Finding Sources of Neutron Emission Timing Inconsistencies at the National Ignition Facility
Finding Sources of Neutron Emission Timing Inconsistencies at the National Ignition Facility lead image
To date, the National Ignition Facility (NIF) has achieved ignition seven times, setting its record with 5.2 megajoules of energy output. To continue meeting and exceeding this performance, multiple factors require intricate collaboration between physicists, computational scientists, spectroscopists, and target engineers to get experiments just right, down to picoseconds and micrometers.
Swadling et al. aimed to resolve a long-standing discrepancy in the timing of peak fusion neutron emission — known as the “bang time” — of 400-700 picoseconds between ignition experiments and simulations.
Since NIF’s inception, continuous campaigns and large suites of instruments have been diagnosing NIF’s energy and power, and it has become customary to add a correction factor to the laser drive in simulations to match the measured bang time. Accurate bang time predictions are essential to ensure images are captured at the right times and the laser drive is cut off at the right moment to maximize the laser’s coupling efficiency with the target.
“Over the years, many people have put forward many, many different hypotheses for the underlying cause of this discrepancy, so we took a large-scale survey of all these different ideas to try and see if they held water,” said author George Swadling.
The researchers tested simplified, subscale hohlraums — gold cavities that house the NIF target — which they iteratively made more and more complex to determine the source of the bang time discrepancy. Their results point to errors in the models used to simulate X-ray emission from the high temperature plasma formed at the hohlraum wall.
NIF implosion experiments last just eight to ten nanoseconds, so “errors of a few hundred picoseconds on the timescale of our experiments are actually pretty significant,” said Swadling.
Source: “Resolving discrepancies in bang-time predictions for indirect-drive ICF experiments on the NIF: Insights from the Build-A-Hohlraum Campaign,” by George Forester Swadling, William Anthony Farmer, Hui Chen, Nicholas Aybar, Michael Rubery, Marilyn B. Schneider, Duane Allen Liedahl, Nuno Lemos, Eleanor Tubman, James S. Ross, Denise Hinkel, Otto L. Landen, Mordecai D. Rosen, Sonja Rogers, Katya Newman, Dexter Yanagisawa, Nicholas T. Roskopf, Scott Vonhof, Loosineh Aghaian, Michael Mauldin, Benjamin L Reichelt, Justin Haruaki Kunimune, Physics of Plasmas (2025). The article can be accessed at https://doi.org/10.1063/5.0259922 .
