LLNL Research Sets New Gold Standard to Narrow ICF ‘Drive Deficit’

For years, LLNL researchers have relied on a mathematical workaround to compensate for a known problem in simulating and designing indirect-drive inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF). This workaround has helped scientists successfully design experiments that have achieved fusion ignition eight times since 2022 in support of the nation’s science-based Stockpile Stewardship Program.

New research recently featured on the cover of the scientific journal Physics of Plasmas has now identified the primary physical contributor to this so-called “drive-deficit” problem—the overestimation in a widely used physics simulation model of predicted x-ray emissions from the gold used in the tiny hohlraums at the center of ICF experiments.

This finding, in turn, will lead to more precise calculations that improve the results of ICF experiments. The findings are comparable to making a correct diagnosis of a disease, a key step to finding a long-term cure, said LLNL physicist Hui Chen, the lead author of the paper, “Key advancements toward eliminating the ‘drive deficit’ in ICF hohlraum simulations.”

“This has been bugging us for decades,” Chen said.

The paper was featured on the cover of the magazine’s April edition and in a companion article, “Identifying a key culprit behind inertial confinement fusion’s ‘drive deficit’ problem,” posted by AIP Publishing’s Scilight.

The research was first outlined in July 2024 in a short letter published in Physical Review E (see “LLNL Researchers Uncover Key to Resolving ICF Hohlraum Drive Deficit”). Physics of Plasmas published the more comprehensive peer-reviewed paper on April 11.

“Between that Physical Review E paper and this current paper, we have a complete series of new data, new results, and a holistic look at the whole problem,” Chen said.

During an ICF experiment at NIF—the first and only facility to achieve fusion ignition in a laboratory—the amplified energy of 192 laser beams is focused into the inside walls of a small pencil-eraser-sized cylinder called a hohlraum. The energy produces a bath of x rays that drive the implosion of a small helium-filled target capsule held in the center of the hohlraum, which is frequently made of gold because of that material’s ability to convert laser energy to x rays.

This rapid compression of the target capsule forces the atoms of the deuterium-tritium fuel mix to fuse, which generates energy. NIF experiments are important for scientists to study nuclear fusion, the same process found in the sun and stars, and in nuclear weapons.

LLNL scientists run simulations to test their theoretical implosion models against actual results of NIF shots. This ongoing quest seeks to calculate precisely how much laser energy is needed to produce a perfectly symmetrical implosion, which in turn produces as much fusion energy as possible.

Simulations, however, have predicted the drive from x-ray energy to be higher than what was measured in experiments. As a result, the peak moment of neutron production, or “bang time,” was occurring about 400 picoseconds sooner than predicted in simulations. This discrepancy is known as the drive deficit.

To compensate for the discrepancy, experiment designers artificially bumped down the amount of laser energy in calculations fed into the simulations. The math workaround made predictions more closely match the actual results for the bang time and other aspects of the fuel capsule’s implosion.

This workaround has sufficed; LLNL first made history by achieving fusion ignition at NIF in December 2022 and has repeated the feat several times since. Yet the problem is like trying to boil water with a thermometer that shows a higher temperature than what is actually achieved in the pot.

So, Chen and a team of LLNL researchers set out on a three-year, eight-experiment effort to find the true cause of the drive deficit. What they found over the course of several experiments was that standard physics codes used in simulations for gold routinely overpredicted x-ray emissions in the hotter area, called the non-local thermodynamic equilibrium (NLTE) region, of plasma directly irradiated by the laser beams.

The experiments show that the discrepancies between the simulations and the actual data were more pronounced in the NLTE region when viewed through the hohlraum’s laser entrance holes.

Halving the Discrepancies

“We conclude that the inaccuracies in the atomic modeling of the NLTE gold plasma are likely responsible for much of the long-standing ‘drive deficit’ problem in hohlraum modeling at least when considering the x-ray drive,” the paper said. “Further work will examine the impact of such a discrepancy on capsule bang time predictions. However, applying the approach presented here to a different campaign suggests that the bang-time discrepancy can be roughly halved.”

New upgrades to NIF’s workhorse diagnostic Dante time-resolved x-ray spectrometer made the research team’s findings possible. Dante is used to measure the energy of the target capsule’s hot spot during the implosion. In addition, high resolution x-ray spectrometers confirmed that the deficit in x-ray emission was compensated for by an increase in the plasma temperature.

“The combination of these two independent measurements elegantly solidified the findings and are leading to more planned and ongoing research to pinpoint the exact mechanisms responsible for the overprediction of the NLTE drive,” said ICF Experiments Group Leader Nino Landen.

And for this research, truncated hohlraums called ViewFactors were used to provide Dante with a wider view of the radiation and plasma conditions surrounding the target capsule at the time of implosion.

Joining Chen on the paper are LLNL colleagues Landen, Douglas Woods, William Farmer, Nicholas Aybar, Duane Liedahl, Stephan MacLaren, Marilyn Schneider, Howard Scott, Judith Harte, Denise Hinkel, John Moody, Mordecai Rosen, James Ross, Sonja Rogers, Nicholas Roskopf, George Swadling, Scott Vonhof, and George Zimmerman.

More Information:

“Identifying a key culprit behind inertial confinement fusion’s ‘drive deficit’ problem,” Scilight, April 11, 2025

“LLNL Researchers Uncover Key to Resolving ICF Hohlraum Drive Deficit,” NIF & Photon Science News , July 23, 2024

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