Physicists uncover key to resolving long-standing inertial confinement fusion hohlraum drive deficit

A team of Lawrence Livermore National Laboratory (LLNL) researchers has made progress in understanding and solving the long-standing “underdrive” problem in indirect-drive inertial confinement fusion (ICF) experiments, a discovery that could pave the way for more accurate predictions and improved performance in fusion energy experiments at the National Ignition Facility (NIF).

The team’s findings were: Published In the journal Physics Review E The paper is titled “Understanding Flaws in ICF Cavity X-Ray Flux Predictions Using Experiments at the National Ignition Facility.” Led by physicists Hui Chen and Todd Woods and a team of experts from LLNL, the study focused on discrepancies between predicted and measured x-ray fluxes in NIF’s laser-heated cavities.

“There has been a lot of effort over the years to determine the physical cause of the radiation-driven undershoot problem,” Chen said. “We are excited about this discovery, which helps solve a decade-old mystery in ICF research. Our findings point the way to improving the predictive capabilities of simulations, which will be crucial for the success of future fusion experiments.”

In the NIF experiment, scientists use a device called a cavitator, about the size of a pencil eraser, to measure the Laser Energy This is converted into X-rays, which compress the fuel capsule and cause a nuclear fusion reaction.

For many years there has been an issue where the predicted X-ray energy (drive) is higher than that measured in experiments. As a result, the time of peak neutron production, or “van time”, occurs about 400 picoseconds earlier in the simulations. This discrepancy is called “underdrive”, because modellers had to artificially reduce the laser drive in their simulations to match the observed van time.

The LLNL researchers found that the model used to predict X-ray energies overestimated the X-rays emitted from the gold in the cavity in a certain energy range. By reducing the X-ray absorption and emission in that range, the model more accurately reproduces the observed X-ray flux in both that energy range and the total X-ray drive, eliminating a large portion of the underdrive. This reduction was necessary due to uncertainties in the rates of certain atomic processes and indicates an area where the gold atomic model needs improvement.

Improving the accuracy of the radiation hydrodynamics code will enable researchers to better predict and optimize the performance of deuterium-tritium fuel capsules in fusion experiments. This adjustment will improve simulation accuracy and enable more precise design of post-ignition ICF and high energy density (HED) experiments, which will be crucial in discussions regarding upgrading NIF and scaling up future facilities.