Picture two materials sandwiched together. The boundary between them may appear flat, but, in reality, it is full of tiny bumps and dents.
Suddenly, the materials are hit with a shockwave. If that wave hits a bump in the material interface, it slows down. If it hits a dent, it accelerates forward. This imbalance creates fast, narrow jets of material - called the Richtmyer-Meshkov (RM) instability.
In a recent paper, published in Physical Review Letters, researchers from Lawrence Livermore National Laboratory (LLNL), Imperial College London and their collaborators used AI to optimize and 3D printing to create a target that effectively negates the RM instability.
"Our target reshapes the shockwave, in both space and time, as it travels through the material," said first author Jergus Strucka, now at the European XFEL. "Instead of a single shock hitting the surface, we introduce voids to break it up into a sequence of smaller pressure pulses that arrive at slightly different times."
The team used a machine-learning design optimization algorithm to search through many possible target structures. The process suggested that a void - a specifically shaped cavity in the material - could reshape the shock as it passes through, effectively weakening and redistributing the wave.
"The challenge is that while these designs look promising in simulations, they are often extremely difficult to manufacture and experimentally test," said Strucka. "Our work is one of the first demonstrations that such AI-optimized structures can actually be built and studied in real experiments."
To assemble such a target, the scientists used a polymer 3D printer to make an inverted version of their target structure. Much like making Jell-O in a mold, they fill the printed structure with gelatin, let it set, then remove it. As a result, one side of the gelatin target has a wavy surface, while the other side contains the voids.
The gelatin structure is deposited onto a thin copper strip. They send a large electrical pulse - equivalent to several lightning strikes - through the copper, which heats, explodes and launches a shockwave into the gelatin.
First, the wave encounters the voids. Then it moves toward the wavy end of the gelatin, where the RM instability would normally grow. But by the time it gets there, the wave has been redistributed.
"To some degree, we are creating another instability using the designed voids that acts against the RM instability and reduces jetting," said study author and LLNL scientist Dane Sterbentz. "By modifying the original pressure pulse as it passes through these voids, we are also creating a sort of secondary pressure wave that can actually act against the unstable jetting."
The same physics of voids should apply in a sphere, making these results potentially useful for improving fill tubes or material interfaces in inertial confinement fusion (ICF) targets. During a fusion ignition experiment, unstable jetting can reduce the symmetry of the imploding capsule and therefore the amount of energy produced.
"For ICF experiments at the National Ignition Facility (NIF), it can be difficult and costly to probe isolated effects like the RM instability," said Sterbentz. "That's where our experimental setup is useful - it allows us to probe the instability in a much simpler system. However, experiments more directly relevant to ICF will have to be further pursued at facilities such as the Omega Laser Facility or NIF."
These findings also extend beyond ICF to a broad swathe of materials research where shockwaves are relevant, including oil and gas extraction and defense applications.
