The biggest laser in the world was used to crush a diamond, offering insights into how the hardest known material behaves when it is exposed to extremely high pressures. The experiment could also reveal new clues about what happens at the cores of giant planets, where conditions of intense atmospheric pressures exist.
Researchers at the Lawrence Livermore National Laboratory in Livermore, California, led by physicist Raymond Smith, blasted a sliver of diamond with a laser beam at a pressure of 725 million pounds per square inch (51 million kilograms per square centimeter). This is the kind of pressure found near the core of giant planets, such as Jupiter or huge, rocky bodies known as “super-Earths.”
The entire experiment took only 25 billionths of a second. The researchers fired 176 laser beams at a small cylinder of gold, called a hohlraum, with a tiny chip of synthetic diamond embedded in it. When the laser beams hit the cylinder, the energy was converted to X-rays. The hohlraum was vaporized, and in the process, the diamond was exposed to pressures tens of millions of times Earth’s atmospheric pressure.
Theoretical calculations predicted that such high pressures should cause a diamond to change its crystal structure. One way to test if this is true is to measure the speed of sound waves in a material. If this speed changes abruptly as the pressure goes up, then the diamond structure has rearranged itself.
But that didn’t happen — the velocity of sound waves changed smoothly.
"If there was a phase transformation you’d expect a discontinuity," Smith said.
The rate of change in the diamond’s density also didn’t match up with earlier theoretical models. Materials typically become denser at high pressures, and diamond is no exception. But how fast its density changed was a surprise, the researchers said.
The experiment was a breakthrough, in that instead of smacking the diamond with high pressure in a stepwise fashion, such as hitting it with successively heavier hammers, the researchers were able to boost the pressure smoothly. This enabled them to crush the diamond and expose it to intense pressure without the substance becoming too hot and melting. (Diamonds can and do melt at sufficiently high temperatures).
Since diamonds are made of carbon, understanding how this material behaves at high pressures can be important in the study of planets around other stars, said Nikku (Madhu) Madhusudhan, a professor of astrophysics at the University of Cambridge.