Study reveals cause of 3-D asymmetry in inertial confinement fusion implosions

Inertial confinement fusion (ICF) implosions require very high levels of symmetry in order to achieve the high densities and temperatures required for fusion-induced self-heating. Even percentage level deviations from perfect spherical symmetry can lead to significant implosion distortions and ultimately degrade fusion performance.

To that end, researchers at Lawrence Livermore National Laboratory (LLNL) have conducted work to better understand why this is happening. The work was published in Physical examination letters and was presented as a suggestion from the editor.

Daniel Casey, LLNL physicist and lead author of the paper, said the work summarized observations of surface density asymmetries seeded by high density carbon capsule thickness (HDC) asymmetries, helping to shed light on the one of the main causes of significant degradation of ICF implosions. at the National Ignition Facility (NIF), the most energetic laser in the world.

“These asymmetries can decrease the energy available to heat the hotspot and reduce the confinement of that energy,” Casey said. “It’s like squeezing a ball a little harder on one side than the other, at some point the ball will try to evacuate weak spots.”

The article reveals that tiny imperfections in the capsule can turn into huge distortions from implosion to peak compression. In fact, some recent experiments described in the article show that the non-uniformity of the sub-percent level (about 0.7 percent) in the thickness of the HDC capsule can develop into variations of about 25 percent of the fuel areal density and produce hot spot speeds on the order of 100 kilometers. per second.

“This result is significant because if we know the causes of these asymmetries in ICF implosions, we are better able to predict them and understand their impact,” said Casey. “Perhaps most importantly, if we know the causes, we can work to correct them.”

The work was conducted by radiography of the pre-shot capsules before the experiment to determine the level of non-uniformity. Then, after performing the experiment, the team looked for signs of asymmetry in the observed residual velocity of the hot spot and the asymmetry of the surface density of the shell.

“This work was made possible in part by advances in the diagnosis of implosion asymmetry through observations of hotspot velocity using neutron spectrometry,” said Casey. “Along with advances in the measurement of shell non-uniformity through neutron activation anisotropies.

“It’s like the analogy of the balloon being squeezed harder on one side, if we find that the hot spot velocity is very high in a certain direction and aligned with a significant non-uniformity of the shell, we know that some aspects of the implosion were not symmetrical enough, “Casey explained.” So the question becomes ‘why this direction?’ “

The team then sought to compare the pre-taken x-rays of the capsule at the speed of the hotspot. They found that variations in capsule thickness deduced from x-rays are often correlated both in direction and in amplitude. This strongly suggests that shell non-uniformities are at least one of the main causes of asymmetry diagnosed by hotspot speed.

Casey said understanding and improving the performance of ICF implosions is an important part of the lab’s research into NIF.

“Now that we have found that the non-uniformity of HDC shells is a significant degradation in implosion performance, we are working to increase the accuracy of our shell metrology and also to improve HDC manufacturing to produce more uniform shells.” , did he declare. .