Understanding explosion precursors: Investigating high-pressure defragmentation by laser ignition experiments

Suddenly there is a fierce flash of light and heat, followed by a rapidly expanding fireball. The burning of high explosives is ubiquitous in popular culture and is also important to ensure the safety and reliability of US stockpiles.

While explosions often get all the credits of burning, defragmentation, which is also a much-known precursor for subsonic speeds, also has the fundamentals to understand the safety and sensitivity of high explosives.

In the new study, researchers at Lawrence Livermore National Laboratory (LLNL) conducted laser ignition experiments on diamond anvil cells and employed large-scale quantum molecular dynamics (QMD) simulations to investigate the products of defragmentation at high pressures. The results could overall improve defragmentation and high explosive models. This work has been featured in the journal Combustion and Flame.

“In general, deflagration precedes an explosion, so understanding dedlagration chemistry is important to understand the process required for an explosion,” says Brad Steel, LLNL scientist and first author.

These experiments and models are intended to determine the product (resulting material) of defragmentation. The composition of the defragmentation products, especially solids, affects the amount of energy and pressure released in the reaction, and whether it transfers to an explosion.

Deflagration is usually studied at relatively low pressure. However, by using laser ignition on the diamond anvil cell, the team was able to obtain data at high pressures comparable to the explosion pressure of the high-pressure LLM-105.

“The experimental approach is a modernized version of the technology first developed at LLNL in the 1990s,” says co-author and project lead researcher Jonathan Crowhurst. “This allows us to investigate the combustion dynamics and chemistry in microscope samples of high explosives at extremely high pressures.”

At these high pressures, the experimental deflagration products were clear. However, in the team’s experiments, only molecular nitrogens that did not explain additional factors that could be present, such as carbon, hydrogen, and oxygen, were detected. To better understand this, they turned to simulations.

Researchers used large-scale QMD simulations to investigate the pressure dependence of product chemistry. They discovered a response mechanism that generates clusters with dilation impairment that contain nitrogen and additional elements.

“Condensed phase chemistry of energy materials is usually simulated using potentials that do not accurately model reaction kinetics. Here, we obtain qualitative agreement with the experiment by modeling reaction kinetics more accurately using QMD,” says Steele. “The main drawback is that this method is so computationally expensive that it requires high performance computing power available in LLNL.”

In both experiments and models, the authors found evidence of pressure reduction during decalcification. The predicted presence of nitrogen and oxygen in the impaired clusters is consistent with delayed gas product formation, and the result is a consequence that can prevent penetration into a complete explosion.

Future work will focus on confirming these findings and incorporating the technology into practical macroscopic models that will help guide better high explosive designs by applying the technology to other energy materials.