LLNL researchers have developed a simplified NIF experimental platform aimed at a better understanding of the basic issues that crop up when NIF target capsules implode during high energy density (HED) and inertial confinement fusion (ICF) experiments. The new platform focuses on the factors that go into achieving a symmetrical implosion—a key requirement for a successful ignition experiment—by separating the effects of x-ray drive asymmetries from hydrodynamic instabilities and other physics effects. The platform also is part of an effort to validate the computer models used to plan HED experiments.
In contrast to traditional NIF ignition shots, in which the laser pulse is divided into three or four separate shocks with different energy levels, the new platform uses only two shocks, as well as a lower overall level of energy—less than one megajoule and 370 terawatts (trillion watts) of peak power instead of nearly two megajoules and about 500 terawatts in an ignition experiment.
Another important difference is the size of the target capsule, which is much smaller than the typical ignition capsule. This dramatically changes the ratio between the capsule diameter and the diameter of the hohlraum—the pencil-eraser-sized cylinder that holds the target capsule and, when irradiated by the NIF lasers, generates a bath of x rays that ablates, or blows off, the outer surface of the target capsule and compresses the hydrogen fuel in a rocket-like implosion.
The new platform, and the results of the tuning experiments to date, are described in a Physics of Plasmas paper published online on April 27. “One of the challenges of obtaining ignition is our models and simulations,” said lead author Shahab Khan. “For instance, some of these models predicted that the National Ignition Campaign (the NIF ignition effort that began in 2006, with experiments conducted from 2009 to 2012) would get ignition, but they were missing important physics. In order to validate and refine our models, we found that it’s easier to start with a simpler platform where asymmetry and ablator instability issues are much less pronounced. Using a smaller capsule (or larger case-to-capsule ratio), less laser energy and two shocks instead of three simplifies the implosion so that we can validate our models.
“The larger case-to-capsule ratio improves symmetry because the x-ray radiation is more isotropic closer to the center of the hohlraum—the larger distance to source creates a more uniform radiation,” Khan added. “Two shocks reduce the convergence of the capsule and make it easier to drive the capsule symmetrically. And the ‘foot’ portion of our pulse is at a higher power than the three- or four-shock systems; this enhances the ablative stabilization and makes our implosion less sensitive to the tent and fill tube.” (The “tent” is the ultrathin membrane that suspends the target capsule in the hohlraum, while the fill tube is used to inject fusion fuel into the capsule. Both features of the NIF target are believed to contribute to hydrodynamic instabilities that limit implosion performance.)
In their effort to isolate particular physics variables, the researchers conducted several experiments in which they adjusted the pulse shape to symmetrically time the shocks at the pole and equator of the hohlraum, as well as to obtain a symmetric in-flight shell and hot spot shape. The platform uses a near-vacuum hohlraum filled with a small amount of helium-4 gas, which allows nearly all of the laser energy to “couple” with the walls of the hohlraum and generate x rays.
“The near vacuum helps in a couple of important ways,” Khan said. “It increases the laser coupling to x-ray drive to nearly 100 percent, meaning that it’s more efficient. Also, the near-vacuum hohlraum allows for symmetry control by manipulating the fraction (CF) of laser energy on the inner cones instead of relying on cross-beam energy transfer used for gas-filled hohlraums.”
Results of experiments using the two-shock platform were compared with several multi-physics ICF design codes commonly used at LLNL and were found to be “well matched,” with yield over clean (YOC) ratios well over 50 percent (“clean” means mix and instability growth aren’t included in the measurements). “The yield over clean ratio is a criterion on how close this experiment is to modeling,” Khan explained. “When this ratio is close to one it means that our models are good and that this experiment was simple enough to be described using physics in one dimension.
“One of the highlights of my paper and this platform is the almost non-existent symmetry swings,” he added. “In other words, the shape of the shell and hot spot are almost the same and do not change much throughout the implosion. This is a desirable trait in a platform, as it reduces residual kinetic energy—energy that doesn’t go into compressing the capsule and fuel.”
The researchers said the results so far demonstrate the value of experiments focusing on specific aspects of implosion physics. The two-shock platform will next be used to study the sensitivity of fuel-ablator mix to symmetry in both first and second shocks as well as to shock merger depth. In addition, a campaign led by Los Alamos National Laboratory (LANL) to test a beryllium ablator is using the HED two-shock platform as a modeling basis.
The platform was developed by a team led by LLNL’s Steve MacLaren and consisting of designers Jay Salmonson and Jessie Pino and experimenters Tammy Ma, George Kyrala and Joe Ralph. Joining Khan on the Physics of Plasmas paper were LLNL colleagues MacLaren, Salmonson, Ma, Kyrala, Pino, Ralph, Ryan Rygg, John Field, Riccardo Tommasini, David Turnbull, Kevin Baker, Robin Benedetti, Dave Bradley, Peter Celliers, Eddie Dewald, Thomas Dittrich, Laura Berzak Hopkins, Nobuhiko Izumi, Peggy Kervin, Sabrina Nagel, Arthur Pak, and Robert Tipton, along with collaborators John Kline of LANL and Andy Mackinnon of Stanford University.
Hydrogen is the most abundant element in the universe, making up nearly three-quarters of all matter. Despite its prevalence, questions about the element remain.
In a paper published on April 15 by Nature Communications, LLNL researchers and their colleagues aim to answer one of those questions—what happens to hydrogen at high pressure?
“This research tells us something about the process of hydrogen’s transformation from insulator to metal at high pressure,” said lead author Paul Davis. Davis conducted the research as a UC Berkeley graduate student with Roger Falcone, sited within the NIF & Photon Science directorate in the former group of Siegfried Glenzer (now a professor at the SLAC National Accelerator Laboratory). Davis now serves as a Science and Technology Policy Fellow at the Department of Defense.
“Because it’s hard to do these kinds of high-pressure experiments,” Davis said, “there tends to be more theoretical and computational work than data available. In particular, no one has been able to do detailed x-ray scattering studies at a range of pressures before. This work helps us confirm theoretical models for materials under extreme conditions.”
In the Nature Communications paper, the team describes how they used x-rays to peer into the interior of a hydrogen target, looking for free electrons to appear in high-pressure shock waves formed when hydrogen is shot with a high-energy laser beam. The electrons are freed from bonded molecules when the hydrogen is sufficiently compressed by the shock.
“Our x-ray scattering technique allows us to measure those electrons directly,” Davis said. “Knowing what pressure that happened at tells us about the material physics at work—how compressed does hydrogen need to be for free electrons to appear, and in what quantities?”
The experiments were conducted at LLNL’s Jupiter Laser Facility using the two-beam Janus laser. One beam launched a shock wave into targets containing deuterium, an isotope of hydrogen used in inertial confinement fusion experiments. The second beam was used to create x-rays that scattered off the shocked hydrogen. A curved crystal spectrometer spread the scattered x-rays into a spectrum, similar to how a prism breaks optical light into its component colors.
“X-ray laser experiments on laser-heated hydrogen are one of the most interesting new research areas that have become possible in recent years.”
“By looking at the details of the spectrum and comparing it to theoretical calculations, we can infer the behavior of the high-pressure target,” Davis said. “In particular, by doing the same thing at several pressures, we can see where free electrons begin to appear in the spectrum, indicating that the hydrogen is turning from an insulator to a metal at that pressure. The challenge of the experiment is that very few x-rays are scattered, especially in hydrogen, which is very low density. Because the experiments only last a few nanoseconds, we’re fighting to capture enough scattered x-rays to make an analysis.”
Collaborators from the University of Rostock in Germany performed sophisticated analysis of the hydrogen at a variety of shock conditions, calculating how many of the deuterium molecules turned into lone atoms—a process called dissociation. The team found that the pressures where their x-ray measurements indicated the appearance of free electrons (“ionization”) coincided with those where they calculated the breaking of molecules into atoms (“dissociation”)—confirming that the processes appear to happen at the same time.
“The change from strong bonding to almost free electrons is mainly driven by pressure,” said Ronald Redmer of the University of Rostock. “To treat this electronic transition correctly is still a challenge for modern quantum physics.” Quantum physics determines fundamental properties of hydrogen, such as electrical conductivity, which are important in understanding planetary science and nuclear fusion. In addition, because hydrogen is the simplest element, it’s an important model system for understanding the physics of materials under extreme conditions.
“This work helps us understand the physics at work inside giant planets like Jupiter,” Davis said. “The details of how hydrogen dissociates under pressure and becomes electrically conductive are important for scientists seeking to understand planetary interiors and the dynamo action that causes their magnetic fields. The very same physics is at work in the targets at the National Ignition Facility, where designers must understand the high-pressure target properties in order to advance toward fusion.”
According to Davis, while the team has demonstrated that their experimental technique works on laser systems, they expect more sophisticated versions to be used at new x-ray laser facilities such as the Linac Coherent Light Source at SLAC.
Higher Fidelity Studies
“New facilities make it possible to do much higher fidelity dynamic x-ray scattering studies, which could be used to answer subtle questions in planetary and material science,” he said.
“X-ray laser experiments on laser-heated hydrogen are one of the most interesting new research areas that have become possible in recent years,” added Glenzer. “These new studies can resolve the ultrafast time scales on which hydrogen transforms into a dense plasma state and measure its properties with high accuracy.”
Davis was joined on the paper by LLNL co-authors Tilo Döppner, Laurent Divol, Arthur Pak, Peter Celliers, Rip Collins, Nino Landen and Ryan Rygg, and by scientists from UC Berkeley, SLAC, UCLA, the University of Rostock and Sandia National Laboratories.
The work was supported by LLNL’s Laboratory Directed Research and Development Program and the U.S. Department of Energy’s Office of Science, Fusion Energy Sciences. Davis was supported by the National Nuclear Security Administration Stockpile Stewardship Graduate Fellowship.
In a paper published in the April 11 online issue of the journal Nature Physics, LLNL scientists and colleagues analyze the series of NIF “high-foot” inertial confinement fusion (ICF) experiments that reached the highest levels of alpha heating ever achieved on any laser facility. Alpha heating, or self-heating, is a key step on the path to ignition.
In alpha-particle self-heating, the deuterium–tritium (DT) fusion reaction products deposit their kinetic energy locally within the fusion reaction region, thus increasing the temperature in the reacting region and continually reinforcing the reaction rate.
The ICF experiments analyzed in the Nature Physics paper were conducted in 2013 and 2014. The alpha-particle heating of the plasma was dominant, with the fusion yield produced exceeding the fusion yield from the work done by the compression of the fuel alone. The analysis provided new insights into implosion performance using a dynamic model of the hotspot at the center of the target capsule.
In the high-foot experiments, the early-time foot of the drive—the initial “picket” of the laser pulse—was approximately doubled as compared to the low-foot drive, thus launching a stronger and faster first shock and substantially reducing the implosion instabilities associated with low-foot experiments.
Recent three-dimensional simulations of the fusion targets used in both high-foot and low-foot experiments have shown reasonable, though not perfect, agreement with the experimental results and suggest that an improved understanding of the implosions is emerging that can be used to guide future work toward ignition. “We have obtained amplification in fusion yield of more than a factor of two due to alpha heating,” said lead author Omar Hurricane, the ICF program’s chief scientist. “What’s really exciting is that our models and the data are converging on a consistent picture as to what needs to be improved in order to go further.”
The series of high-foot experiments achieved the highest yield—26 kilojoules—of any facility-based ICF experiments to date. The researchers estimate that implosion densities and temperatures must still be increased by roughly a factor of two for ignition to occur, and current ICF experiments are working toward that goal. Among the challenges now being addressed are implosion asymmetries driven by a non-uniform x-ray illumination of the capsule by the hohlraum drive, as well as engineering features such as the “tent” diaphragm structure that holds the capsule in the center of the hohlraum and the tube used to fill the capsule with DT fuel. “Laser-plasma interactions that generate hot electrons (the presence of which is measured in our experiments) that preheat the DT fuel are also a potential performance degradation mechanism and may also be responsible for non-symmetric features on the implosion,” the researchers said.
For more information on NIF’s strategy to achieve ignition, see “Climbing the Mountain of Fusion Ignition: An Interview with Omar Hurricane.”
NIF’s chief mission is to provide experimental insights and data for the National Nuclear Security Administration’s science-based Stockpile Stewardship Program. These experiments represent an important step in the continuing demonstration that the stockpile can be kept safe, secure and reliable without a return to nuclear testing. Ignition physics and performance also play a key role in fundamental science and for potential energy applications.
Hurricane was joined on the paper by LLNL researchers Debbie Callahan, Daniel Casey, Eduard Dewald, Thomas Dittrich, Tilo Döppner, Steve Haan, Denise Hinkel, Laura Berzak Hopkins, Oggie Jones, Annie Kritcher, Sebastien Le Pape, Tammy Ma, Andres MacPhee, Jose Milovich, John Moody, Arthur Pak, Hye-Sook Park, Prav Patel, Joseph Ralph, Harry Robey, J. Steven Ross, Jay Salmonson, Brian Spears, Paul Springer, Riccardo Tommasini, Félicie Albert, Robin Benedetti, Richard Bionta, Essex Bond, Dave Bradley, Joseph Caggiano, Peter Celliers, Charlie Cerjan, Jennifer Church, Rebecca Dylla-Spears, John Edwards, David Fittinghof, Maria Alejandra Barrios, Alex Hamza, Robert Hatarik, Bernard Kozioziemski, Gary Grim, John Field, Nobuhiko Izumi, Shahab Khan, Tom Kohut, Nino Landen, Pierre Michel, Alastair Moore, Sabrina Nagel, Tom Parham, Ryan Rygg, Daniel Sayre, Marilyn Schneider, Dawn Shaughnessy, David Strozzi, Richard Town, David Turnbull, Alan Wan, Klaus Widmann, and Charles Yeamans along with colleagues from the Laboratory for Laser Energetics at the University of Rochester, Los Alamos National Laboratory, General Atomics, and the Massachusetts Institute of Technology.