approximately one nanosecond (billionth of a second). This increased laser propagation time allows one to drive a larger capsule, which absorbs more energy and is predicted to improve implosion performance by as much as a factor of eight in neutron yield.”
The new design is based on a June 2017 NIF shot which produced a record neutron yield. The expansion rate and absorption of laser energy by the bubble was quantified for both cylindrical and shaped hohlraums, and the predicted performance was compared. The design has not yet been fine-tuned, the researchers said, “which would be expected to increase the performance further. Future work is ongoing to design an initial series of tuning experiments to establish the initial shock timing, symmetry, laser backscatter, etc. These experiments will be essential for quantifying the potential benefits of this design.”
Joining lead author Harry Robey on the paper were LLNL colleagues Laura Berzak Hopkins, Jose Milovich, and Nathan Meezan.
Among the many discoveries on matter at high pressure that garnered him the Nobel Prize in 1946, scientist Percy Bridgman identified five different crystalline forms of water ice. Bridgman’s development of equipment that could produce extremely high pressures ushered in 100 years of research into how ice behaves under extreme conditions.
One of the most intriguing properties of water is that it may become “superionic” when heated to several thousand degrees at high pressure, similar to the conditions inside giant planets like Uranus and Neptune. This exotic state of water is characterized by liquid-like hydrogen ions moving within a solid lattice of oxygen—in effect, water behaving as both a liquid and a solid at the same time.
Since this was first predicted in 1988, many research groups in the field have confirmed and refined numerical simulations, while others used static compression techniques to explore the phase diagram of water at high pressure. While indirect signatures were observed, no research group has been able to identify experimental evidence for superionic water ice—until now.
In a paper published on Feb. 5 by Nature Physics, a research team from LLNL, the University of California, Berkeley, and the Laboratory for Laser Energetics (LLE) at the University of Rochester provides experimental evidence for superionic conduction in water ice at planetary interior conditions, verifying the 30-year-old prediction.
Using shock compression, the team identified thermodynamic signatures showing that ice melts near 5,000 kelvin (K) at 190 gigapascals (GPa)—almost two million times Earth’s atmospheric pressure—which is 4,000 K higher than the melting point at 50 GPa, nearly the surface temperature of the sun.
“Our experiments have verified the two main predictions for superionic ice: very high protonic/ionic conductivity within the solid and high melting point,” said LLNL physicist Marius Millot, lead author of the paper. “Our work provides experimental evidence for superionic ice and shows that these predictions were not due to artifacts in the simulations, but actually captured the extraordinary behavior of water at those conditions,” he said. “This provides an important validation of state-of-the-art quantum simulations using density-functional-theory-based molecular dynamics.”
“Driven by the increase in computing resources available, I feel we have reached a turning point,” added LLNL physicist Sebastien Hamel, a co-author of the paper. “We are now at a stage where a large enough number of these simulations can be run to map out large parts of the phase diagram of materials under extreme conditions in sufficient detail to effectively support experimental efforts.”
Planetary Science Insights
The work also has important implications for planetary science because Uranus and Neptune might contain vast amounts of superionic water ice. Planetary scientists believe these giant planets are made primarily of a carbon, hydrogen, oxygen and nitrogen (C-H-O-N) mixture that corresponds to 65 percent water by mass, mixed with ammonia and methane.
Many scientists envision these planets with fully fluid convecting interiors. Now, the experimental discovery of superionic ice should give more strength to a new picture for these objects—a relatively thin layer of fluid and a large mantle of superionic ice. In fact, such a structure was proposed a decade ago, based on dynamo simulations, to explain the unusual magnetic fields of these planets. This is particularly relevant as NASA is considering launching a probe to Uranus and/or Neptune in the wake of the successful Cassini and Juno missions to Saturn and Jupiter.
“Magnetic fields provide crucial information about the interiors and evolution of planets,” said Raymond Jeanloz, a co-author of the paper and professor in Earth & Planetary Physics and Astronomy at the University of California, Berkeley. “So it is gratifying that our experiments can test—and in fact, support—the thin-dynamo idea that had been proposed for explaining the truly strange magnetic fields of Uranus and Neptune. It’s also mind-boggling that frozen water ice is present at thousands of degrees inside these planets, but that’s what the experiments show.”
Using diamond anvil cells at LLNL, the researchers applied 2.5 GPa (25,000 atmospheres) of pressure to pre-compress water into the room-temperature form of ice called ice VII, a cubic crystalline form that is different from regular ice-cube hexagonal ice and is 60 percent denser than water at ambient pressure and temperature. They then transported the samples to Rochester and used LLE’s OMEGA-60 laser to perform laser-driven shock compression of the pre-compressed cells.
They focused up to six intense beams of the laser, delivering a one-nanosecond (billionth of a second) pulse of ultraviolet light to one of the diamond cells. This launched strong shock waves of several hundred GPa into the sample to simultaneously compress and heat the water ice.
“Because we pre-compressed the water, there is less shock-heating than if we shock-compressed ambient liquid water,” Millot said, “allowing us to access much colder states at high pressure than in previous shock-compression studies, so that we could reach the predicted stability domain of superionic ice.”
The team used interferometric ultrafast velocimetry and pyrometry (measurements of velocity and temperature) to characterize the optical properties of the shocked compressed water and determine its thermodynamic properties during the brief 10-20 nanosecond duration of the experiment, before pressure-release waves decompressed the sample and vaporized the diamonds and the water.
“These are very challenging experiments, so it was really exciting to see that we could learn so much from the data—especially since we spent about two years making the measurements and two more years developing the methods to analyze the data," Millot said.
“The next step will be to determine the structure of the oxygen lattice,” said LLNL physicist Federica Coppari, another co-author of the paper. “X-ray diffraction is now routinely performed in laser-shock experiments at OMEGA and it will allow (us) to determine experimentally the crystalline structure of superionic water. This would be very exciting, because theoretical simulations struggle to predict the actual structure of superionic water ice.”
Looking ahead, the team plans to push to higher pre-compression and extend the technique to other materials, such as helium, that would be more representative of planets like Saturn and Jupiter.
Other co-authors of the paper were LLNL’s Peter Celliers, Dayne Fratanduono, Damian Swift, and Jon Eggert along with Ryan Rygg and Gilbert Collins, previously at LLNL and now at LLE. The experiments were supported by target fabrication efforts by LLNL’s Stephanie Uhlich, Antonio Correa Barrios, Carol Davis, Jim Emig, Eric Folsom, Renee Posadas Soriano, Walter Unites, and Timothy Uphaus.
“Newly Discovered Form of Water Ice Is ‘Really Strange’,” The New York Times, Feb. 5, 2018
“Laser Experiment Hints at Weird In-Between Ice,” Science News, Feb. 5, 2018