A better understanding of the formation of toroidal turbulences—known as vortex rings—could help nuclear fusion researchers compress the fuel more efficiently, bringing it closer to becoming a viable energy source.
Developed by researchers at the University of Michigan, the model could help design the fuel capsule, minimizing the energy lost trying to ignite the reaction that makes stars shine. In addition, the model could aid other engineers who must manage fluid mixing after the shock wave has passed, such as those designing supersonic jet engines, as well as physicists trying to understand supernovae.
“These swirling loops move outward from the collapsing star, filling the universe with material that will eventually become nebulae, planets and even new stars — and inwards during fusion explosions, disrupting the stability of the burning fusion fuel and reducing the efficiency of the reaction,” said Michael Wadas, PhD student in mechanical engineering. at UM and the corresponding author of the study.
“Our research, which shows how these 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 said.
Nuclear fusion pushes atoms together until they fuse. This process releases many times more energy than the fragmentation of atoms or fission, which is what powers nuclear power plants today. Researchers can create this reaction, fusing forms of hydrogen into helium, but at the moment, much of the energy used is wasted in the process.
Part of the problem is that the fuel cannot be accurately compressed. The instability creates jets that pierce the hotspot, and fuel escapes between them—Wadas compared it to trying to crush an orange with your hands, and how juice seeps out between your fingers.
The researchers have shown that the vortex rings that form at the leading edge of these jets are mathematically similar to the smoke rings and vortices behind jellyfish and the rings of plasma that fly on the surface of a supernova.
Perhaps the most famous fusion technique is a spherical array of lasers all pointing at a spherical capsule of fuel. This is how experiments are conducted at the National Ignition Facility, which has repeatedly broken energy production records in recent years.
The laser energy vaporizes the layer of material around the fuel—a nearly perfect diamond shell developed in the lab in the latest record-breaking December 2022. As that shell vaporizes, it pushes the fuel in while the carbon atoms fly out. This generates a shock wave that pushes the fuel so hard that the hydrogen fuses.
While spherical fuel pellets are some of the roundest things ever made by man, each one has a deliberate flaw: a fill tube, where the fuel enters. This, the researchers explained, is the most likely place for a jet driven by the ring’s vortex to form, like a straw stuck in that powdery orange, when pressure kicks in.
“Fusion experiments are happening so quickly that we only have to delay formation of the jet by a few nanoseconds,” said Eric Johnsen, associate professor of mechanical engineering at UM, who led the study.
The study combined Wadas and Johnsen’s fluid mechanics expertise as well as knowledge of nuclear and plasma physics in the lab of Caroline Currans, associate professor of nuclear engineering and radiological sciences.
“In high-density energy physics, many studies refer to these structures, but they have not clearly identified them as vortex rings,” Wadas said.
With deep research body knowledge of structures seen in merger experiments and astrophysical observations, Wadas and Johnsen were able to build on and extend this existing knowledge rather than trying to characterize them as entirely new features.
Johnson is particularly interested in the possibility that vortex rings could help drive mixing between heavy and lighter elements when stars explode, as some mixing process must have occurred to produce the formation of planets like Earth.
The model can also help researchers understand the energy limits that a vortex ring can carry, and how much fluid can be pushed before the flow becomes turbulent and difficult to model as a result. In the ongoing work, the team is validating the vortex ring model through experiments.
Publication of the research in the journal Physical review letters.
Michael J. Wadas et al., Saturation of vortex loops ejected from shock-accelerating interfaces, Physical review letters (2023). DOI: 10.1103/PhysRevLett.130.194001
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