Oct. 15, 2015
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Shaped laser pulse experiments boost understanding of NIF performance

Breanna Bishop, bishop33 [at] llnl.gov, 925-423-9802

In a recent series of papers, Lawrence Livermore National Laboratory (LLNL) scientists examined the role of ablation-front instability and capsule convergence ratio in ignition science experiments at the National Ignition Facility (NIF).

The papers address different aspects of an "adiabat-shaping" experimental campaign, which was performed to controllably assess the effects of ablation-front instability growth vs. fuel adiabat (compression) on implosion performance.

The researchers said this work marks an important step forward in the quest for ignition by demonstrating that ablation-front instability growth can be managed while simultaneously achieving high areal density, a necessary requirement for inertial confinement fusion (ICF) ignition. Furthermore, the results help to implicate ablation-front growth as one of the dominant sources of performance degradation during the National Ignition Campaign (NIC).

The campaign was designed to understand the physics of layered deuterium-tritium (DT) implosions as related to the four-shock low-foot (LF) approach used during the National Ignition Campaign that featured a lower foot, or initial laser picket, in the laser pulse and the three-shock high-foot (HF) approach that featured a higher foot in the laser pulse. The HF was designed to reduce both ablation front instability and capsule convergence ratio that together led to a substantial increase in yield.  The question was how much of the improved performance was due to the lower hydrodynamic instability and how much was from the lower convergence ratio. These experiments were designed to help shed light on this question.

Two new drives were designed for this mini-campaign: a four-shock adiabat-shaped drive derived from the LF drive and a three-shock adiabat-shaped drive derived from the HF drive. Adiabat shaping is based on a strong first shock that decays as it propagates through the shell of the target capsule toward the DT fuel. It sets the ablation front on a higher adiabat (beneficial for stabilization of hydrodynamic instabilities) while keeping the adiabat in the DT fuel at a lower adiabat (needed for high compression). The concept of adiabat shaping was previously proposed and developed in direct-drive ICF experiments at the Omega laser facility (NIF experiments are primarily indirect drive).

In a Physical Review Letters paper titled "Improved performance of high areal density indirect drive implosions at the National Ignition Facility using a four-shock adiabat shaped drive (link is external)," lead author Dan Casey and colleagues describe a significant improvement in implosion performance using a four-shock adiabat-shaped drive with a small increase in early-time laser pulse energy. This modest laser pulse change (a less than 1 percent change in total energy) leads to reduced ablation front instability growth comparable to the high foot experiments, while simultaneously allowing for a low fuel adiabat, resulting in high implosion areal density and convergence ratio comparable to the NIC.

"This adiabat-shaped version of the LF showed a three to 10 times improvement in yield compared to similar experiments during the National Ignition Campaign," Casey said, but still lower than the high foot experiments. "This result helps to separate the deleterious impact of ablation front growth from convergence on implosions during the NIC and supports the hypothesis that seeds at the ablation front—for example, the capsule support tent— played a significant role in limiting (implosion) performance."

Since the NIC, researchers have learned that the "tent" that supports the capsule inside the hohlraum seeded a large perturbation that was amplified by ablation front instability and was likely the dominant factor degrading the performance of NIC capsules. Reducing this with high foot was an important factor in improving performance.

In a Physics of Plasmas paper titled "Stabilization of high-compression, indirect-drive inertial confinement fusion implosions using a 4-shock adiabat-shaped drive (link is external)," lead author Andrew MacPhee and colleagues describe using the recently developed laser pulse profile in separate focused physics experiments to directly confirm the reduction in the ablation front instability growth, while maintaining conditions for a low fuel adiabat needed for increased compression.

Finally, in another Physics of Plasmas paper titled, "First results of radiation driven, layered deuterium-tritium implosions with a 3-shock adiabat-shaped drive at the National Ignition Facility, (link is external)," lead author Vladimir Smalyuk and colleagues discuss using the campaign’s new three-shock adiabat-shaped drive, derived from the HF pulse, to increase the overall capsule convergence ratio and compression.

"The fuel areal density was improved by approximately 25 percent with this new drive compared to similar high-foot implosions," Smalyuk said, although in this case neutron yield remained comparable to the high foot experiments rather than increasing. A leading hypothesis is that asymmetry in the X-ray drive from the hohlraum currently prevents increases in performance that might otherwise be expected from increased fuel areal density or equivalently, convergence ratio. Improving the hohlraum drive symmetry is now a major focus of the program.

Casey was joined by LLNL co-authors Jose Milovich, Vladimir Smalyuk, Dan Clark, Harry Robey, Arthur Pak, Andrew MacPhee, Kevin Baker, Chris Weber, Tammy Ma, Hye-Sook Park, Tilo Döppner, Deborah Callahan, Steve Haan, Prav Patel, Luc Peterson, Charles Yeamans, John Edwards, Otto Landen, Kai Lafortune, Brian MacGowan, Clay Widmayer, Daniel Sayre, Robert Hatarik, Essex Bond, Sabrina Nagel, Robin Benedetti, Nobuhiko Izumi, Shahab Khan, Benjamin Bachmann, Brian Spears and Charles Cerjan and colleagues from General Atomics, Los Alamos National Laboratory and the Massachusetts Institute of Technology.

MacPhee was joined by LLNL colleagues Peterson, Casey, Clark, Haan, Landen, Milovich, Robey, Smalyuk and Oggie Jones.

Smalyuk was joined by LLNL co-authors Robey, Döppner, Jones, Milovich, Bachmann, Baker, Bond, Callahan, Casey, Cerjan, Clark, Haan, Lafortune, Landen, MacGowan, MacPhee, Pak, Patel, Peterson, Weber, Widmayer, Yeamans, Peter Celliers, Sham Dixit, John Edwards, Alex Hamza, Omar Hurricane, Ken Jancaitis, Jeremy Kroll and Laura Berzak Hopkins, and colleagues from General Atomics and the University of Rochester’s Laboratory for Laser Energetics..