New model aims to better understand vortex rings in nuclear fusion

 "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," explains Michael Wadas, a doctoral candidate in mechanical engineering at U-M and corresponding author of the study.

"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.

In nuclear fusion reactors, atoms are pushed together until they merge to release exceptional amounts of energy, hence the interest in them. However, this is currently very energy intensive and wasteful as the fuel cannot currently be neatly compressed. Often, as the researchers point out, instabilities in the process can form jets that penetrate into the hotspot of the reaction chamber. This causes some fuel to spurt out between them, much like what would happen if you squished an orange in your hands.

The researchers have discovered that the vortex rings created at the front of these jets bear a mathematical resemblance to smoke rings, the swirls left in the wake of jellyfish, and the plasma rings emitted from a supernova's surface. "Fusion experiments happen so fast that we really only have to delay the formation of the jet for a few nanoseconds," said Eric Johnsen, an associate professor of mechanical engineering at U-M, who supervised the study.

The study brought together the fluid mechanics expertise of Wadas and Johnsen as well as the nuclear and plasma physics knowledge in the lab of Carolyn Kuranz, 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," said Wadas. 

It is hoped now that researchers can gain insights into the capabilities of vortex rings by utilizing the model as it allows them to determine the maximum energy that can be transported by the ring, as well as the amount of fluid that can be displaced before the flow becomes turbulent and difficult to simulate. At present, the team is conducting experiments to validate the accuracy of the vortex ring model.

You can review the study for yourself in the journal Physical Review Letters.

Study abstract:

Structures evoking vortex rings can be discerned in shock-accelerated flows ranging from astrophysics to inertial confinement fusion. By constructing an analogy between vortex rings produced in conventional propulsion systems and rings generated by a shock impinging upon a high-aspect-ratio protrusion along a material interface, we extend classical, constant-density vortex-ring theory to compressible multifluid flows. We further demonstrate saturation of such vortex rings as the protrusion aspect ratio is increased, thus explaining morphological differences observed in practice.