Lawrence Livermore National Laboratory

Riding ‘Unicorns’ Into the Future for NIF

For more than a year, NIF scientists have successfully been bringing “Unicorns” to life, only to obliterate their creations with the world’s highest-energy lasers.

It’s been a groundbreaking team effort to build these particular Unicorns—the nickname of a new NIF target diagnostic platform developed under the leadership of Michael Stadermann, group leader for NIF Targets Science and Technology, working with target fabrication engineer Sean Felker of the Laser Systems Engineering and Operations Division.

A Unicorn TargetA NIF Unicorn target utilizes an antenna-like fill tube atop the capsule to assess stability.

Unicorns carry import for NIF’s mission as well: The original legendary beasts were thought by the ancients to impart magical powers, while NIF Unicorn team members believed their incarnation would provide an important clue toward reaching the ultimate inertial confinement fusion (ICF) goal supporting NIF’s Stockpile Stewardship mission: Ignition (see “Ignition: A Look Ahead”).

From the outside, the NIF Unicorn looks deceptively simple: It’s smaller than a fingernail, some 10 millimeters from its conical golden bottom to its single slender horn-like tube atop its ruby-red globed capsule. Scientists use the tube to inject hydrogen fuel into the target capsule to create NIF conditions.

“The Unicorn target has substantially improved our understanding of the importance of the fill tube,” said Stadermann, a chemist who joined Livermore in 2004. While the fill tube is tiny, about 10 microns in diameter (about one-sixth the diameter of a human hair), studies have shown that its presence in the capsule causes significant hydrodynamic perturbations that can interfere with NIF implosion performance.

Results from the capsule team’s findings, along with computer simulations, led researchers to conclude that a smaller fill tube was important. And building experiments based partly on that conclusion in late February, scientists achieved a higher neutron yield per energy input on a shot performed with a diamond capsule using a five-micron fill tube.

While this shot was not a Unicorn shot, its design was influenced by the guidance obtained from Unicorn’s preliminary results, Stadermann explained. “The Unicorn shots are a bit like trying out several sets of running shoes,” he said. “No records will be broken with these tryouts; they show what works better and what doesn’t. The diamond shot was the actual race, and a new high yield was achieved, and we used the shoes that the Unicorn said would be better.”

Unicorn Target Shows Fill Tube EffectThe Unicorn target highlighted the effects on NIF implosions from the fill tube. Coupled to experiments, this was part of a continuing effort to benchmark high-mode fill tube simulations with measured data.

Giving Birth to Unicorns

The team’s initial challenge was to find a better way to hold a target capsule inside a hohlraum without attaching a “tent” around the capsule that had been suspected of impacting performance.

They experimented with test targets using modified tents and no tents, with fill tubes and with no fill tubes, varying the size and the composition of the components. To measure the impact of these features on the implosion, a set of targets for Hydro-Growth-Radiography, or HGR, experiments was developed and shot. One of these, a target with multiple fill tubes nicknamed the “Porcupine,” showed a larger-than-expected perturbation from the fill tubes. To get a better understanding of this feature, the Unicorn was born: An HGR target with a single fill tube.

The HGR targets feature a cone that supports the capsule instead of a tent, allowing the new system to directly face a diagnostic and provide a view to the target as clear and open as high noon. The “horn” at the top is a glass tube 20 times thinner than a human hair, and it sits on a capsule that has been made of diamond, beryllium or ruby-red plastic.

The Physics Challenge

ICF Program scientists have been rooting out possible causes of performance disruption standing in the way of achieving ignition. While conducting a post-shot analysis last year, they noticed a microscopic jet plume correlated with the location of the fill tube. The plume was penetrating the hot plasma core, the source of nuclear yield in ICF, and they focused on this area as a possible cause of performance disruption.

Researchers led by Vladimir Smalyuk, co-leader of the ICF Capsule Science Group, ran experiments breaking down the early, or front end, of the NIF implosions and discovered a tell-tale shadow in the x-ray image created by laser beams on the wall of the hohlraum; it was illuminating the fill tube shadow on the capsule surface like finger puppets on a wall.

Unicorn Target in Target AssemblyA rendering of the NIF Unicorn hohlraum diagnostic, shown at center inside a target assembly. The Unicorn has been a valuable new tool to help scientists locate areas that can limit capsule performance in NIF. The Unicorn utilizes x-ray radiography imaging to measure hydrodynamic instabilities in inertial confinement fusion implosions.

They were able to zoom in on the problem by running their experiments through a new diagnostic tool, a Kirkpatrick-Baez microscope that specializes in capturing high-resolution x-ray images, which revealed that the impact was a hydrodynamic perturbation larger than the fill tube itself.

Once the team isolated the problem, the next step was to solve it. Fill tubes had always been considered an acceptable intrusion in the ICF implosion, like the weld seam on a pipe. The challenge facing the fusion research team was to splt the tiny fill tube by half again, creating a tube just five microns in diameter which was dubbed the baby Unicorn by the target fabrication team.

Led by Jay Crippen, the specialized target makers at General Atomics’ Inertial Fusion Technologies division developed the new target by drilling smaller holes into the diamond-lined target capsule and attaching the half-size fill tube. Initial experiments with the thinner tube showed significant reduction in the perturbation caused by both the fill tube and the x-ray shadow—some five times lower, Snalyuk said.

“We are very happy, because it was a good physics experiment,” he said. “It was a challenge, and then you get there, to solve the challenge, and then you have a whole new challenge. That is the work.”

While the target developed from what the Unicorn demonstrated has so far shown promise, it did not eliminate the disruption-causing feature, Smalyuk said, the the physics team remains cautiously optimistic about the new approach

Successful Innovation in Practice

What also proved a breakthrough for the target fabrication team was not just what the scientists found with the Unicorn shot results, but how they found it.

In a new research & development approach for the group, Stadermann and Felker said they worked simultaneously and in close collaboration with three teams, from the kickoff through to the experimental phase. More than a dozen material scientists, physicists, and engineers met weekly, with continued contact in between.

They deliberately altered the project approach from individual teams working separately and communicating through single team leads to a model in which all team members, designers, experimental leads, material scientists, target fabrication engineers, and target and component fabricators meet to discuss ideas and challenges. “We all came together for one project from the beginning at the same time, feeding ideas, accelerating the process,” said Felker.

The collaboration proved so successful, Stadermann said, they have copied the model with subsequent projects.

“The Unicorn series will carry us to the point where we’ve understood that we’ve mitigated some of these (perturbation) factors and then we’ll leave it behind,” said Stadermann, noting that Unicorn is more of a diagnostic tool than a functioning target. “Once we’ve figured it out with these tools, we’ll do the real target with these solutions that we’ve found that then we hope will take us closer to ignition.”

Echoing Smalyuk, Felker added, “The work is always significant at some level because we’re solving the last problem until ignition—until we find the next problem, and work on solving that one.”

Along with Stadermann, Felker, and Crippen, key members of the Unicorn team included LLNL’s Abbas Nikroo, John Bigelow, Elias Piceno, Chris Choate, and Jeremy Kroll, along with Neal Rice, Noel Alfonso, and Casey Kong from General Atomics. The LLNL physics team was composed of Smalyuk, Laura Berzak Hopkins, Eduard Dewald, Harry Robey, Warren Hsing, Louisa Pickworth, Andrew MacPhee, Chris Weber, Jose Milovich, Bruce Hammel, and Sebastian LePape.

The future now belongs to the Unicorn, which was begat from the Porcupine—technologies following the long scientific tradition of assigning pop-culture nicknames to serious work. Developing new scientific machinery and nicknaming it makes the job fun for the science and engineering staff, explained Felker.

And concepts like Unicorn targets can help capture the collective imagination about the scientific advancements at LLNL and NIF.

“When I told my daughters I made a diamond unicorn and a baby diamond unicorn, they were so excited,” Stadermann grinned. “Then I told them I had to shoot it.”

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