Inertial insulation fusion is a method for generating energy through nuclear fusion, albeit one plagued by all sorts of scientific challenges (though progress is being made). Researchers at LeHigh University are trying to overcome a specific insect with this approach by conducting experiments with mayonnaise placed in a rotating figure-eight structure. They describe their latest findings in a new paper published in the journal Physical Review E with an eye toward increasing energy yields from fusion.
The work builds on previous research in the LeHigh lab of mechanical engineer Arindam Banerjee, which focuses on investigating the dynamics of fluids and other materials in response to extremely high acceleration and centrifugal force. In this case, his team was exploring what is known as the “threshold of instability” of elastic/plastic materials. Scientists have debated whether this is due to initial conditions, or whether it is the result of “more catastrophic local processes,” according to Banerjee. The question is relevant to a number of fields, including geophysics, astrophysics, explosive welding, and yes, inertial confinement fusion.
How exactly does inertial insulation melting work? As Chris Lee explained to Ars in 2016:
The idea behind inertial confinement fusion is simple. To join two atoms, you must bring their nuclei into contact with each other. Both nuclei are positively charged, so they repel each other, meaning that force is needed to persuade two hydrogen nuclei to touch. In a hydrogen bomb, the force is generated when a small fission bomb explodes, compressing a hydrogen nucleus. This fuses to create heavier elements, releasing a large amount of energy.
Being killers, scientists prefer not to detonate nuclear weapons whenever they want to study fusion or use it to generate electricity. Which brings us to the inertial confinement fusion. In inertial confinement fusion, the hydrogen core consists of a spherical ball of hydrogen ice inside a heavy metal shell. The cover is illuminated by powerful lasers, which burn away a large portion of the material. The reaction force from the vaporized material exploding outwards causes the remaining shell to explode. The resulting shock wave compresses the core of the hydrogen pellet so that it begins to melt.
If the insulation joint ends there, the amount of energy released would be small. But the energy released due to the initial burning of the melt in the center generates enough heat for the hydrogen on the outside of the pellet to reach the required temperature and pressure. So in the end (at least in computer models), all the hydrogen is consumed in a fiery death and massive amounts of energy are released.
That’s the idea anyway. The problem is that hydrodynamic instabilities tend to form in the plasma state – Banerjee compares it to “two materials [that] interpenetrate each other like fingers” in the presence of gravity or some accelerating field—which in turn reduces energy yields. The technical term is a Rayleigh-Taylor instability, which occurs between two materials of different densities, where the density and gradient of pressure move in opposite directions turns out to be an excellent analogue to investigate this instability in accelerated solids, without the need for a laboratory setup with high temperature and pressure conditions, because it is a non-Newtonian fluid.
“We use mayonnaise because it behaves as a solid, but when subjected to a pressure gradient, it starts to flow,” Banerjee said. “Just like with a traditional molten metal, if you put stress on mayonnaise, it will start to deform, but if you remove the stress, it returns to its original shape. So there is an elastic phase followed by a stable plastic phase. The next stage is when it starts to leak, and that’s where the volatility starts.”
More mayo, please
His team’s experiments in 2019 involved pouring Hellman’s Real Mayonnaise—no miracle whip for this crew—into a Plexiglas container and then creating wave-like disturbances in the mayonnaise. One experiment involved placing the container on a spinning wheel in the shape of a figure eight and tracking the material with a high-speed camera, using an image processing algorithm to analyze the footage. Their results supported the claim that the instability threshold depends on the initial conditions, namely amplitude and wavelength.
This latest paper sheds more light on the structural integrity of the fusion capsules used in inertial insulation fusion by taking a closer look at the material properties, amplitude and wavelength conditions, and the acceleration rate of the materials. such as they reach the Rayleigh-Taylor instability threshold. . The more scientists know about the phase transition from the elastic to the stable phase, the better they can control the conditions and maintain an elastic or plastic phase while avoiding instability. Banerjee et al. were able to identify the conditions to maintain the elastic phase, which could inform the design of future inertial confinement fusion pellets.
That said, the mayonnaise experiments are an analog, orders of magnitude away from the real-world conditions of nuclear fusion, which Banerjee readily admits. He hopes, however, that future research will improve the predictability of what happens inside the pellets in their high-temperature, high-pressure environments. “We are another cog in this giant wheel of researchers,” he said. “And we’re all working to make inertial fusion cheaper and therefore more accessible.”
DOI: Physical Review E, 2024. 10.1103/PhysRevE.109.055103 (About DOIs).