Lawrence Livermore National Laboratory


Reduced Instability Growth with High-adiabat High-foot Implosions at NIF

Recent advances in reducing hydrodynamic instabilities in NIF inertial confinement fusion (ICF) implosions were described by LLNL researchers in a Physical Review E paper published online on July 25. The instabilities are a major obstacle in the quest to achieve ignition, as they cause preexisting capsule defects to grow and ultimately quench the fusion burn in NIF implosions.

Unstable growth at the ablation front has been dramatically reduced in recent implosions with high-foot (high initial laser pulse) drives, helping to improve the performance of layered deuterium-tritium (DT) implosions when compared to previous “low-foot” experiments. This has demonstrated the value of stabilizing ablation-front growth and provided directions for future ignition designs.

In their paper, the researchers reported on direct experimental measurements of the growth at the ablation front as a function of mode and time for two different drives. “Most importantly,” they said, “we show that we can sensitively change the ablation-front instability level and measure it at the onset, before stagnation where it is much harder to diagnose. This capability in our implosion tuning process will allow us to more rapidly improve NIF capsule performance and move toward higher yield implosions.” Lead author Dan Casey was joined on the paper by LLNL colleagues and by Abbas Nikroo and Denise Hoover of General Atomics and A.S Moore of the UK’s atomic Weapons Establishment.

Instability Growth Diagrams(a) Schematic diagram of instability growth at several different radii showing density contours from HYDRA simulations. Also shown is a simulated stagnated hot spot showing the consequences of instability growth. (b) Schematic of an indirectly driven hydrogrowth radiography target. The capsules are machined with sinusoidal perturbations and backlit using a V foil x-ray backlighter. (c) Slit image radiograph of side-by-side, preimposed, single-mode ripples of l = 60 and 90, for the low-foot pulse at 20.6 ns, which corresponds to an imploded radius of R=613 μm. (d) Also shown is a slit image radiograph at 14.8 ns, driven with the high-foot pulse, which corresponds to an imploded radius of R=600 μm for the same side-by-side modes.

Measuring Hydrodynamic Instability Growth in NIF Implosions

The results of NIF experiments in which pre-imposed “ripples” on the surface of NIF target capsules are used to measure hydrodynamic instability growth in NIF implosions were reported by LLNL researchers in a Physics of Plasmas paper published online on July 21. The hydrodynamic growth radiography (HGR) platform determines the growth of pre-imposed sinusoidal modulations of the capsule surface as a function of wavelength for two ignition-relevant laser drives: the “low-foot” drive used during the National Ignition Campaign (NIC), and the new high-foot (high initial laser pulse) shape, for which the predicted instability growth is much lower.

Diagram of the HGR ExperimentsDiagram of the HGR experiments (side view not to scale)

Rayleigh-Taylor (RT) and Richtmyer–Meshkov (RM) hydrodynamic instabilities can significantly lower inertial confinement fusion (ICF) capsule performance by degrading the ablator’s ability to compress the fusion fuel and/or by mixing ablator material into the fuel. High levels of fuel-ablator mixing were observed in many of the implosions studied during the NIC. One purpose of the HGR experiments is to compare direct measurements of ICF capsule RT/RM growth with the simulations used to predict the growth before NIC, as part of the effort to understand why NIC capsules failed to approach ignition conditions even at ignition-relevant implosion velocities. The measured growth is consistent with model predictions, including much less growth for the high-foot drive, demonstrating the instability mitigation aspect of the new pulse shape.

“The results (so far) have largely validated the tuned drive approach to modeling instability growth used at NIF,” theresearchers said. “Experiments in the near term will subject the models to increasingly stringent tests at both higher convergence and higher mode numbers. Future experiments are being designed in this newly qualified platform to investigate a variety of issues that will directly impact ICF capsule performance, including 3D surface roughness, features such as the (capsule) support tent (inside the hohlraum), instability aspects of alternate ablators, pulse shapes, and more.”

Lead author Kumar Raman was joined on the paper by LLNL colleagues Vladimir Smalyuk, Daniel Casey, Steve Haan, Omar Hurricane, Jeremy Kroll, Luc Peterson, Bruce Remington, Harry Robey, Dan Clark, Bruce Hammel, Nino Landen, Marty Marinak, David Munro, and Jay Salmonson and by Denise Hoover and Abbas Nikroo of General Atomics and Kyle Peterson of Sandia National Laboratories.