Members of the General Atomics (GA) Metrology Research and Development team work with the 4Pi Integrated Metrology System in a cleanroom at GA headquarters. Credit: General Atomics
(Download Image)Automated Target Measurements Contribute to LLNL's Ignition Success
An advanced technique for characterizing high-quality target capsules for experiments at Lawrence Livermore National Laboratory (LLNL)’s National Ignition Facility (NIF) has played an important role in LLNL’s recent success in repeatedly achieving fusion ignition.
Developed by researchers at General Atomics (GA) in San Diego, the 4Pi Integrated Metrology System “has been transformative for inertial confinement fusion (ICF) research,” said NIF Target Fabrication Program Manager Michael Stadermann, “in that it helps us identify the best capsules for ICF experiments.”
“If you want to have a high-performing experiment, you want to be good to within 99.8 percent.”—Sal Baxamusa
GA’s Metrology Research and Development Team regularly partners with LLNL’s Target Fabrication Team to characterize the tiny high-density carbon, or diamond, capsules at the heart of every ICF implosion. Their work in developing the 4Pi System was recently recognized with the 2024 “Team of the Year” R&D 100 Professional Award from R&D World magazine.
The 4Pi System uses an array of advanced instruments to rapidly examine the entire surface and internal alignment of an ICF capsule, enabling researchers to choose the highest-quality candidates for ignition experiments.
“We are overjoyed with the news of this recognition and are proud that our team has played a significant role in supporting successful ignition experiments at NIF,” said Haibo Huang, project lead for 4Pi and director of GA’s Center of Excellence in Advanced Diagnostics. “Our goal is to create cutting-edge technologies that help overcome the most complex obstacles that ICF scientists face to support national security missions.”
LLNL and GA’s long-standing partnership under the National Nuclear Security Administration’s ICF and High Yield research program is designed to help facilitate a safe, secure, and effective nuclear deterrent without underground testing.
The 4Pi system “has allowed us to select capsules that give us high confidence that these implosion experiments are going to be high performing,” said LLNL chemical engineer Sal Baxamusa, deputy program manager for Target Fabrication. “Rather than getting representative images of what a capsule might look like, we actually see every square millimeter of the capsule with the 4Pi.
“The full surface of a sphere has 4Pi steradians, which are two-dimensional analogs of a circle’s 360 degrees,” Baxamusa explained. “So when you say you’re measuring it in 4Pi, it means that you’re measuring the entire surface of the capsule.”
In the past, Baxamusa said, determining which capsules have the best chance of contributing to a successful implosion was a challenge for the Target Fabrication Team and the ICF researchers conducting the experiments.
The synthetic diamond capsules, about the size of a peppercorn, are almost perfectly round, with a surface 100 times smoother than a mirror. They must, however, be as free from flaws and irregularities as possible to ensure a symmetric implosion that traps as much energy as possible in the hot spot at the center of the target, triggering a self-sustaining thermonuclear fusion “burn” that leads to ignition (see “How NIF Works”).
NIF’s capsules are produced in batches of 20 to 25 by a partnership involving LLNL, GA, and Diamond Materials GmbH of Freiburg, Germany.
Elements of the 4Pi system were introduced at GA several years ago, Baxamusa said, but could examine only a limited number of capsules. “We used to have to very carefully choose the capsules that were going to get this kind of measurement,” he said. “There wasn’t a whole lot we could do with the information other than report it to the researchers and use it to understand what happens during the implosion.”
Rapid Characterization
That changed about a year ago when 4Pi was automated, greatly speeding the characterization process while also avoiding measurement errors when a capsule is manually moved from one instrument to another. The 4Pi system combines up to eight technologies, including robotics, automation, batch evaluation, and machine learning, within one common coordinate system.
All five NIF experiments that achieved ignition—producing more fusion energy than the amount of laser energy delivered to the target—have utilized a target characterized and selected by the 4Pi system, either before or after automation.
“It’s all robotic now,” Baxamusa said. “That’s the big change that has us really excited. Now we’re at a point where every single capsule that’s manufactured is actually measured as it enters the production line.
“This has a lot of advantages,” he continued. “One advantage for me is I can now get a lot of statistics on our capsule manufacturing techniques. I’m no longer getting some selection of a capsule; I know exactly what they all look like.
“4Pi gives us the data to then make many quality improvements on the upstream (manufacturing) side. Now we really have the information to choose what we need to improve and how we would improve it.”
Many of the instruments in the 4Pi system have been in operation for years and were used in earlier NIF experiments, including the August 2021 shot that brought NIF to the threshold of ignition.
Measuring Key Parameters
The 4Pi system provides information on two key parameters of capsule quality: the number of microscopic “pits” on the capsule’s surface and the alignment of the shell’s outer and inner layers. Both are potentially important factors in causing hot-spot drift, known as Mode 1 or P1, that can dampen the fusion energy yield in NIF experiments.
“Ideally you want the centers of those two spheres to be right on top of each other,” Baxamusa said. “If you want to have a high-performing experiment, you want to be good to within 99.8 percent. (A capsule) that’s (only) in the 99.5 percent region, we would throw that capsule away and never look at it again. That gives you a sense of the precision, not just in the manufacturing, but in the measurement that’s needed.”
Along with the measurements made possible by the 4Pi system, Baxamusa said a key enabler of the successful ignition experiments was the joint production of “hybrid” capsules fashioned in a sequence of steps as the shells were shuttled between LLNL, Diamond Materials, and GA.
The members of the GA Metrology Research and Development Team—Haibo Huang, Pavel Lapa, Masashi Yamaguchi, Kurt Boehm, and Kevin Sequoia—are collaborating with LLNL’s Baxamusa, Michael Stadermann, Dan Casey, Art Pak, Sean Hayes, and Juergen Biener and GA researchers Shahin Pajoom and Mark Ratledge in the continuing research and development of inertial fusion target fabrication.
More Information:
R&D World announces 2024 R&D 100 Professional Award Winners, R&D World, August 7, 2024
Target Evolution Is a Key to NIF’s Continued Success, NIF & Photon Science News, April 20, 2023
High-Quality Diamond Capsule Enhanced NIF’s Record-Energy Shot, NIF & Photon Science News, December 1, 2021
General Atomics and NIF: Partners on a Mission, NIF & Photon Science News, November 18, 2020
