Measuring Instabilities in NIF Implosions at Peak Velocity

One of the most important challenges to achieving ignition at NIF is understanding and controlling the instabilities that cause target capsule shell material to mix with fusion fuel during inertial confinement fusion (ICF) implosions. In a Physical Review Letters paper published online on July 11, LLNL researchers and their colleagues reported on a new experimental technique that has enabled the first measurements of the total hydrodynamic instability growth near the peak velocity of a NIF implosion (about 900,000 mph), after the initial acceleration phase is complete.

The research joins several other new experimental approaches developed for NIF with the goal of pinpointing the sources of hydrodynamic (fluid) instability growth and improving the capability to computationally model them.

Even though a NIF implosion lasts only a few billionths of a second, it has two distinct phases, noted LLNL physicist Bruce Hammel. “It’s unstable as it accelerates in,” he said, “and it’s unstable as it decelerates.”

The first phase consists of a rocket-like acceleration as the hohlraum x-ray drive ablates, or blows off, target capsule material and accelerates the capsule and fusion fuel inward. This is followed by a rapid deceleration when the shock front “rebounds” from the target’s center and moves back through the capsule in the opposite direction, slowing the implosion.

“During the first phase the low-density ablated material is ‘pushing’ on the higher-density shell,” Hammel said. “This is an unstable process in which the low-density material bubbles through the high-density material to ‘get around it.’ In an ICF implosion, this means that bumps on the capsule surface and other imperfections are amplified as the implosion accelerates inward.

“During the second phase, the lower-density ‘hot spot’ in the center of the capsule pushes back against the denser shell and stops the implosion; this is also unstable,” he said. “The net result is that some of the shell material can mix with the fuel in the hot spot, cooling it and reducing the amount of fusion reactions that can occur.”

As in previous NIF hydrodynamic growth radiography (HGR) experiments, special target capsules with pre-imposed sinusoidal modulations, or “ripples,” on the capsule surface were used (see “Controlling Hydrodynamic Mixing in NIF Implosions”). The experiments measure the growth of hydrodynamic instabilities as a function of the frequency, or “mode” of the ripples (the number of ripples around the capsule). Unlike the previous HGR experiments, however, which were able to measure growth only part of the way through the acceleration phase, the new studies are the first to measure the state of the shell at the end of the acceleration phase, when the implosion has reached peak velocity.

“There are two unknowns in terms of how perturbed the shell is when it implodes,” Hammel said. “One unknown is the growth factors—how much does a ripple get amplified, and what is that amplification factor as a function of the frequency of the ripples, or mode number. The other unknown is what is the state of the capsule when you start. What are the initial perturbations that get amplified by the growth factors?

“We carefully measure the capsules after fabrication with metrology, but there are still uncertainties, including the surface roughness,” he said. “We wanted to understand how perturbed the shell is when it finishes that acceleration phase so we can get a snapshot of how much the ripples or, ultimately, the inherent roughness on a shell, have grown once that first phase is completed.”

Along with measuring the growth of perturbations that arise from imperfections in the capsule’s surface, the research also is aimed at capturing the extent of instability growth caused by factors like the fill tubes used to introduce fuel into the capsule, and the ultrathin diaphragms called “tents” that suspend the capsule inside the hohlraum. “We really want to characterize the entire experiment,” said LLNL physicist Louisa Pickworth, who performed and analyzed the experiments and was principal author of the Physical Review Letters paper. “That’s what this (experimental) platform is for. It is one of the merits of this platform.”

To get a snapshot of the state of the implosion at peak velocity, the researchers employed a novel technique designed by Hammel. They added a small amount of argon to the gas in the plastic capsule to enhance the x-ray emissions from the central hot plasma created during the shock rebound phase of the implosion. In a process called self-radiography, the x rays were used to radiograph perturbation growth from inside the shell, as opposed to the external backlighter/reentrant cone configuration used in the previous HGR experiments.

The research measured perturbations at mode 40, which is at or near the peak of the growth factor curve for these implosions. “In contrast to previous experiments that measured hydrodynamic growth when the capsule was only partially imploded,” Hammel said, “these new experiments measured the total growth from the acceleration phase, when the capsule reached peak implosion velocity.”

By measuring the growth at peak velocity, the authors achieved the highest direct measurement of hydrodynamic growth in any ICF experiment to date—a factor of 7,000 amplification in areal density. Simultaneous measurements of the areal density growth factors at both the waist and pole of the capsule were an order of magnitude larger than previous HGR measurements, and differed by about a factor of two between the waist and the pole, showing asymmetry in the measured growth factors.

The researchers said the next step will be to take measurements during and at the end of the deceleration phase, when the target capsule is fully compressed. “It gets much harder to understand what happens in the capsule after the first phase is completed,” Hammel said, “because it gets even smaller by another factor of four or five, and the perturbations of interest are small enough in wavelength that you can’t resolve them with the instrumentation that we have.”

“The hot spot at peak compression in a layered implosion experiment is approximately 40 microns in diameter,” Pickworth added, “and the spatial resolution we have in the diagnostics we’re running at the moment is about 10 microns, so we can only achieve a small number of spatial resolution elements across that entire hot spot. We have been working hard to build new x-ray diagnostics which have better spatial resolution.”

Pickworth said higher-mode perturbations could soon be observed through the use of new high-resolution imaging systems, such as the Kirkpatrick-Baez (KB) microscope, recently demonstrated on NIF (see “Promising New X-Ray Microscope Poses Logistics Challenges”). The new diagnostic combines better than eight-micron-resolution imaging with narrow-band energy responsiveness in addition to a larger photon collection efficiency when compared to similar imaging systems at NIF. This diagnostic will improve the measurements discussed in the paper, allowing higher modes and lower amplitudes to be measured.

“The KBO has great spatial resolution for these sorts of small objects, and it also has the ability to look in a narrow energy band,” Pickworth said. “It’s difficult to do that on ICF implosions.”

Pickworth and Hammel were joined on the paper by LLNL colleagues Vladimir Smalyuk, Andy MacPhee, Howard Scott, Harry Robey, Nino Landen, Maria Alejandra Barrios, Marilyn Schneider, Tom Kohut, Dean Holunga, Curtis Walters, Ben Haid, and Matthew Dayton and by Sean Regan of the Laboratory for Laser Energetics at the University of Rochester and Martin Hoppe, Jr., of General Atomics.

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