Three peer-reviewed papers highlight scientific results of National Ignition Facility record yield shot
On the one-year anniversary of achieving a yield of more than 1.3 megajoules at LLNL’s National Ignition Facility, the scientific results of this record experiment have been published in three peer-reviewed papers: one in Physical Review Letters and two in Physical Review E. This stylized image shows a cryogenic target used for these record-setting inertial fusion experiments. Credit: James Wickboldt/LLNL

After decades of fusion research, a yield of more than 1.3 megajoules was achieved on August 8, 2021.

The results of this record experiment have been published in three peer-reviewed papers on the one-year anniversary. Many individuals who have worked over many decades to enable this significant advance are acknowledged in a paper by more than 1000 authors.

"The record shot was a major scientific advancement in fusion research, which establishes that fusion ignition in the lab is possible at NIF," said Hurricane. "Achieving the conditions needed for ignition has been a long-standing goal for all insturment fusion research and opens to a new experimental regime where alpha-particle self- heating outstrips all the cooling mechanisms in the fusion plasma."

The results from August 8, 2021 are described in the papers. Alex Zylstra, lead experimentalist and first author of the experimental Physical Review E paper, said that in 2020 and early 2021.

He said that they made improvements to get to the shot. The success of the August shot can be attributed to improvements to the physics design and quality of target.

The experiment included a few changes. The first author of the other Physical Review E paper said that reducing coasting-time with more efficient hohlraums was key in moving between the burningplasma and ignition regimes. Improved capsule quality and a smaller fuel fill tube were some of the changes.

Three peer-reviewed papers highlight scientific results of National Ignition Facility record yield shot
This three-part image shows the cut-away 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 Aug. 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. Credit: Lawrence Livermore National Laboratory

The team has been doing a series of experiments to understand the experimental sensitivities of the new regime.

Each experiment can be impacted by many variables. The 192 laser beams don't perform the same from shot to shot, the quality of targets varies, and the ice layer on each target is different. The experiment gave an opportunity to test and understand the variability of this new experimental regime.

While the August 2021 experiment did not reach the same level of fusion yield as the repeat attempts, all of them demonstrated capsule gain greater than unity with yields in the 430– 700 kJ range, significantly higher than the previous highest yield of 170 kJ. The data gained from these and other experiments provides crucial clues as to what went right and what changes are needed in order to exceed its performance in the future. The team is using the experimental data to further understand the fundamental processes of fusion ignition and burn.

The team is working to leverage the accumulated experimental data and simulations to move towards a more robust regime where general trends can be better separated from variability in targets and laser performance.

Efforts to increase fusion performance and robustness are underway via improvements to the laser, improvements to the targets and modifications to the design that further improve energy delivery to the hotspot. Increasing the amount of fuel and compression of the fusion fuel are some of the ways this can be done.

Hurricane said it was exciting to have an "existence proof" of ignition in the laboratory. Since the end of nuclear testing, no researchers have been able to access the regime that we are operating in.

More information: H. Abu-Shawareb et al, Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.075001

A. B. Zylstra and her co- authors wrote about their experience at the National Ignition Facility. There is a book titled "PhysRevE.106.025 202."

The design of a fusion experiment exceeded the criterion for ignition. There is a book titled "PhysRevE.106.025201."

Journal information: Physical Review Letters , Physical Review E