Inertial confinement fusion is one method of generating energy through nuclear fusion, but it is plagued by a variety of scientific challenges (although progress is being made). Researchers at Lehigh University are attempting to overcome certain weaknesses of this approach by experimenting with mayonnaise in a rotating figure-of-eight device. They explain their latest findings as follows: New Paper A study aimed at increasing the energy yield from nuclear fusion has been published in Physical Review E.
The research builds on mechanical engineer Arindam Banerjee’s previous work at Lehigh Lab. Banerjee focuses on investigating the response of fluids and other materials to extremely high accelerations and centrifugal forces. In this case, his team was exploring what’s called the “instability threshold” of elastic/plastic materials. Scientists have debated whether this is caused by initial conditions or the result of “more localized catastrophic processes,” according to Banerjee. The question has implications for a variety of fields, including geophysics, astrophysics, explosive welding, and inertial confinement fusion.
How exactly does inertial fusion work? As Chris Lee explained for Ars in 2016:
The idea behind inertial confinement fusion is simple: to fuse two atoms, their nuclei must be brought into contact with each other. Both nuclei are positively charged, so they repel each other. This means that a force is needed to bring two hydrogen nuclei into contact. In a hydrogen bomb, a small fission bomb explodes, compressing the hydrogen nuclei, creating a force that fuses them to produce heavier elements, releasing a huge amount of energy.
Scientists are boring people, and they don’t want to detonate a nuclear weapon every time they want to study nuclear fusion or use it to generate electricity. Enter inertial confinement fusion. In inertial confinement fusion, a hydrogen core consists of a spherical hydrogen ice pellet inside a heavy metal case. The case is irradiated with a powerful laser, burning up most of the material. The reaction of the vaporized material exploding outwards causes the remaining shell to explode. The resulting shock wave compresses the center of the hydrogen pellet’s core, and fusion begins.
If confinement fusion had stopped there, the amount of energy released would be negligible, but the energy released by the initial fusion burn in the core creates enough heat to bring the hydrogen on the outside of the pellet up to the necessary temperature and pressure, so that eventually (at least in computer models) all the hydrogen burns off and disappears, releasing a huge amount of energy.
That’s the idea, anyway. The problem is that in a plasma state, hydrodynamic instabilities tend to form, which Banerjee calls “the process of creating a plasma between two substances.” [that] In the presence of gravity or acceleration fields, the particles tend to “stick into each other like fingers”, reducing the energy yield. The technical term is Rayleigh-Taylor instability, and it occurs when density and pressure gradients move in opposite directions between two substances of different densities. Because mayonnaise is a non-Newtonian fluid, it turned out to be a good analogue for investigating this instability in accelerating solids, without the need for high-temperature, high-pressure laboratory equipment.
“We use mayonnaise because it behaves like a solid, but when it is subjected to a pressure gradient it starts to flow.” Banerjee said:“Like conventional molten metal, when you apply pressure to mayonnaise it starts to deform, but when you remove the pressure it goes back to its original shape. That is, an elastic phase is followed by a stable plastic phase. The next phase is when it starts to flow, and that’s where instability begins.”
More mayonnaise please
A 2019 video shows the spinning wheel Rayleigh-Taylor instability experiment at Lehigh University.
In experiments his team conducted in 2019, they poured Hellman’s Real Mayonnaise (they didn’t use Miracle Whip) into a Plexiglas container and created turbulent, wave-like movements in the mayonnaise. In one experiment, they placed the container on a wheel that rotated in a figure-eight shape, tracked the material with a high-speed camera, and analyzed the footage using image-processing algorithms. The results supported the claim that the instability threshold depends on the initial conditions, namely the amplitude and wavelength.
This latest paper sheds more light on the structural integrity of fusion capsules used in inertial confinement fusion by taking a closer look at the material properties, the amplitude and wavelength conditions, and the rate of acceleration of the material when the Rayleigh-Taylor instability threshold is reached. The more scientists know about the phase transition from the elastic to the stable phase, the more they can control the conditions to maintain either the elastic or plastic phase and avoid instability. Banerjee et al. were able to identify the conditions for maintaining the elastic phase, which could aid in the design of future pellets for inertial confinement fusion.
Banerjee acknowledges that the mayonnaise experiment is an analogue, an order of magnitude different from real-world conditions for nuclear fusion. Still, he hopes that future research will improve predictability of what happens inside the pellets at high temperatures and pressures. “We’re just one cog in a giant wheel of researchers,” he says. He said“And we’re all working to make inertial fusion cheaper and more feasible.”
DOI: Physical Review E, 2024. 10.1103/Physics Revision E.109.055103 (About DOIs)
