Experiments on Lawrence Livermore National Laboratory's National Ignition Facility (NIF) are providing scientists with new insights into the turbulent after-effects of a supernova explosion. The studies also could inform efforts to achieve self-sustaining nuclear fusion on NIF and other high-energy laser systems.
When stars of a certain mass collapse and then violently explode, material called ejecta expands rapidly and is then decelerated by the surrounding circumstellar medium. This results in Rayleigh-Taylor (RT) hydrodynamic instabilities -- the mixing of denser with less-dense material.
The same instabilities can affect the performance of inertial confinement fusion (ICF) and high-energy density (HED) science experiments such as those conducted on NIF, the world’s highest-energy laser system. The instabilities can cause too much target capsule material to mix with the fuel, quenching the fusion reaction.
The supernova studies, conducted by an international team of researchers led by the University of Michigan (UM) and LLNL physicists Hye-Sook Park and Channing Huntington, found that high energy fluxes and the resulting heat could reduce RT growth in supernova remnants (SNRs) -- something previous astrophysics models had not considered. The results were reported in a Nature Communications paper published online on April 19.
"Rayleigh-Taylor instabilities have been studied for more than 100 years," said Carolyn Kuranz, director of UM’s Center for Laser Experimental Astrophysical Research and an associate research scientist of climate and space sciences and engineering. "These instabilities are important in supernova dynamics, but the effects of these high-energy fluxes, these mechanisms that cause heating, have never been studied in this context." The researchers said realistic models of SNRs "must account for the effects of thermal conduction to accurately predict their evolution at epochs immediately following the shock breakout."
"These heating mechanisms reduce mixing and can have a dramatic effect on the evolution of a supernova," Kuranz said. "In our experiment, we found that mixing was reduced by 30 percent and that reduction could continue to increase over time."
That finding is potentially important for NIF because "the Rayleigh-Taylor instability is a very basic ingredient in understanding NIF’s performance," Park said. "The effect of large energy flux and the basic science of the RT theory and application are quite relevant to ICF and HED science."
Involving NIF users
The supernova studies originated shortly after NIF became operational as a way to begin involving academic institutions in NIF experiments -- an effort that has grown into the current Discovery Science program in which external institutions compete for time on NIF to conduct experimental campaigns (see "NIF Users Bring Ideas and Energy to Discovery Science").
"We designed that experiment way back in 2009," Park said. "This is one of the original Discovery Science programs -- the first one, actually. The University of Michigan was doing supernova RT (SNRT) experiments on OMEGA (the OMEGA Laser at the University of Rochester), so we decided we could think of a similar experiment, but with a new physics goal, on NIF."
U- was studying RT instabilities in the context of the cosmic shocks produced by supernova explosions, "and NIF is really good at generating high-energy fluxes, using the hohlraum, that are transported via thermal heat conduction and radiation transport -- that’s one of the things that we can do easily," Park said.
"So the idea was to study the effect of high-energy flux on RT growth that may be important to supernova evolution. Using NIF, we decided to study the difference in RT growth between the low-flux (230-electron-volt, or eV) and high-flux (325-eV) radiation drive cases."
There were setbacks in the early years, however; NIF diagnostics at the time weren’t up to the task of providing the data required by the researchers. "A 325-eV hohlraum is really hot (about 3.8 million degrees Centigrade), generating a huge amount of background ‘noise,’" Park said. "A typical ICF deuterium-tritium (fusion) shot uses about 290 eV (3.4 million degrees C). Our 325-eV hohlraum is 400,000 degrees hotter. So our signal from our initial experiment got swamped by the background on the time-integrating X-ray film."
The experiments regained momentum when Park became aware of a new experimental platform developed by Los Alamos National Laboratory (LANL) (see "'Shock/Shear’ Experiments Shed Light on Turbulent Mix"). "We designed a target to fit in the halfraum (the half-hohlraum used in the shock/shear experiments)," Park said, "and we did a first set of experiments with a low-flux drive.
"We got great data with the low-flux hohlraum," she said, "but we also needed to do a high-flux drive case with which to compare it. The experiment is all about comparing low drive and high drive. This (shock/shear) platform was never done at high drive, and unless you carefully design the target it wouldn’t work. We had to incorporate additional features to generate a high-flux drive that didn’t generate significant background and that didn’t hamper our observations."
With a new target designed and deployed, the researchers were able to conduct a successful series of nine shots using both low-flux and high-flux drives that demonstrated the effect of high-flux radiation on the subsequent RT growth. They used the results to explore how the large energy fluxes present in supernovae could affect the structure of SNRs and Rayleigh–Taylor growth.
"In analyzing the comparison with supernova SN1993J, a Type II supernova," they said, "we found that the energy fluxes produced by heat conduction appear to be larger than the radiative energy fluxes, and large enough to have dramatic consequences."
According to Park, future experiments in the campaign will attempt to measure the temperature and density in the shock-formation region. "Then we can be more creative about creating more heat flux in the NIF environment to more closely mimic the astrophysical conditions," she said. "There are a lot of windows of opportunity."
Joining Kuranz, Park and Huntington on the paper were Aaron Miles, Bruce Remington, Harry Robey, Kumar Raman, Steve MacLaren, Shon Prisbrey, Russell Wallace and Dan Kalantar from LLNL; along with John Kline, Kirk Flippo, Willow Wan and Forrest Doss from LANL; Eric Harding from Sandia National Laboratories; Christine Krauland and Emilio Giraldez from General Atomics; and researchers from UM and Florida State University; Ben Gurion, University of the Negev in Israel; and MJ Grosskopf, Simon Fraser University, Canada.
bishop33 [at] llnl.gov