Finding density asymmetries that can limit NIF fusion ignition performance

In December 2022 and 10 times since, the National Ignition Facility (NIF) achieved fusion ignition: It imploded a deuterium-tritium (DT) capsule to generate more fusion energy than the laser energy delivered to the target.

Asymmetries in the target can limit ignition performance, and one of the tools NIF uses to diagnose these asymmetries is its Nuclear Imaging System, which measures emissions from the imploding capsule along three lines of sight. Using these sets of data, researchers can reconstruct a 3D density distribution of the compressed fuel — a computationally intensive process that Ricketts et al. compared with multiple faster, cheaper 2D reconstructions.

“We’ve used these 2D images for years now to help with ignition performance, but how do they really compare to the 3D world that we know we live in?” said author Sidney Ricketts.

The 2D approach relies on a method called fluence compensation, which adjusts the image for uneven neutron production so it highlights the distribution of compressed fuel. But the underlying assumptions produce a unitless, qualitative representation of the density, so the researchers asked whether combining these qualitative images from more than one line of sight can recreate the same 3D asymmetries as the computationally intensive approach.

They found the result depends on the physics being studied: The fluence-compensated technique is valuable for understanding compression asymmetries when the laser parameters are changed, but 3D methods are still needed to evaluate how instabilities are forming.

Ricketts hopes to test if their method can quantify how much DT makes it into the central compression region during ignition and says there is still a lot of physics to be learned from NIF’s limited data sets.

“This is kind of navigating a new sort of physics regime,” Ricketts said. “We’ve only gotten these igniting capsules a handful of times, and that’s the first time that has ever happened in a laboratory environment … so I’m trying to do my best to pull as much information out of the data as I can.”

Source: “Three-dimensional reconstruction of cold fuel shells using down-scattered neutron imaging at the National Ignition Facility,” by S. A. Ricketts, P. L. Volegov, M. S. Freeman, M. Durocher, C. R. Danly, K. D. Meaney, R. H. Dwyer, V. Geppert-Kleinrath, C. H. Wilde, M. P. Selwood, and D. N. Fittinghoff, Physics of Plasmas (2026). The article can be accessed at https://doi.org/10.1063/5.0344898 .

This paper is part of the Papers from the 67th Annual Meeting of the APS Division of Plasma Physics Collection, learn more here .