A decades-long journey to ignition

In December 2022, one of the most challenging scientific goals to-date was realized at the National Ignition Facility – nuclear fusion ignition. Ignition is achieved when more fusion energy is produced than the amount of laser energy delivered to the target, resulting in an energy excess.

This scientific feat was the result of a multi-decade effort involving hundreds of scientists and engineers, including many who developed diagnostics to improve laser design, radiation drive, drive symmetry, shock timing, and implosion velocity along with reducing observed degradations.

Kilkenny et al. discuss the journey to ignition by examining diagnostics in terms of validation of theory or falsification.

“Falsifiability is the root reason we do experiments — to prove theory wrong and suggest how to improve it,” said author J.D. Kilkenny.

Early design work recognized that calculations alone could not achieve the fine tuning required to achieve ignition. Experimental setups were needed, ideally scans that vary just one parameter at a time. Four high-level variables, or the known unknowns of shape, adiabat, velocity, and mix, had to be optimized. The directions of adjustments were determined by trend analysis or evaluating imaging diagnostics.

Additional measurements revealed some surprising unknown unknowns, such as the effect of target filling, support structures, and hot spot drift velocity. These previously considered, but underestimated, factors were degrading performance.

“Over the years, we made major improvements in our measurement capability, particularly in x-ray and neutron imaging, allowing us to make the necessary fixes for these degrading factors and guiding us to demonstrate ignition and burn in the laboratory,” said Kilkenny.

Source: “The crucial role of diagnostics in achieving ignition on the National Ignition Facility (NIF),” by J. D. Kilkenny, A. Pak, O. L. Landen, A. S. Moore, N. B. Meezan, S. W. Haan, W. W. Hsing, S. H. Batha, D. K. Bradley, M. Gatu-Johnson, A. J. Mackinnon, S. P. Regan, and V. A. Smalyuk, Physics of Plasmas (2024). This article can be accessed at https://doi.org/10.1063/5.0211684 .