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Model Devised to Study Vortex Rings in Nuclear Fusion

Better comprehending the development of swirling, ring-shaped disturbances—called vortex rings—can aid nuclear fusion scientists compress fuel more resourcefully, increasing its feasibility as an energy source.

Model Devised to Study Vortex Rings in Nuclear Fusion
This graphic shows what happens when a shockwave passes through the interface between two different fluids. The top half of the image shows the starting situation. The top section, in dark teal, shows the vorticity of the fluid, or what parts are engaging in these swirling flows (none at the start). The second layer shows the density of the fluid. The navy blue is less dense, while the yellow and green are the same density—they just sit on opposite sides of the shockwave. The denser fluid protrudes into the less dense fluid, and the initial interface between the two fluids is marked by the dotted line. From that starting point, the shockwave passes through. A jet pushes into the denser fluid, with a vortex ring running ahead of it, traveling in the opposite direction of the shockwave. The swirling flows are shown in light teal in the vorticity panel, while the edges of the vortices are shown in orange. Image Credit: Michael Wadas, Scientific Computing and Flow Physics Laboratory, University of Michigan.

The model formed by scientists at the University of Michigan could help in the development of the fuel capsule, which reduces the energy lost while attempting to kickstart the reaction that underpins how stars emit light. The model can also aid other engineers who need to control the mixing of the fluids following the passing of a shock wave, like those engineering supersonic jet engines, along with physicists trying to comprehend supernovae.

These vortex rings move outward from the collapsing star, populating the universe with the materials that will eventually become nebulae, planets, and even new stars—and inward during fusion implosions, disrupting the stability of the burning fusion fuel and reducing the efficiency of the reaction.

Michael Wadas, Study Corresponding Author and Doctoral Candidate, Mechanical Engineering, University of Michigan

Our research, which elucidates how such vortex rings form, can help scientists understand some of the most extreme events in the universe and bring humanity one step closer to capturing the power of nuclear fusion as an energy source,” he added.

Atoms are pushed together until they combine through nuclear fusion. This method releases several times more energy than splitting atoms apart, or fission, which drives nuclear plants that are present today. Scientists can produce this reaction by combining forms of hydrogen into helium, but currently, a lot of energy utilized in the process is wasted.

The fuel cannot be neatly compressed, which is a part of the problem. Instabilities result in the creation of jets that pierce into the hotspot, and the fuel sprays out between them—Wadas related it to an attempt to squish an orange with the hands and how the juice would flow through the gaps between your fingers.

Scientists have shown that the Vortex rings that form at the leading edge of these jets are mathematically identical to smoke rings, which are the eddies behind jellyfish, and the plasma rings that fly off the surface of a supernova.

Possibly the most popular method of fusion is a spherical array of lasers directed toward a spherical capsule of fuel. This is the way experiments are set up at the National Ignition Facility, which has broken records repeatedly for energy output in the near past.

The material layer around the fuel is vaporized by the energy from the lasers.  When that shell vaporizes, the fuel is driven inward while the carbon atoms fly outward, producing a shockwave, pushing the fuel so hard that the hydrogen is fused.

Although the spherical fuel pellets are some of the most ideally round objects humans have ever created, all atoms have a deliberate fault: a fill tube, from where the fuel can get an entry. Similar to a straw stuck in a crushed orange, this could be the best spot for a vortex-ring-led jet to form when the compression begins, the scientists described.

Fusion experiments happen so fast that we really only have to delay the formation of the jet for a few nanoseconds,” stated Eric Johnsen, Associate Professor of Mechanical Engineering at U-M, who also supervised the study.

The research brought along the fluid mechanics expertise of Wadas and Johnsen along with the nuclear and plasma physics knowledge in the lab of Carolyn Kuranz, who is an associate professor of nuclear engineering and radiological sciences.

In high-energy-density physics, many studies point out these structures, but haven’t clearly identified them as vortex rings,” added Wadas.

Understanding the deep body of research into the structures observed in fusion experiments and astrophysical observations, Wadas and Johnsen could draw on and expand that available knowledge instead of attempting to explain them as fully new features.

Johnsen is specifically keen on the potential that vortex rings could aid in driving the mixing between light and heavy elements when stars burst, as some mixing process needs to have happened to generate the composition of planets such as Earth.

Also, the model can aid scientists in comprehending the limits of the energy that a vortex ring can carry, and the amount of fluid that can be pushed before the flow turns turbulent and harder to model as a consequence. In the present work, the researchers are validating the vortex ring model using experiments. 

The study is financially supported by Lawrence Livermore National Laboratory and the Department of Energy, using computational resources offered by the Extreme Science and Engineering Discovery Environment through the National Science Foundation and the Oak Ridge Leadership Computing Facility.


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