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Measuring Plasma Conditions in Z-Pinch Fusion
Z-pinches, a type of plasma confinement system, are candidates for achieving net gain fusion energy in a compact device. Livermore researchers working in partnership with the University of California at San Diego (UCSD), the University of Washington, Sandia National Laboratories, and private-sector fusion company Zap Energy have reported advancements in understanding plasma pressure profiles within flow-stabilized Z-pinches. Their findings, revealing insights into plasma conditions that coincide with fusion neutron production, were published July 2, 2024, in Physics of Plasmas.
The sheared-flow-stabilized Z-pinch configuration uses electric currents to generate magnetic fields that assemble and compress a flowing filament of plasma. By introducing a sheared-flow velocity within the plasma stream, scientists aim to maintain stability and sustain plasma long enough to create fusion energy. To measure plasma conditions at different locations in Zap Energy’s fusion Z-pinch experiment, the team used a Livermore–UCSD-designed portable Thomson scattering diagnostic developed with support from the Department of Energy Advanced Research Projects Agency–Energy. Measurements showed that the plasma column had higher density and less shot-to-shot variation closer to the central electrode, enabling the reconstruction of a full pressure profile.
The team’s measurements provide insight into the performance of fusion energy concepts and link expected, simulated plasma conditions and neutron yields to actual outputs, which is essential to move toward practical fusion energy solutions. Says Livermore physicist Clément Goyon, “The ability to accurately measure and understand plasma conditions is crucial for optimizing fusion performance and advancing this transformative energy technology.”
Contact: Clément Goyon (925) 423-4163 (goyon1 [at] llnl.gov (goyon1[at]llnl[dot]gov)).
Solution for Drive-Deficit Problem
In National Ignition Facility indirect-drive inertial confinement fusion (ICF) experiments, scientists use a device called a hohlraum—approximately the size of a pencil eraser—to convert laser energy into x rays, which then compress a fuel capsule to achieve fusion. A long-standing problem with this approach has been drive deficit, when the predicted x-ray energy converted from laser energy by the hohlraum is higher than what is yielded in experiments. This problem results in the time of peak neutron production, or “bangtime,” occurring around 400 picoseconds too early in simulations, requiring modelers to artificially reduce the laser drive in simulations to match the observed bangtime. A team of Livermore researchers has made advancements in understanding and resolving this issue. Their findings were published July 15, 2024, in Physical Review E.
The team found that the models used to predict x-ray energy were overestimating the x rays emitted by the gold in the hohlraum in a specific energy range. Reducing x-ray absorption and emission in that range led to models better reproducing the observed x-ray flux overall, eliminating most of the drive deficit. This adjustment helps improve the accuracy of the simulations, enabling more accurate design of ICF and high-energy-density experiments following ignition.
“Significant effort has been invested over the years to pinpoint the physical cause of the radiation drive-deficit problem,” says Livermore physicist and lead author Hui Chen. “We are excited about this discovery as it helps resolve a decade-long puzzle in ICF research. Our findings point the way to an improvement in the predictive capabilities of simulations, which is crucial for the success of future fusion experiments and the core mission of stockpile stewardship.”
Contact: Hui Chen (925) 423-5974 (chen33 [at] llnl.gov (chen33[at]llnl[dot]gov)).
Efficient Synthesis of Radioactive Compounds
Compounds of the heavy elements at the bottom of the periodic table, such as americium, curium, and others, are traditionally difficult to synthesize and analyze. Only seven crystal structures of molecular compounds containing curium were reported between its discovery in 1944 and 2020, whereas thousands are known for the lighter elements. Livermore researchers Ian Colliard and Gauthier Deblonde have developed a technique to streamline the synthesis of molecular compounds with heavy elements at 1,000 times greater efficiency than before, without compromising data quality. Results from a demonstration with a series of 20 were published in two papers, both highlighted on the covers of their respective journals—the June 14, 2024, issue of Chemical Communications and the July 22, 2024, issue of Journal of the American Chemical Society.
The new pathway can help scientists perform serial chemistry with radioactive elements, hasten research and development for nuclear waste management and radiopharmaceuticals, and lead to innovative ways of separating elements and studying radioisotopes. In fact, the researchers have discovered five new curium compound structures in the last two years, significantly outpacing the previous discovery rate. The pathway’s efficiency also addresses the minimal supply of research isotopes produced by the Department of Energy.
A common method in heavy element chemistry has been to perform experiments with nonradioactive surrogates, such as the lanthanides. Deblonde says, “We provide an unequivocal set of experimental proofs that heavy element chemistry cannot be predicted solely based on lanthanide chemistry. The results also show a clear fundamental difference between actinide and lanthanide elements, which will lead to new developments in the chemistry of heavy elements, even the rarest ones like actinium and elements beyond curium.”
Contact: Ian Collard (925) 423-8773 (collard1 [at] llnl.gov (collard1[at]llnl[dot]gov)) and Gauthier Deblonde (925) 423-2068 deblonde1 [at] llnl.gov ((deblonde1[at]llnl[dot]gov)).
