High-energy-density science blooms at NIF

Four years beyond the original deadline for reaching its goal, and more than seven years since it was commissioned, the National Ignition Facility (NIF) has yet to produce fusion energy greater than what’s delivered to initiate the laser-driven reaction. But even as scientists at the $3.5 billion Lawrence Livermore National Laboratory facility continue their quest for ignition, a small but growing band of academic researchers has been harnessing the unmatched compression and other diagnostic capabilities of the 1.8 MJ laser to explore fundamental questions in condensed matter, astrophysics, planetary science, and other areas (see “NIF may never ignite, DOE admits ,” June 2016, Physics Today online).

“We’re trying to grow the community and get more basic science from NIF,” says Bruce Remington, program leader for NIF discovery science. The number of shots at NIF devoted to academic research has increased almost fivefold in just two years, to 38 last year, and 8% of NIF’s experimental time, some 18 days a year, was set aside for those experiments. Nine projects were selected from the 36 proposals received in last year’s solicitation.

John Browne, chair of NIF’s Management Advisory Committee, says NIF opens “a broad space of parameters for the science community to look at temperatures and pressures they can’t get any other way.”

Raymond Jeanloz, a physicist at the University of California, Berkeley, and one of NIF’s first academic users, says the facility offers “truly a new regime” for a high-energy-density science community that he estimates at about 200 experimentalists. NIF’s energy range is 60 times as high as the University of Rochester’s 30 kJ Omega, the next most energetic US laser, and can exert 10¹⁵ Pa of pressure on a 2-mm-diameter target. Even if poor coupling or other inefficiencies degrade that by a few orders of magnitude to 10 TPa, it’s equivalent to the pressure at the center of Jupiter and far exceeds the 2.3 TPa of pressure that keeps the hydrogen atom from collapsing on itself, he says.

At such pressures, experimentalists might observe a new regime of chemistry, “keV chemistry,” in contrast to the more typical eV energies of standard chemical bonding. “We think at these conditions the core electrons, the inner guts of atoms, actually get involved in chemical bonding. There is theoretical support for that prediction, but very little experimental work has been done in the area,” Jeanloz says. Those conditions also can be found deep inside giant and supergiant planets, but also in the more extreme conditions that prevail in brown dwarfs.

NIF’s productivity has risen dramatically, with the total number of shots, most of which are for the weapons program, more than doubling, from 191 in fiscal year 2014 to 417 in FY 2016. And the lab has been able to make greater use of the laser’s full design energy, says NIF director Mark Herrmann, due to an improved understanding of optically induced damage to the laser’s large lenses. That new knowledge has led to changes in the initial fabrication of optics, increased uniformity of beam intensity across their aperture, and a more automated optics recycling process.

The NIF academic research program also provides lab scientists who work in the classified weapons realm an opportunity to publish in the open literature.

A big challenge for scientists working on NIF is obtaining a fine-grained picture of phenomena that occur when NIF’s 192 beams strike targets. The time and size scales are extremely small: Resolving what occurs inside hohlraums, the cylindrical containers that house peppercorn-sized capsules of fusion fuel, requires nanosecond-scale measurements at 50 µm resolution in a high-energy x-ray environment. Tracking implosions of the capsules requires measurements at the scale of a few microns in a high-neutron-flux environment, says General Atomics vice president Joseph Kilkenny, who’s long been stationed at NIF.

More than 100 scientists and engineers at the lab and at other institutions have helped develop NIF’s 70 separate diagnostics. Recent advances include hybridized CMOS detectors used to make movies through the entrance holes of hohlraums and a dilation x-ray imager, which stretches out photon signal pulses to provide 10 ps temporal resolution of capsule implosions. Conventional gated x-ray cameras achieve 100 ps resolution at best.

Some diagnostics are used for making measurements at Sandia National Laboratories’ Z facility and at Omega, which also conduct experiments for the Department of Energy’s inertial confinement fusion program.

Beyond diamond densities

Experiments just getting under way by one international collaboration will attempt to synthesize a new phase of carbon known as BC-8 that’s predicted to occur at pressures around 1 TPa. Amy Lazicki Jenei, a Livermore researcher who’s part of the eight-institution team, says she and her collaborators also hope to see if BC-8 is metastable at ambient pressure and temperature. If so, it’s possible that a new high-strength form of carbon could be grown the way synthetic diamonds are made today, using chemical vapor deposition. Other phases of carbon are predicted at the even higher pressures occurring in the interiors of carbon planets, a putative type of planet that forms from carbon-rich, oxygen-poor protoplanetary disks.

The group has already determined by using NIF’s in situ x-ray diffraction diagnostic that diamond does not undergo a phase change at up to 800 GPa.

Other planetary science and astrophysics questions are being probed at NIF. In the hunt for exoplanets that could support life, a consideration is whether a planet has a solid iron core surrounded by a liquid iron outer core. That combination is what generates the dynamo and resulting magnetosphere that shields a planet’s surface from the charged particles that stream from its parent star. Inner-core pressures in large exoplanets dubbed super Earths are predicted to range up to 3.5 TPa—10 times the pressure in Earth’s center. A new NIF–university collaboration will subject small amounts of iron to pressures up to 2 TPa to see if the metal remains solid under those conditions.

An ongoing project begun in 2015 explores the dynamics of stellar birth in such clumpy dust clouds as the “pillars of creation” columns in the Eagle Nebula. When a new star lights up, its UV radiation sweeps away the dense cloud through radiative ablation. That mechanism is also what drives implosions in NIF fusion experiments, where intense x rays cause the surfaces of fuel capsules to blow off at high velocity. The experiments involve zapping and vaporizing three or four hohlraums in sequence. The resulting relatively long-lived 60 ns radiation source is useful for studies that require a steady light source to re-create the relevant physics.

Other science experiments aim to discover the contribution of radiative shock in supernovae, the conditions inside brown dwarfs and stars, and whether collisionless shock is responsible for the generation and amplification of the magnetic fields that pervade the universe.

Ignition work continues

The knowledge gained from experiments at NIF is important whether or not ignition is achieved, Herrmann says. “If we get ignition, we will open up whole new vistas of things we can do. If we don’t get ignition, figuring out why we can’t get it and what’s wrong with our simulations that predict we can is important for the [weapons] enterprise.”

Browne says that simulations have now improved to the point that researchers can credibly predict what will occur during shots. The question is whether that predictive capability will extrapolate to the ignition scale.

While optimistic about ignition at NIF, Herrmann notes that other similar facilities are being built in China, France, and Russia, some of which could be larger than NIF. But ignition won’t be easy even at double NIF’s energy, he cautions. “We will have to learn more to make it work even with more energy.”

Since the US stopped underground testing in 1992, the weapons labs’ directors have certified the US stockpile of nuclear weapons every year, evidently without the need for NIF to achieve ignition. Still, says Browne, a former director of Los Alamos National Laboratory, ignition would give the labs “a way of looking at real honest-to-goodness weapons problems that you might have seen if you had been doing nuclear testing” and would add confidence that the weapons simulations can predict those problems. (See the article by Victor Reis, Robert Hanrahan, and Kirk Levedahl, Physics Today, August 2016, page 46 .)

On top of refining the number and intensity of the nanoseconds-long laser pulses that constitute each shot, ignition scientists have been trying out capsule shells fashioned from diamond. The initial capsules were plastic; other candidate materials include beryllium. Researchers also have been trying to figure out how to minimize the hydrodynamic instabilities that arise from the presence of 30-nm-thick films that hold the capsules in place inside the hohlraums and reduce instabilities created by the fill tube through which cryogenic deuterium and tritium are fed into the capsules just prior to shots.

Experiments have also been conducted to determine the optimum density of helium gas used inside the hohlraum. Too much gas produces more laser–plasma instabilities; too little causes the walls to collapse before the implosion can occur.

The next step is to try out larger hohlraums. The farther the capsule is from the inner walls, the greater the opportunity for the x rays radiated by the hohlraum to smooth out and become uniformly deposited on the capsule. But there’s a trade-off: If the hohlraum gets too big, the energy delivered to the capsule will be insufficient to drive the implosion.

Riccardo Betti, assistant director for academic affairs of the University of Rochester’s Laboratory for Laser Energetics, says NIF now has three or four combinations of hohlraums, gas concentrations, and capsules to experiment with. “There is no silver bullet, but they have a better understanding and a choice of paths,” says Betti, who reviewed NIF’s progress toward ignition in a paper published in Nature Physics last year. The best NIF results date to 2014, when the lab recorded a shot in which half of the fusion reactions came from alpha-particle heating in the fusion reaction, and half were caused by the implosion compression. Ignition, he notes, amounts to “runaway alpha heating.”

Much of NIF’s time since the initial ignition push ended in 2012 has been devoted to other nuclear weapons–related topics, including the behavior of plutonium under extreme pressures. In one technique, some of NIF’s beams are used to squeeze plutonium and others create a bright x-ray source to probe the metal’s atomic structure. Herrmann says those experiments have been able to reach previously unattainable pressures that are relevant to nuclear weapons. The exact pressures are classified, but Herrmann says they are “considerably higher” than those that diamond anvil cells can attain.

A large body of experiments in support of the nuclear weapons stockpile address hydrodynamic instabilities and radiation hydrodynamics in a high-energy-density regime. That research is also of fundamental scientific interest, and many of the experimental techniques can be used to characterize materials that aren’t weapons-related.

Browne says those weapons experiments have increased confidence in the continuing reliability and safety of the aging weapons arsenal. “It’s had a direct impact on how the weapons program has tried to understand actual things they face in certifying the stockpile,” he notes.

Jeanloz sees an enormous amount of science in inertial confinement fusion to be done at NIF regardless of whether ignition is within its grasp. A lot more research will need to be done if ignition is achieved to make reaching it easier, more efficient, and reproducible. “The richness of the field is not whether or not you plant your flag on that mountaintop called ignition,” he says, “but really the path toward there, and filling in a better understanding of material properties, and the dynamics, including hydrodynamic instabilities.”