Better Understanding Burning Plasmas in Inertial Confinement Fusion

In 2022, the National Ignition Facility, which performs laser-based inertial confinement fusion experiments with deuterium and tritium fuel ions, achieved the landmark accomplishment of fusion ignition, creating more energy than was consumed. Researchers have since been trying to better understand the dynamics of thermonuclear burn propagation and the formation of burning fusion plasmas, which could ultimately help lead to more efficient nuclear energy gain.

During nuclear fusion, some energetic neutrons collide with and deposit energy onto surrounding fuel ions in a process called “up-scattering.” These neutrons have been named reaction-in-flight (RIF) neutrons. Using the lab’s neutron time-of-flight detector suite, Jeet et al. provided the first detailed measurements of a novel nuclear signal, the energy spectrum of the RIF neutrons.

“The spectrum of RIF neutrons is sensitive to how nuclear fusion reaction products scatter and deposit energy into the fusion fuel, and so could provide new insight into the fusion burn process,” said author Justin Jeet. “Making these measurements represented a significant challenge owing to the very small fraction — less than 0.01% — of RIF neutrons produced in the experiments and was only enabled by innovations in neutron time-of-flight diagnostics.”

The authors highlighted the lab’s improved diagnostic capacities, which now can span several orders of magnitude in neutron signal sensitivity.

They also showcased a first set of comparisons between the high-quality spectral measurements that are now possible and radiation hydrodynamic simulations for various experiments, from marginal burn to ignition-scale implosions.

“As we progress further into the era of ignition-class implosions, the measured RIF spectra may become a key metric to assess performance,” said Jeet. “It also may be used to validate, or constrain, current models used in describing hydrodynamic fusion.”

Source: “Diagnosing up-scattered deuterium-tritium fusion neutrons produced in burning plasmas at the National Ignition Facility,” by J. Jeet, B. D. Appelbe, A. J. Crilly, L. Divol, M. Eckart, K. D. Hahn, E. P. Hartouni, A. Hayes, S. Kerr, Y. Kim, E. Mariscal, A. S. Moore, A. Ramirez, G. Rusev, and D. J. Schlossberg, Review of Scientific Instruments (2024). The article can be accessed at https://doi.org/10.1063/5.0219671 .