Papers and Presentations

Experiments Reveal New Details Inside NIF Implosions

Two new experimental techniques are shedding new light on the effects of hydrodynamic instabilities and asymmetries in the later stages of NIF inertial confinement fusion (ICF) implosions. The techniques are uncovering details about how fuel-capsule support structures and fill tubes may affect ignition, according to researchers from LLNL, General Atomics, and the Massachusetts Institute of Technology.

In a Physics of Plasmas paper published on Aug. 14, the researchers detailed how the experiments found “unexpected 3-D structures” caused by the ultrathin membranes that support the target capsule inside the hohlraum as NIF’s lasers fire (see “How NIF Targets Work”).

This simulation using LLNL’s HYDRA simulation code shows the growth of a pre-imposed ripple in density at the ablation front as the capsule is compressed from a radius of 900  microns to 300 microns. In the most damaging form of growth, a jet can penetrate into the hot spot during the deceleration phase, bringing with it high-atomic-number material from the shell. This can manifest in the x-ray images as bright spots; a simulated x-ray image using a 2-D HYDRA code with a single axial jet is shown.

The paper also detailed how a high-Z (high atomic number) dopant coating the capsule’s inner surface produces x-ray emissions used to measure the role of perturbations and radiative losses near the peak of ICF compression, “one of the major questions in ICF,” according to the paper.

While more is known about the acceleration phase of NIF implosions, the new experimental platforms should give scientists “a more complete picture of the hydrodynamic growth as we enter into the deceleration phase,” said LLNL physicist Louisa Pickworth, the paper’s lead author. “That’s very important because it tells us the state of the (target capsule) shell and the quality of our implosions.”

Pickworth provided the paper as part of an invited presentation at the American Physical Society Division of Plasma Physics in 2017. The paper also was an extension of research presented in earlier papers, including one from 2016 published in Physical Review Letters about measuring instabilities in NIF implosions at peak velocity.

Pickworth and many LLNL colleagues, including Bruce Hammel and Vladimir Smalyuk, have long studied ways to measure the deceleration stage of ICF implosions after NIF’s laser beams heat the inside of a hohlraum to more than three million degrees Celsius, triggering the fusion of hydrogen atoms in a tiny target capsule fueled with the hydrogen isotopes deuterium and tritium. Ongoing research on various fronts is trying to better understand hydrodynamic (fluid) instabilities, which have been shown to reduce neutron production during ICF experiments.

For one technique, the researchers added a small amount of argon to the gas fuel mixture inside the capsule to “self-backlight” the shell to enhance images of perturbations near peak velocity and into the deceleration phase.

The technique is analogous to a light bulb revealing the textures and folds of a lampshade by providing illumination from the inside, Pickworth said. The x-ray source inside the capsule “allows us to see density distribution in a portion of the shell,” she said. “It’s an important step forward in a very complicated field of inertial confinement fusion,” Smalyuk added.

Using this technique, experiments with pre-imposed perturbations showed up to a 7,000-times growth in the areal density of hydrodynamic instability, “the largest ever observed in ICF implosions,” according to the paper. There was also a difference between the growth shown on the capsule’s equator compared to its pole, “possibly pointing to the different amount of pre-heat between these two directions,” the paper said.

In addition, the researchers made another discovery.

“These experiments discovered unexpected three-dimensional structures originating from the capsule support structures,” the report said. “These new 3-D structures became one of the primary concerns for the indirect drive ICF program that requires their origin to be understood and their impact mitigated.”

A schematic of the capsule support system, the “tents,” for standard tents (a) and polar contact tents (b). The departure angle from the capsule of the tent is shown in (c) with the contact radius indicated. In the polar contact tents, the contact radius is reduced.

In the second technique, which complemented the first, researchers used a high-Z dopant in the inner layer of the capsule. This method was developed to visualize and diagnose problems caused if a “catastrophic growth” were to launch large amounts of ablator material into the hot spot that might otherwise be hard to distinguish during the deceleration phase.

“This made it possible to clearly visualize the temporal evolution of high-mode perturbations, including the tents and fill tubes, as well as low-mode asymmetries that affect the overall shape of the implosion,” the report said.

The method also allowed studies of how much those disturbances reduced neutron yield.

Testing Tents

The researchers then applied this method to tests of new polar contact tents, part of months-long efforts to counter the undesirable distortions caused by the gossamer-thin membranes that support the target capsule inside the hohlraum. Those include using 43-nanometer-thick, carbon-coated polyimide foil that comes in contact with the capsule at its top and bottom poles. This method was designed to reduce the contact area between the tents and the capsule.

The researchers expected to see a “small circular region” at the points where the tents were in contact with the capsule, Pickworth said. “What we didn’t expect to see is a radial pattern. We saw this fractured, cobweb image.”

The radial pattern suggested that the tent, designed to be stiff enough to support the capsule, either bent or fractured into segments, the paper said. Nevertheless, more recent implosions with polar contact tents showed improved neutron yield performance, which warranted the further development of that tenting method, the paper said.

“It’s a work in progress,“ Pickworth said. Click here for a separate Physics of Plasmas paper by Hammel and colleagues providing more details on the polar contact tent studies.

Pickworth, Hammel, and Smalyuk were joined on the paper, “Development of new platforms for hydrodynamic instability and asymmetry measurements in deceleration phase of indirectly driven implosions on NIF,” by LLNL colleagues Harry Robey, Riccardo Tommasini, Laura Benedetti, Laura Berzak Hopkins, David Bradley, Matthew Dayton, Sean Felker, John Field, Steven Haan, Benjamin Haid, Robert Hatarik, Edward Hartouni, Dean Holunga, Nobuhiko Izumi, Steve Johnson, Shahab Khan, Thomas Kohut, Nino Landen, Sebastien Le Pape, Andrew MacPhee, Edward Marley, Nathan Meezan, Jose Milovich, Sabrina Nagel, Abbas Nikroo, Arthur Pak, Bruce Remington, Howard Scott, Paul Springer, Michael Stadermann, Curtis Walters, Klaus Widmann, and Warren Hsing, along with Martin Hoppe Jr. and Neal Rice of General Atomics and Richard Petrasso and Brandon Lahmann of MIT.

—Benny Evangelista