A NIF campaign aimed at measuring the growth of hydrodynamic instabilities in a twice-shocked interface is producing new insights into the turbulent conditions that exist under circumstances ranging from inertial confinement fusion (ICF) implosions to supernova explosions.
Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities occur when fluids of different densities are accelerated together (RT) or shocked (RM) and begin to interpenetrate at their boundary. In an ICF experiment, RT and RM instabilities occur during two distinct phases of the implosion—as the hohlraum x-ray drive accelerates the material in the outer layers (the ablator) of the target capsule into the fusion fuel inside the capsule, and when the shock front “rebounds” from the target’s center and moves back through the capsule in the opposite direction.
These instabilities can lower ICF capsule performance by substantially degrading the ablator’s ability to compress the fusion fuel and by mixing ablator material into the fuel.
To better understand the dynamics of RT and RM instabilities, LLNL researchers have developed a unique ReShock campaign to “study the fundamental physics of turbulent mixing when an interface is shocked twice,” said campaign leader Steve MacLaren. Previous shock-tube experiments have studied the mixing of gases and liquids, MacLaren said, but “instead of heavy and light classical fluids we’re looking at two different densities of plasma. Our objective is to measure the growth of mixing, the interpenetration of the two plasmas, as a function of time following the second shock.”
The campaign began in the fall of 2014 and has conducted a few dozen shots to “understand how well the experimental platform is developing the observables we want to measure,” MacLaren said. “We had to evolve the target design to overcome edge effects and other artifacts that would interfere with our ability to do the measurements. Comparisons of 2-D and 3-D simulations with data from the initial experiments were critical to this process of design evolution.”
The experiments make use of a platform developed by Los Alamos National Laboratory’s Shock/Shear campaign on NIF (see “‘Shock/Shear’ Experiments Shed Light on Turbulent Mix”), in which a pair of “halfraums” (half-hohlraums) are used to produce opposing shocks. The NIF & Photon Science/Weapons and Complex Integration target fabrication team collaborates with General Atomics to make the targets. The process involves machining sinusoidal perturbations, or “ripples,” with two different wavelengths side-by-side at the interface between planar layers of high and low density materials that are mated to the halfraums. The experiments measure the growth of the hydrodynamic instabilities as a function of the wavelength of the ripples.
The signature of the RT and RM instabilities is a “bubble” and “spike” structure, in which fingers of dense material (spikes) interpenetrate the less dense fluid (bubbles), leading to the mixture of the layers of fluid. “What we found was that after the first shock the growth of spikes was dependent on the initial amplitude of the ripples,” MacLaren said. “After the reshock the growth rate was independent of initial amplitude.” This effect had been predicted in theory but had not been observed for a single-mode perturbation in classical shock-tube studies due to the difficulty of controlling the fluid initial condition. In contrast, “we get to start our experiment with materials in a solid state, and the shocks turn everything into a plasma state,” MacLaren said. “This was the first time this was done in a single mode in the plasma state, and it confirms the theory of independence of initial conditions.
“We also developed a technique to simultaneously measure the spike growth and the bubble growth by splitting the target in half,” he said. “Instead of changing the amplitude of the ripples, we changed the materials.” In one half hohlraum, heavy material was doped with iodine; in the other half, lighter material, a nickel-doped carbon foam invented for the experiments by the Advanced Materials Synthesis group in LLNL’s Physics and Life Sciences Directorate was used. “This technique allows us to view both penetrations simultaneously in order to capture the full width of the mixing region,” MacLaren said.
Along with helping to improve and validate the models used in radiation hydrodynamic simulations, data from the ReShock experiments also will inform studies of RM and RT instabilities in core-collapse supernovas.
“Laser-driven experiments can probe hydrodynamic instability evolution in various phases of supernova explosions,” said Channing Huntington, a member of the ReShock team. “Only on NIF can we drive an RT unstable surface with a radiative blast wave, similar to the configuration believed to be present during the early phase of a supernova explosion.” Experiments to measure the effect of radiation in an RT-unstable surface were conducted in a radiative supernova Raleigh-Taylor (RadSNRT) campaign led by the University of Michigan in Fiscal Year 2015. Additional NIF Discovery Science experiments aimed at isolating RT growth following a blast wave are scheduled to begin in June.
University of Oxford graduate student Jena Meinecke has literally prepared for years and traveled around the world for the chance to conduct experiments on NIF. Meinecke is one of about 600 members of the NIF user community —researchers who are interested in experiments on the world’s most energetic laser. Her quest: probe the origins of magnetic fields in the universe.
“There’s only one laser on Earth that can tell us the origins of magnetic fields in our universe, and that’s the NIF laser,” Meinecke said. “We will have three days of shots on NIF to gather our data.”
Meinecke has used other high-energy lasers on her path to NIF to probe physics relevant to shocks and plasma interactions from colliding galaxy clusters and supernova explosions (see “Lasers Ignite ‘Supernovae’ in the Lab”). Using these scaled experiments, she has generated seed magnetic fields—the “grandparents” of the fields scientists today study throughout the universe. But to be able to understand the origins of magnetic fields—how they first formed in the early universe and how they reached the levels that we observe today—she will need to measure one of the most coveted phenomena in laboratory astrophysics: the turbulent dynamo process (see “Lasers Provide Insights Into Formation of Galactic Magnetic Fields”).
Enter NIF. “It’s a question that’s consumed me for years,” Meinecke said, as well as being a research focus area for the Oxford group of Professor Gianluca Gregori, in which Meinecke works. “And NIF is the only laser on Earth capable of creating plasmas with magnetic energy on par with the kinetic energy.”
Meinecke fielded questions from the public as part of the “Ask Me Anything” (AMA) feature on the popular website Reddit on Friday, Feb. 12. Click here to see the questions and her answers.