Researchers from Sandia National Laboratories used the speed of sound to determine the shock melting pressure of diamond – and found it to be remarkably high. Not exactly the result researchers hoping to use diamond in inertial confinement fusion (ICF) fuel capsules were hoping to hear.
ICF is a technique that uses high-powered lasers to detonate the outer layer (called the ablator) of a small fuel capsule. This creates an inward-traveling shock wave that raises the temperature and pressure at the center of the capsule to fusion-igniting levels.
The ablator must be made of a material that can absorb the x-ray energy emitted in ICF but that also has a low atomic mass. Diamond and beryllium have been identified as prime candidates for use. Although diamond is attractive from a manufacturing and fabrication point of view, this research shows that it’s probably not the right choice for ICF applications.
The experiment showed that shock waves stronger than 10 million times atmospheric pressure are needed to completely melt diamond. Beryllium, however, needs less than 3 million times. In addition, diamond is a solid-liquid mix over a much larger range of pressures than beryllium is, placing further constraints on the design of ICF capsules made with diamond.
The researchers determined the pressure needed to melt diamond by hitting samples with aluminum/copper plates traveling at up to 54,000 mph. This created shock waves that caused a pressure wave to travel through the diamond at the speed of sound. The speed of sound is highly dependent on the phase of the diamond, so researchers were able to measure the pressures where melting started and was completed.
Recent advances in laser power and efficiency make ICF an attractive candidate for alternative energy sources. In addition, it has promising applications in defense and fundamental physics research.