Aug. 2, 2019
The view from the bottom of the target chamber of the National Ignition Facility, which is working on fusion ignition.
The challenge of fusion ignition
Nuclear fusion has the potential to provide the world with clean, plentiful power. However, it always seems to be 20 years away from any given starting point in history.
Nuclear fusion powers the sun and stars through reactions that turn hydrogen nuclei into helium nuclei. But if that process could be harnessed on earth, the world would have a safe and virtually limitless source of clean energy.
In a series of experiments last year, the Lawrence Livermore National Laboratory’s National Ignition Facility did finally pass a landmark, with the implosion releasing more energy from fusion reactions than was put in to make it happen. Computation and machine-learning algorithms may help researchers build on this achievement through improved experimental designs.
Omar Hurricane, chief scientist of the ICF program at Lawrence Livermore, describes himself as a “pessimistic optimist.” After 50 years of research, scientists now understand the extreme conditions required for fusion. They still can't create them in the lab but are gradually getting closer.
“I am optimistic that the laboratory fusion problem is a solvable one,” he says. “But Mother Nature makes the fusion problem diabolically hard because the required conditions are so extreme.”
Three of the images collected at Argonne National Laboratory’s Dynamic Compression Sector, highlighting diffracted signals recorded on the X-ray detector.
Going for the gold
Gold is an extremely important material for high-pressure experiments and is considered the "gold standard" for calculating pressure in static diamond anvil cell experiments. When compressed slowly at room temperature (on the order of seconds to minutes), gold prefers to be the face-centered cubic (fcc) structure at pressures up to three times the center of the Earth.
However, researchers from Lawrence Livermore National Laboratory (LLNL) and the Carnegie Institution of Washington have found that when gold is compressed rapidly over nanoseconds (1 billionth of a second), the increase in pressure and temperature changes the crystalline structure to a new phase of gold. This well-known body-centered cubic (bcc) structure morphs to a more open crystal structure than the fcc structure.
"We discovered a new structure in gold that exists at extreme states — two thirds of the pressure found at the center of Earth," said lead author Richard Briggs, a postdoctoral researcher at LLNL. "The new structure actually has less efficient packing at higher pressures than the starting structure, which was surprising considering the vast amount of theoretical predictions that pointed to more tightly packed structures that should exist."
By using laser-generated, hologram-like 3D images flashed into photosensitive resin, LLNL researchers have discovered they can build complex 3D parts in a fraction of the time of traditional layer-by-layer printing.
Building objects with light
Despite the range of 3D printing technologies, all share some basic similarities. With one or two notable exceptions, 3D printers build parts one layer at a time from the bottom up. All are able to build extremely complex — and even previously unmanufacturable — geometries. All achieve accuracies that, in many cases, compete with traditional manufacturing processes. And, unfortunately, all are relatively slow, sometimes taking hours and even days to complete a single workpiece.
But Lawrence Livermore researchers and collaborators have created a 3D printer that could make parts almost instantaneously, not unlike the Replicator depicted on the TV series "Star Trek." The new replication process is called Computed Axial Lithography, or CAL.
LLNL engineers are collaborating with UC Berkeley on the CAL method. “It works a lot like computerized tomography (CT) scanning, except in reverse,” said LLNL researcher Maxim Shusteff. “When you get a CT scan, the machine takes a series of X-ray images and uses powerful software to reconstruct them into a virtual image of your body. With our volumetric printing technology, we determine what series of images would be needed to construct a 3D object, and then project them into a semi-transparent resin, thus creating that object.”
Shusteff noted that the light sources are not beams, as are other types of AM technology. Nor are they applied in layers, but rather images that you can think of as frames in a movie.
One of the DOE-funded projects involves LLNL researchers working with semiconductor chip manufacturer Applied Materials to improve a process for depositing thin-film materials on wafers that are used in high-efficiency LED lights and other products. Photo courtesy of Applied Materials.
Energy innovation through HPC
The Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) recently awarded $2 million in federal funding for seven new high performance computing (HPC) projects, as part of the High Performance Computing for Energy Innovation Initiative.
The initiative helps to leverage the national laboratories’ high performance computing capabilities to address challenges in manufacturing and materials through state-of-the-art modeling, simulation and data analysis. Lawrence Livermore leads the program.
Within the High Performance Computing for Manufacturing Program, EERE’s Advanced Manufacturing Office (AMO) has selected five projects including two from LLNL: Ferric, Inc. will partner with LLNL to develop analytical tools that will combine traditional electromagnetic finite-element analysis with micromagnetic simulation; and Applied Materials will continue to work with LLNL on Phase II of developing predictive modeling capabilities for the advanced film deposition technique, High Power Impulse Magnetron Sputtering.
Researchers from LLNL and SLAC National Accelerator Laboratory have found a way to negate the formation of pores that can initiate cracking in a finished part under stress.
Poring through metal AM parts
Lawrence Livermore National Laboratory researchers, along with scientists at the SLAC National Accelerator Laboratory (SLAC) have discovered a solution to a major type of defect in metal 3D-printed parts.
By combining high-performance computer simulations with X-ray imaging of the laser powder-bed fusion metal additive manufacturing (AM) process obtained with SLAC’s synchrotron, researchers have found a way to negate the formation of pores — tiny holes created under the surface of a build that can initiate cracking in the finished part under stress.
The mitigation strategy involves reducing the power of the laser as it slows down to make its turn along the serpentine path it takes to scan and build a metal layer. By varying the laser power throughout the build, they found, they could keep the laser’s depression depth shallow and constant.
“We found in a lot of parts that at the end of that area where the turnaround is, there’s a huge concentration of pores, which affect the material quality. We’ve had simulations that show this, and we needed some experimental validation,” LLNL researcher Aiden Martin explained. “We now have a mitigation strategy where we measure the speed of the laser as it makes that turn, and we change the power dynamically during the turnaround. If we know the speed, we can adjust the power of the laser, and that keeps that depression nice and stable at the surface, and we don’t get that depression that forms the pore at the end.”
