It’s no wonder that a team of scientists at General Atomics won the “Team of the Year” R&D 100 Professional Award for their development of the 4Pi System. This metrology tool played a pivotal role in the historic achievement of fusion ignition at the National Ignition Facility (NIF) in December 2022. Scientists demonstrated for the first time that a controlled fusion reaction could produce more energy than it consumed — a feat long considered the “holy grail” of clean energy research, pursued since the 1940s following Nobel laureate Hans Bethe‘s research.

The 4Pi System’s ability to analyze the tiny fuel capsules used in Inertial Confinement Fusion (ICF) experiments helped pave the way for this breakthrough. Haibo Huang, Ph.D., director of the Center of Excellence for Advanced Diagnostics and Sensors at General Atomics, highlighted two specific challenges the system addressed:

First, 4Pi helped solve the wall thickness variation problem. The 4Pi System can accurately measure and map the wall thickness of fusion target capsules made of nanocrystalline diamond (HDC), which appear opaque to visible light. While X-rays can see through, it can’t measure to the nanometer sensitivity required. Huang’s team turned to mid-infrared wavelengths to accurately measure and map the wall thickness. Huang explained: “If it’s lopsided, then after the X-ray energy impinges on the surface, it’s going to drift in one direction, and that drift deprives the energy that could otherwise be used to compress the capsule to a higher temperature and pressure,” he noted. “So this is one key reason why ignition was not achieved for almost a decade.”

Second, it can detect and map surface defects. The 4Pi System can identify and quantify tiny surface defects, such as pits, on the capsule surface. These defects, even at microscopic scales, can cause instability growth during the implosion process. Huang described the problem: “Nuclear fusion is an unstable problem… There’s something called Rayleigh-Taylor instability growth, and because of that, any small imperfection in the ablator material itself will cause exponential instability growth during the implosion, injecting a spike of ablator material into the DT ice layer inside. The mixed-in material becomes so hot, it behaves like “meteors” radiating away the much needed energy, thereby quenching the ignition.”

Huang describes the impact of the instrument as “transformative.” “It can be applied back and forth. You can select the best capsule,” he said. “You can feed the information back to the shell production team so that they can improve their fabrication process. You can give the results to the PIs (principal investigators) of the experiment so they can simulate performance and predict the yield and enable them to improve their code.”

The minds behind the machine: Meet the 4Pi team

Central to the 4Pi System’s success is the team’s multidisciplinary expertise. Kurt Boehm, Ph.D., serves as the engineering project manager. He has a background in mechanical engineering and system design and translating complex physics requirements into practical specifications. Kevin Sequoia, Ph.D., contributes as a data scientist, developing algorithms for processing the vast data sets generated by the system. His work was instrumental in identifying the best target capsules for experiments. Pavel Lapa, Ph.D., and Masashi Yamaguchi, Ph.D., both Instrumentation physicists, combined their experience to refine instrument control systems and ensure seamless data acquisition.

“Our team brings together a diverse range of expertise, allowing us to tackle complex challenges and innovate in ways that wouldn’t be possible individually,” Huang said.

The team did not only collaborate internally. “Our mission and vision is to be at the nexus of comprehensive solutions for anything that’s measurement-wise that people need,” Huang explained. “I would just say our desire is to be a one-stop shop for metrology solutions, supporting our customers to accomplish their missions.”

General Atomics operates a DIII-D tokamak system, a national fusion facility, which is unique for a private company. Huang emphasized that General Atomics is “not just a job shop” but a “true scientific partner of the national labs,” investing millions annually in metrology research and target fabrication.

The partnership between General Atomics and national labs goes beyond building instruments. They also collaborate on fundamental research. For example, Huang mentioned collaboration with National Institute of Standards and Technology (NIST) to refine X-ray fundamental parameters in support of the national program investigating how a material’s opacity changes under extreme conditions—research that has implications for both fusion energy and astrophysics.

Developing the 4Pi system

The quest to achieve fusion ignition presented a significant challenge: the need for precise and comprehensive measurement of the inertial confinement fusion (ICF) capsules used in experiments at the National Ignition Facility (NIF). Traditional metrology methods were insufficient, as they typically involved sampling or measuring only parts of the capsule’s surface. Dr. Haibo Huang recognized this limitation early on. “Going forward, we would need to have a full-body analysis of the entire ablator capsule, not just sampling. In addition, different instruments will be used to map different properties of the capsule, and present the data on a common coordinate system to gain deeper insight.”

This realization led to the conceptualization of the 4Pi Integrated Metrology System. The name “4Pi” references the total solid angle in a sphere—symbolizing the system’s ability to analyze the entire surface area of the spherical capsules. “We want to measure the whole surface instead of doing a trace, want to quantify all imperfections inside a capsule. This led to the development of the 4Pi System,” says Huang.

The 4Pi Integrated Metrology System represents a significant advance in the field of inertial confinement fusion (ICF) research. This sophisticated platform provides full 4Pi surface coverage of ICF capsules, enabling comprehensive analysis not possible with traditional methods. By combining data from multiple instruments, including holography, FTIR, X-ray tomography, AFM, and microscopy, within a common (theta, phi) coordinate system, the 4Pi System allows for data fusion from various metrology tools.

Notable metrology tools integrated into the 4Pi System include the Lyncee Tec Digital Holographic Microscope for rapid full-surface defect detection in high-density carbon (HDC) capsules, a compact Fourier transform infrared spectroscopy (FTIR) for measuring wall thickness and variations in capsules, a near infrared microscope specialized in measuring thin wall thickness, a second generation digital microscope with 3-laser wavelengths for improved step height measurement and integrated AFM for detailed local defect mapping, a darkfield microscopy for inspecting interior defects on transparent capsules, and a Lasik tool for removing protruding defects or writing engineered structure onto otherwise perfect capsules.

The 4Pi system is a 12-axis platform that provides full surface coverage of ICF capsules. Each capsule is approximately 2mm in diameter—about the size of a BB—and is fabricated at sub-micron tolerances. The system achieves high precision, with 2μm positional repeatability and less than 1μm wobble control.

The most recent development is the addition of a robotic batch loader which enabled 24/7 production operation, and more than quadrupled the 4Pi metrology throughput. This allows every capsule in a production batch to be fully measured by multiple instruments and select out the very best capsule for high yield experiments. Since the robotic automation, ignitions had been achieved at least four more times, at much increased tempo.

One key challenge was integrating various advanced instruments into a cohesive system capable of analyzing ICF capsules at sub-micron tolerances. “We designed the entire post-processing software on our own because we wanted to suppress the artifacts we don’t want and maximize the signals that we care about,” Huang noted.

Future plans

Looking ahead, Huang and his team are exploring new frontiers in capsule analysis. “In the future, what if we have a capsule that’s so opaque that we cannot see through it with X-ray or infrared? Can we now start to ‘hear’ it? Similar to what LIGO brings “hearing” to astronomy as opposed to “seeing” using telescopes,” Huang mused. “We are constructing a Laser Ultrasound Spectroscopy (LUS) system to “hear” the thickness of targets too opaque for photons to go through. On another front, we will repurpose the FTIR microscope for a new application. We will map oxygen uptake variations in Glow Discharge Polymer (GDP) capsules. According to national lab simulations, GDP has potential to outperform HDC when the NIF yield increases by another ten times to 30MJ.”

The team is also investigating ways to compensate for imperfections in capsule geometry. As Huang explained, “We have been talking about the possibility… let’s say if the wall thickness is lopsided, can we align it in such a way that we can use laser to offset that lopsidedness?” This approach could potentially “achieve a spherical implosion with specific laser power distribution, as long as we know how the shell is oriented,” he said.

“Our mission is to make science fiction come true and become science. So we always think ahead and try to solve problems ahead of time,” Huang concluded.

Editor’s note: This article was updated on October 9, 2024, with corrections and clarifications provided by Haibo Huang, Ph.D.. The updates include more precise descriptions of X-ray capabilities, technical details about the 4Pi System’s components and recent developments, and refined explanations of fusion processes and metrology challenges.