Reducing the drive-deficit problem in fusion simulations

Inertial confinement fusion (ICF) recently achieved ignition, or a self-sustaining fusion reaction. In direct-drive ICF experiments, a fuel pellet is directly heated by lasers. An alternative approach, called indirect drive, relies on imploding a capsule through X-rays generated in a surrounding enclosure called a hohlraum.

This approach is the one that achieved ignition at the National Ignition Facility. However, it also exhibits a persistent discrepancy: The simulated coupled energy to the capsule is consistently higher than what is measured, resulting in the predicted peak neutron production time being too early. This “drive-deficit” problem makes optimizing the system with simulations alone virtually impossible.

Farmer et al. developed a model framework that addresses the problem, resulting in more accurate predictions for igniting indirect drive experiments.

“We identified several mechanisms that could be responsible for this discrepancy: There is more energy absorbed by the wall, the laser doesn’t create as many X-rays as expected, and the capsule isn’t quite as efficient,” said author William Anthony Farmer.

By adopting a new atomic kinetics model and adjusting the opacity of the hohlraum wall and the fusion capsule in their simulations, the authors reduced the energy transferred to the capsule in a way that maintains agreement with measurements.

In the future, the team plans to investigate how well this new approach scales to different radiation temperatures and hohlraum sizes.

“These results are already impacting the ICF program by giving our simulation team better predictions out of the gate for their integrated fusion experiments,” said Farmer. “They are also giving us a solid basis to examine other discrepancies.”

Source: “A physics-based simulation framework for resolving ignition-scale hohlraum drive discrepancies,” by William Anthony Farmer, C. R. Weber, D. P. Higginson, D. T. Woods, N. A. Aybar, H. Chen, S. B Hansen, D. E. Hinkel, H. P. Le, S. A. MacLaren, J. D. Moody, M. D. Rosen, J. S. Ross, M. B. Schneider, C. R. Schroeder, D. J. Strozzi, and G. F. Swadling, Physics of Plasmas (2026). The article can be accessed at https://doi.org/10.1063/5.0349061 .