Physicists reported recently that they have successfully used the lasers built for fusion reactions at the National Ignition Facility in Lawrence Livermore National Laboratory to compress a synthetic diamond to pressures of 50 million Earth atmospheres (5 terapascals). For the first time scientists measured pressure-density curves of matter at trillion pascal pressures, an extreme environment found in the core of gas giants and super Earth planets.
A tiny sample of synthetic diamond, millimeter-sized and in the shape of a cylinder, was held upright and put into the crosshairs of 176 high powered fusion laser beams. The beams have total peak power of 2200 gigawatts (GW). In comparison, a nuclear power plant only produces as much as energy at a rate of 0.5 to 2 GW. Since power is the energy output over time, the laser beams can only run a very short time at such power, so the total output of energy is not high.
Half the beams are focused on the top half of the cylinder and the other half on the bottom. This squeezes the cylinder when the lasers fire. Upon firing, the physicists measured the rate of diamond material moving under the tremendous heating and counter-reactions. As the cylindrical piece of diamond is compressed, its middle bulges out at extremely high velocities. The measured peak velocity was 109,000 miles per hour, or about 45 kilometers per second.
They found that at the peak pressure of 5 trillion pascals, or equivalently 50 million Earth atmospheres, the density of the diamond had more than tripled. Therefore the diamond was compressed to three times a smaller volume than before, making its density equal to that of lead.
The results were compared to a type of computer simulation called density functional theory (DFT). DFT is based on a branch of physics known as quantum mechanics. While it is an approximate method, meaning that accuracy of representing the underlying physics is sacrificed for purposes of speed, it is quite successful in predicting many complex aspects of matter. The researchers used two types of theories in DFT and showed that the measured results fall right in between the computer predictions.
One mystery is that one simulation predicted the diamond system would under phase changes, that is, changes of symmetry properties of the material. Examples of phase changes are the transition from disordered liquid to crystalline ice, or from a cubic crystal to a hexagonal crystal. However, the measured pressure-density curve was very smooth, lacking any evidence of a phase change, which was hypothesized to show up as discontinuities or sudden jumps in the curves at some point.
The results directly impact our understanding of the core of giant exoplanets by providing one of the first pieces of experimental evidence against which the computer models will be refined. More excitingly, it paves the way for study of even more extreme environments, for example at the center of stars.
The research team was led by senior author Gilbert W. Collins and primary author Ray F. Smith, both scientists at Los Alamos National Labs (LLNL).
This research was published in Nature on Jul 17th 2014.