A study by scientists at the University of California San Diego has identified how diamond capsules used in nuclear fusion experiments can develop structural flaws under the high pressures required for the process. 

“The findings can help guide improved capsule designs and models to achieve more uniform implosions, and thus maximize the energy output of fusion experiments,” said the researchers in a press release.

The study is relevant to research at facilities like the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, where inertial confinement fusion is studied as a potential energy source. 

In these experiments, powerful lasers compress a diamond capsule containing deuterium and tritium fuel. The goal is to create a symmetrical implosion that subjects the fuel to the high pressures and temperatures needed for nuclear fusion to occur. 

“By using a high-power pulsed laser to simulate these extreme conditions, researchers found that diamonds can form a series of defects, ranging from subtle crystal distortions to narrow zones of complete disorder, or amorphization,” noted the press release.

“These imperfections can disrupt the implosion symmetry, which in turn can reduce energy yield or even prevent ignition.”

Generating a shock wave

The study details the physical processes occurring within the diamond on extremely short timescales. The laser-driven compression generates a shock wave that creates high pressure and associated high shear stresses within the material in approximately one nanosecond.

“Diamond is an inherently brittle material, lacking dislocation activity under ambient conditions,” added the study.

“This brittleness at room temperature makes it challenging to examine its behavior under shock conditions and complicates post-shock microscopy analysis due to sample fragmentation.”

The experiments were conducted on single-crystalline diamond specimens at various shock pressures. The results showed that at a pressure of 69 gigapascals (GPa), the diamond only exhibited elastic deformation, retaining its defect-free lattice. 

“At a pressure of 115 GPa, defects are generated in the structure by the high shear stresses, which are relaxed by stacking faults, dislocations, and twins,” explained the research team.

First experimental observation

This work marks the first experimental observation of shock-induced amorphization in diamond, a material response that had previously been predicted by molecular dynamics simulations but not seen in a laboratory setting.

The study notes that materials with “open” crystal structures, like diamond, are susceptible to this type of structural collapse under pressure. The atomic packing factor of diamond’s cubic structure is 0.34, considerably lower than that of common metals (0.68 to 0.74). 

“It is recognized that shear stresses superimposed on hydrostatic pressure play an important role in phase transformation and solid-state amorphization,” highlighted the study.

An enhanced understanding of how and why these defects form provides data that can be used to refine the computer models that simulate the implosion process.

“The results from this study on deformation mechanisms may contribute to a more comprehensive constitutive understanding not only of diamond but also of covalently bonded materials in general,” concluded the study.