A recent breakthrough by a team of researchers at Lawrence Livermore National Laboratory (LLNL) has shed light on the long-standing “drive-deficit” issue in indirect-drive inertial confinement fusion (ICF) experiments. This discovery marks a significant step forward in the quest for more accurate predictions and improved performance in fusion energy experiments conducted at the National Ignition Facility (NIF). The findings of this study, titled “Understanding the deficiency in ICF hohlraum X-ray flux predictions using experiments at the National Ignition Facility,” and led by physicist Hui Chen and Tod Woods, have been published in the prestigious journal Physical Review E.
During NIF experiments, researchers utilize a hohlraum, which is about the size of a pencil eraser, to convert laser energy into X-rays. These X-rays are then used to compress a fuel capsule in order to achieve fusion. However, for years, scientists have been facing a significant challenge where the predicted X-ray energy (drive) exceeded the energy measured in actual experiments. This discrepancy led to the “bangtime,” or the time of peak neutron production, occurring approximately 400 picoseconds too early in simulations. This discrepancy was referred to as the “drive-deficit.” Modelers had to make adjustments to artificially reduce the laser drive in the simulations to match the observed bangtime.
The team of LLNL researchers discovered that the models used to predict the X-ray energy emitted by the gold in the hohlraum were overestimating the X-rays in a specific energy range. By reducing X-ray absorption and emission in that particular range, the models were able to more accurately reproduce the observed X-ray flux, both within that energy range and in the total X-ray drive, effectively eliminating most of the drive deficit. This adjustment was crucial due to uncertainties in the rates of certain atomic processes and highlighted areas where the gold atomic models needed improvement. Enhancing the precision of radiation-hydrodynamic codes enables researchers to make better predictions and optimizations for the performance of deuterium-tritium fuel capsules in fusion experiments. Improving the accuracy of simulations is vital for the design of ICF and high-energy-density (HED) experiments post-ignition and plays a key role in scaling discussions for NIF upgrades and future facilities.
The breakthrough made by the LLNL research team offers promising prospects for the field of fusion energy research. By unraveling the complex “drive-deficit” problem, researchers have unlocked new possibilities for enhancing the precision and efficiency of fusion energy experiments at the NIF. The insights gained from this study not only contribute to a better understanding of the fundamental processes involved in inertial confinement fusion but also pave the way for advancements in fusion energy technologies. As scientists continue to refine their models and simulations, the potential for achieving sustainable and abundant fusion energy becomes increasingly within reach.
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