One year later: Three peer-reviewed papers tell the story of NIF’s record yield shot

August 11, 2022, 12:00PMNuclear News
A stylized image of a cryogenic target used in NIF experiments. (Image: James Wickboldt/LLNL)

Just over one year ago, on August 8, 2021, researchers achieved a yield of more than 1.3 megajoules (MJ) at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) for the first time, achieving scientific ignition and getting closer to fusion gain.

The scientific results of the historic experiment were published exactly one year later in three peer-reviewed papers: one in Physical Review Letters and two (an experimental paper and a design paper) in Physical Review E. In recognition of the many individuals who worked over decades to enable the ignition milestone, more than 1,000 authors are included on the Physical Review Letters paper.

“The record shot was a major scientific advance in fusion research, which establishes that fusion ignition in the lab is possible at NIF,” said Omar Hurricane, chief scientist for LLNL’s inertial confinement fusion program. “Achieving the conditions needed for ignition has been a long-standing goal for all inertial confinement fusion research and opens access to a new experimental regime where alpha-particle self-heating outstrips all the cooling mechanisms in the fusion plasma.”

Run-up to the record: The papers describe the results of the August 2021 shot and the design and experimental adjustments that made it possible following experiments in 2020 and early 2021 that reached the “burning plasma” regime for the first time.

“From that design, we made several improvements to get to the Aug. 8, 2021, shot,” said LLNL physicist Alex Zylstra, lead experimentalist and first author of the experimental Physical Review E paper. “Improvements to the physics design and quality of target all helped lead to the success of the August shot, which is discussed in the Physical Review E papers.”

The August experiment incorporated a few changes, including improved target design. “Reducing the coasting-time with more efficient hohlraums compared to prior experiments was key in moving between the burning plasma and ignition regimes,” said LLNL physicist Annie Kritcher, lead designer and first author of the design Physical Review E paper. “The other main changes were improved capsule quality and a smaller fuel fill tube.”

This three-part image shows the cutaway characteristic target geometry (a) that includes a gold-lined depleted uranium hohlraum surrounding an HDC capsule with some features labeled. The capsule, ~2 mm in diameter, at the center of the ~1 cm height hohlraum, occupies a small fraction of the volume. Laser beams enter the target at the top and bottom apertures, called laser entrance holes. In (b), total laser power (blue) vs. time and simulated hohlraum radiation temperature for the August 8, 2021, experiment are shown with a few key elements labeled. All images are 100 square microns. Imaging data is used to reconstruct the hotspot plasma volume needed for inferring pressure and other plasma properties. (Composite image: LLNL)

Ignition according to Lawson: As explained in the abstract of the Physical Review Letters paper, “An ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin ‘burn propagation’ into surrounding cold fuel, enabling the possibility of high energy gain.”

While “scientific breakeven”—or unity target gain—was not achieved, the August 8, 2021, experiment did produce “capsule gain” and achieved ignition by nine different formulations of the so-called Lawson criterion.

In the article “Fusion Turns Up the Heat,” published August 8 in Physics, the online magazine of the American Physical Society, Matthew Zepf of the Helmholtz Institute Jena explains that “they produced a plasma in which self-heating locally surpasses not only the external heating but also all loss mechanisms.”

Zepf further explains that the conditions required for a fusion machine to reach that point were first formulated in 1955 by engineer and physicist John Lawson. In the case of NIF and its deuterium-tritium fuel, the necessary threshold can be defined as 1025 eV ⋅ s ⋅ m−3, a triple product of plasma density, temperature, and time. That threshold was achieved.

The record stands: Since the experiment last August, the NIF team has been executing a series of experiments to attempt to repeat the performance and to understand the experimental sensitivities in this new regime, according to LLNL. While repeat attempts have not reached the same level of fusion yield as the August 2021 experiment, all have demonstrated capsule gain with yields in the 430–700 kJ range, significantly higher than the previous highest yield of 170 kJ from February 2021.

“Many variables can impact each experiment,” Kritcher said. “The 192 laser beams do not perform exactly the same from shot to shot, the quality of targets varies, and the ice layer grows at differing roughness on each target. These experiments provided an opportunity to test and understand the inherent variability in this new, sensitive experimental regime.”

Next steps: Looking ahead, the team wants to move further beyond the ignition cliff so that result trends can be better isolated from target and laser performance variability, according to LLNL. By improving the laser, targets, and design, NIF researchers hope to improve energy delivery to the hotspot while maintaining or even increasing pressure on the hotspot.

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