"We're still working on the same problem, which is the structural integrity of fusion capsules used in inertial confinement fusion, and Hellmann's Real Mayonnaise is still helping us in the search for solutions," says Arindam Banerjee, the Paul B. Reinhold Professor of Mechanical Engineering and Mechanics at Lehigh University and Chair of the MEM department in the P.C. Rossin College of Engineering and Applied Science.
Fusion reactions, which power the sun, could provide an almost limitless and clean energy source if harnessed on Earth. However, replicating the sun's conditions is extremely challenging. Researchers, including Banerjee and his team, are tackling this problem from various angles.
Inertial confinement fusion triggers nuclear fusion by rapidly compressing and heating capsules filled with hydrogen isotopes. Under extreme temperatures and pressure, these capsules form plasma, a charged state of matter that generates energy.
"At those extremes, you're talking about millions of degrees Kelvin and gigapascals of pressure as you're trying to simulate conditions in the sun," says Banerjee. "One of the main problems associated with this process is that the plasma state forms these hydrodynamic instabilities, which can reduce the energy yield."
Banerjee's team first addressed this issue, known as Rayleigh-Taylor instability, in a 2019 paper. This instability occurs between materials of different densities when density and pressure gradients are opposite, creating an unstable stratification.
"We use mayonnaise because it behaves like a solid, but when subjected to a pressure gradient, it starts to flow," he says. Using mayonnaise eliminates the need for extreme temperatures and pressures, which are difficult to control.
The team used a custom-built rotating wheel facility in Banerjee's Turbulent Mixing Laboratory to replicate the flow conditions of plasma. Once the acceleration reached a critical point, the mayonnaise began to flow.
They discovered that before instability occurs, the mayonnaise goes through several phases. "As with a traditional molten metal, if you put a stress on mayonnaise, it will start to deform, but if you remove the stress, it goes back to its original shape," Banerjee says. "So there's an elastic phase followed by a stable plastic phase. The next phase is when it starts flowing, and that's where the instability kicks in."
Understanding the transition between the elastic and stable plastic phases is crucial because knowing when plastic deformation begins can help predict when instability will occur. Researchers aim to control the process to maintain the elastic or stable plastic phase.
In their latest paper, published in Physical Review E, the team, including former graduate student and first author Aren Boyaci '24 PhD, now at Rattunde AG, investigated the material properties, perturbation geometry, and acceleration rate in Rayleigh-Taylor instability.
"We investigated the transition criteria between the phases of Rayleigh-Taylor instability, and examined how that affected the perturbation growth in the following phases," Boyaci says. "We found the conditions under which the elastic recovery was possible, and how it could be maximized to delay or completely suppress the instability. The experimental data we present are also the first recovery measurements in the literature."
This finding is significant as it could inform the design of fusion capsules to prevent instability.
However, a question remains about how the team's data applies to actual fusion capsules, which have significantly different property values from the soft solids used in experiments.
"In this paper, we have non-dimensionalized our data with the hope that the behavior we are predicting transcends these few orders of magnitude," Banerjee says. "We're trying to enhance the predictability of what would happen with those molten, high-temperature, high-pressure plasma capsules with these analog experiments of using mayonnaise in a rotating wheel."
Banerjee and his team are contributing to the global effort to realize fusion energy. "We're another cog in this giant wheel of researchers," he says. "And we're all working towards making inertial fusion cheaper and therefore, attainable."
Research Report:Transition to plastic regime for Rayleigh-Taylor instability in soft solids
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